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Regional Climate Change Effects: Useful Information for Transportation Agencies

3 Projected Climate Change by Geographic Region

This section begins with a brief review of national-scale changes in temperature, precipitation, sea level, and storm activity. This information is drawn largely from the USGCRP (2009) report. These national-scale descriptions are followed by the core of this report: a series of sections providing the current state of the knowledge concerning the aforementioned variables for every region in the country.11 When possible, related regional-scale information about extreme climate stressors is also provided to assist highway planners in relating climate change impacts on infrastructure of concern. For example, a given projection of changes in extreme events such as increases in heavy precipitation or, conversely, drought, may stress a given infrastructure if the projected conditions fall outside the design range. (The Climate Change Effects Typology Matrix in Appendix C provides detailed information of the entire, and broader, set of studies collected for this effort.) After discussion with climate experts, a set of inclusion criteria was produced that determined which of the collected studies would be discussed in this section (Section 2 and Appendix A provide further discussion of this step). As discussed in Section 2, there is a large degree of uncertainty associated with climate projections and, accordingly the projections are compiled from a number of climate models and across emission scenarios to help bracket the range of plausible futures. These projections are provided at varying degrees of uncertainty: "mean range," the "likely range," and the "very likely" range (as discussed in Section 2). Temperature projections for all ranges and regions are projected to rise. The plausible ranges for precipitation projections, however, may extend from a reduction to an increase, which complicates the explanation of regional precipitation patterns.

3.1 National projections

The following bullets provide a general overview of how temperature and precipitation are projected to change for the United States (USGCRP 2009):

3.1.1 Temperature

The likely range for global annual mean temperatures is projected to rise this century by 2 to 11.5°F by 2100; this range is based on the multi-model ensemble results across each of the six SRES (USGCRP 2009; IPCC 2007a).13 This warming is not projected to be evenly experienced around the globe. The greatest warming is projected to occur over land and in most high northern latitudes (IPCC 2007b). A number of studies suggest that irreversible, severe, and widespread impacts would be associated with a 2°F increase in average global temperatures above 1980-1999 levels (USGCRP 2009).

Within the United States, the annual mean temperatures by the end of the century are projected to warm by approximately 7 to 11°F under the higher A2 emissions scenario and by approximately 4 to 6.5°F under the lower B1 emissions scenario (USGCRP 2009). Summer months are projected to experience greater warming nationally compared with winter months. Figure 4 illustrates the warming projected for mid-century and end-of-century under a higher (A2) and lower (B1) emissions scenarios for the continental United States. In addition to increased mean summer temperatures, extreme heat days will grow in number while the number of extreme cold days will decrease. By the end of the century, extreme heat events that currently have a 5% chance of occurring each year are projected to have a 50% chance of occurring each year under a moderate (A1B) emission scenario (USGCRP 2009). This study further finds that in addition to more frequent occurrence of extreme heat waves, very hot days are projected to be about 10°F hotter than they are today (USGCRP 2009).

Figure 4
2040-2069 (A2 Emission Scenario) 2070-2099 (A2 Emission Scenario)
Figure 4 - click for long description Figure 4 - click for long description
2040-2069 (B1 Emission Scenario) 2070-2099 (B1 Emission Scenario)
Figure 4 - click for long description Figure 4 - click for long description

Figure 4. Projected mean summer temperature change (°F) relative to 1971 to 2000 based
on the projections of statistically downscaled CMIP3 projections (see Appendix B for additional
temperature projections).
14

3.1.2 Precipitation and Storm Events

Similar to temperature, projected precipitation is discussed in terms of seasonal averages and extreme precipitation events (i.e., heavy downpours). Discussion of annual precipitation, however, is limited in this report as it masks important variability, and provides, in some cases, misleading information. Seasonal precipitation, on the other hand, illustrates important trends while smoothing out the effects of heavy downpours. At a daily scale, discussions of heavy precipitation events, generally defined as greater than 2 inches per day, provide information about isolated storm activity;15 however, no processed information is readily available regarding other storm variables such as wind strength and direction, nor the type of storm causing the event. The combination of projected seasonal precipitation and precipitation events provide compelling information for system planning.

There is greater uncertainty associated with future changes in total precipitation compared with temperature, because precipitation is more heavily influenced by both small-scale phenomena and climate variability not captured by climate models. The uncertainty represented in sections 3.3 through 3.11 through the provided likely and very likely ranges encompasses the extent of plausible seasonal precipitation futures. The USGCRP report (2009) states that the confidence of seasonal precipitation projections for the United States is highest for winter and spring when precipitation is projected to increase significantly for the northern region as the boundary between the southern warm moist air and the northern cold air shifts northward. In addition, the northern regions are projected to experience more precipitation falling as rain and less as snow. Conversely, the southern regions are projected to experience significant reductions in precipitation during the winter and spring months, particularly in the Southwest.

Extreme precipitation events are projected to increase in frequency and intensity, while the amount that falls in light precipitation events is projected to decrease (USGCRP 2009). By the end of the century, heavy downpours that have a 5% chance of occurring in a given year are projected to have a 7 to 25% chance of occurring in a given year, depending on location (USGCRP 2009). In addition, heavy downpours that have a 5% chance of occurring today are projected to be 10% heavier under the lower emission scenario (B1) to 25% heavier under the higher emission scenario (A2) than it is now (USGCRP 2009). An apparent paradox of increased moisture leading to both increased drying conditions and increased heavy precipitation events is actually consistent with a warmer atmosphere. As temperatures increase, the air can hold more water vapor, allowing for increased amounts of evaporation. As more moisture enters the atmosphere, rises, and cools aloft, the water vapor condenses back to a liquid, leading to a greater amount of precipitation and increasing the energy associated with the storm (i.e., energy is released when water vapor condenses to a liquid).

The United States is home to an impressive and diversified set of storms. In the Great Plains, for example, convective storms can become so severe as to produce damaging strong winds, large-sized hail, and tornadoes. In the Southeast, convective storms in Florida produce a significantly high number of lightning strikes per year. In the Northeast, particularly New England, nor'easters16 can produce intense and damaging conditions. The Gulf states and the Southeast are vulnerable to tropical storms and hurricanes. The Pacific coast experiences coastal storm and flooding events resulting from the (in)famous Pineapple Express.17

The storm events currently experienced in the United States will likely evolve in response to a changing climate. Though the current research is somewhat insufficient to draw conclusive projections, some broad relationships between the plausible future conditions and the impacts these conditions may have on storm development have been discussed in the CCSP's (2008b) Weather and Climate Extremes in a Changing Climate report. This discussion provides information for three types of storm events: convective storms (e.g., thunderstorms), extratropical storms (e.g., cyclonic storms forming along a mid-latitude or high-latitude front), and tropical storms and hurricanes18 (e.g., organized thunderstorms with cyclonic motion originating in the tropics).

Convective storms that are very localized may in fact increase in intensity in response to the increase in atmospheric moisture, but decrease in duration or frequency. Projections of changes in convective storms are unclear and may improve with the application of nested models (regional models driven with GCM data that in turn feed information back to the GCM model).

The physical mechanisms associated with extratropical storms are not yet entirely understood even for present-day events; for example, major El Niños are understood to influence storm behavior, but it is unclear how the natural variability of other large-scale circulations affects these storms. To complicate matters, the characteristics such as sea-level pressure or strong surface winds used to define an extratropical storm differ between climate studies. Even under these circumstances, some consistent changes across studies of extratropical storms have been identified and suggest strong storms will be more frequent, while the overall number of storms may decrease.

The recent scientific consensus on tropical cyclonic activity (i.e., tropical storms and hurricanes, also known as typhoons in the Pacific, a cyclone in India, and a tropical cyclone in Australia) describes the globally averaged intensity of tropical cyclones as increasing by 2 to 11% by the end of the century (Knutson et al. 2010). On the other hand, the globally averaged frequency of tropical cyclones is consistently projected by modeling studies to decrease by 6 to 34%; however, results from higher resolution models suggest increases in both the frequency of intense storm activity and the precipitation rate within the storm center. It remains uncertain whether past changes in tropical storm activity are influenced by natural variability or human activity (Knutson et al. 2010; CCSP 2008b). It should be noted that precipitation presented by region in the following subsections encompasses changes in precipitation associated with storm events.

3.1.3 Sea-Level Rise

Detailed national information is not uniformly available for sea-level rise projections. Many of the state-of-the-science studies discussing sea-level rise provide only global projections. A number of factors determine sea level changes at any given location. On the global scale, the two most important factors are the expansion of ocean water as it warms, and changes in the amount of water in the oceans due to the accumulation and melting of ice sheets and glaciers and changes in the amount of water stored in reservoirs19. These factors are generally considered in the production of global sea level projections.

According to the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4), globally averaged sea level will rise by at least 5" by mid-century and 7" to 23" by end-of-century (IPCC 2007a). Due to significant uncertainty associated with future changes (e.g., melting rates) in the volume of glaciers and ice sheets at the time that report was written, the IPCC essentially excluded major contributions from those factors in its quantitative projections of sea level. Methods have been developed since the publication of the IPCC AR4 results that attempt to address these issues, though comparisons across recent studies can be difficult due to differing analytical approaches and Earth system components that are included in each study. Table 3 1 describes the global sea-level rise projections estimated by a number of recent studies for the end of the century, providing a range of projected global sea-level rise of 7 to 79". It is obvious that there is great variability in the results, but note that most of the estimates are significantly higher than those by the IPCC (2007a). The low end of the IPCC (2007a) projections is considered very conservative, for example, and assumes negligible contribution from melting of Greenland (DWR 2008). The continued rise in global sea level would greatly affect coastal locations, particularly those already vulnerable to storm surge. The impact of sea-level rise could be further exacerbated if coastal landforms that serve as storm-surge barriers are lost to extreme storm events such as a hurricane (USGCRP 2009).

Locally, other factors including vertical land motion (i.e., subsidence or uplift of land), sedimentation and erosion, ocean circulation, gravitationally induced changes, and ocean density (affected by regional changes in ocean salinity and ocean temperature) can also play a role (with vertical land motion often dominant), thus complicating the work of making projections at the sub-global level. Many of these factors are not well understood and are the subject of current research efforts, so any regional or local projections made at this time are fraught with uncertainty and should be considered carefully. It should be noted that the term "relative sea-level rise" refers to the changes in land elevation with respect to the level of the ocean determined by tide gauge measurements. Several different modeling and analytical tools are used to predict changes in each of these factors, which must be summed to determine local changes. Estimates of regional and local relative sea-level rise have been collected and are discussed in sections 3.3 through 3.11, as available. It should be noted there is no study that considers all these factors, nor is there a consistent methodology applied across these studies, so local projections should be considered carefully.

Table 3-1
Study Projection,
2100
Methodology
IPCC (2007a) 7" to 23" (18cm to 59cm) Accounts for thermal expansion and conservative estimates of changes in ice/snow melt.
Rahmstorf (2007) 20" to 55" (0.5m to 1.4m) Assumes a linear relationship between 20th century observed temperature and sea-level rise to obtain a proportionality constant of 3.4 mm/year per °C that was used in Rahmstorf's estimate of future sea level. This projection relies upon the assumption that the past statistical relationship remains constant in the future and uses the global mean temperature projections of the IPCC Third Assessment Report (TAR) were used.
Grinsted et al. (2009) 35" to 51" (0.9m to 1.3m) Uses four inversion experiments to relate 2,000 years of global temperatures to sea level and validated model parameters with satellite altimetry. The global mean temperature projections of 6 IPCC AR4 emission scenarios were used.
Rohling et al. (2008) 63" (1.6m) Uses paleoclimate data of the last interglacial period, when global mean temperatures were at least 2°C warmer than today and comparable to current projected temperatures.
Pfeffer et al. (2008) 31" to 79" (0.785m to 2.008m) Uses thermal expansion projected by IPCC AR4 together with kinematic scenarios (e.g., varying the velocities of outlet glaciers) to estimate the change in surface mass balance of ice of Greenland and Antarctica, and discharge of melting ice sheets and glaciers.

Table 3 1: Global projections of sea-level rise compared with 1990 levels, except Rohling et al. (2008) which describes projections per century. All results have been converted to inches, with study results provided in parenthesis. These studies do not account for additional regional factors that may cause regional sea-level change to be greater or less than the global average.

The IPCC factored in regional changes of ocean density and ocean circulation to estimate local sea-level change. Figure 5 illustrates the variation in sea-level rise when these factors are considered. Local sea-level changes will also be affected by other changes in smaller scale circulation patterns, and vertical land movement, not included in this figure.

Figure 5
Figure 5 - click for long description
Figure 5: The intent of this figure is to simply illustrate the potential variability of regional differences, and should not be relied on for planning purposes. This figure shows projected local sea-level change (in meters) due to ocean density and circulation change relative to the global average (i.e., positive values indicate greater local sea-level change than the global average) during the 21st century, calculated as the difference between averages for 2080 to 2099 and 1980 to 1999, as an ensemble mean over 16 AOGCMs forced with the SRES A1B scenario. Stippling denotes regions where the magnitude of the multi-model ensemble mean divided by the multi-model standard deviation exceeds 1.0 (IPCC 2007a). This figure does not include other local factors such as land uplift or subsidence.

3.1.4 Local Applications of Regional Data

Most of the regional projections provided in sections 3.3 through 3.11 are averaged across each region. Due to regional terrain or other phenomena, local variability within a region may be large and may affect the robustness of using an averaged regional projection. For example, Figure 6 demonstrates the large local variability in extreme heat events20 projected to occur at the end of the century for the contiguous United States.21 In this example, a regional average of the inter-mountain West would only be a very rough approximation of the local conditions.

Figure 6

Figure 6 - click for long description
Figure 6: Extreme heat days (days where the maximum temperature exceeds 90°F) (USGCRP 2009).

The confidence of these projections also varies regionally. Figure 7 presents projected changes in annual average precipitation for North America by 2080-2099 relative to precipitation in the recent past (USGCRP 2009). The hatched areas on the maps demonstrate projections where confidence is highest (that is, at least two out of three models agree on the sign of the projected change in precipitation). According to Figure 7, during the summer months, two large pockets of considerable reduction in precipitation are evident within the continental United States, though there is only high confidence in the Pacific Northwest drying. Overall, confidence is higher for the winter and spring seasons where the northern regions are projected to experience significantly more precipitation in response to the northward movement of the boundary between warm, moist southern air and cold, continental northern air (USGCRP 2009).

Figure 7
Figure 7 - click for long description
Figure 7: Projected future changes in precipitation by 2080-2099, relative to average seasonal precipitation 1961-1979 under the A2 emission scenario and simulated by 15 climate models (CMIP3 data; USGCRP 2009). Hatched areas show areas with highest confidence in the projected change.

3.2 Regional Summaries

Sections 3.3 through 3.11 provide regional discussions of climate projections of temperature and precipitation based largely on the results from the USGCRP (2009) report (however, the USGCRP report focuses on impacts and does not provide this information uniformly for all regions). The time ranges presented in the regional summaries (i.e., "near-term", "mid-century", and "end-of-century") are developed from the collection of peer-reviewed studies in the Climate Change Effects Typology Matrix. As noted earlier, the USGCRP (2009) time ranges fall within these time ranges but are not identical to them.

The USGCRP (2009) projections are summarized in the tables provided in each section as well as illustratively with the maps in Appendix C (these maps also illustrate the regional boundaries). The tables and maps provide the mean, likely range, and very likely range of multi-model ensemble results for "low" and "high" emission scenarios (see Section 2 for more information). If the collection of peer-reviewed studies within the Climate Change Effects Typology Matrix provide additional information and meet the criteria outlined in Figure 3, then the discussions of the USGCRP (2009) projections are enhanced with these additional studies. In many cases, these additional studies provide insight into changes of extreme events such as heat waves. It should be noted this report does not attempt to judge these studies, beyond meeting the inclusion criteria, and merely presents them.

The regional summaries provide climate projections of temperature and precipitation. Table 3-2 and Table 3-3 provide the observed annual and seasonal means averaged over the 1961 to 1979 time period.22 The means are provided as whole numbers to reflect the lack of precision in calculating a regional mean, given insufficient station density required to provide higher confidence. This information is provided for the continental United States and was not readily available for the other U.S. regions. Table 3-2 illustrates the large 15°F difference in annual mean temperatures experienced across the continental United States. There are large differences in seasonal mean temperatures experienced within each region, with summers exhibiting the warmest average temperatures and winters the coldest, with fall being slightly warmer than spring months.23 The change in the observed annual mean temperature averaged over 1993 to 2008 relative to the 1961-1979 baseline is also provided in the continental regional summaries (likewise for the observed annual mean precipitation).

Table 3-2
Region Mean Temperature ( °F)
Annual Winter Spring Summer Fall
Southeast 63 47 63 78 65
Northeast 47 24 45 67 50
Midwest 47 21 46 69 50
Great Plains 52 29 51 73 53
Southwest 55 39 53 72 57
Pacific Northwest 47 31 45 63 48

Table 3-2. 1961-1979 annual and seasonal mean temperature (°F) for the continental U.S. regions.

Table 3-3 provides observed mean precipitation for each continental U.S. region. Annually, the Southeast experiences the greatest amounts of precipitation. The Southwest is particularly drier in comparison. Overall, winter tends to be a drier season for most regions, with the other seasons somewhat comparable to each other. The Pacific Northwest is an exception with winter as the wettest season and summer as the driest.

Table 3-3
Region Mean Precipitation (inches)
Annual Winter Spring Summer Fall
Southeast 50 11 12 15 11
Northeast 41 9 10 11 10
Midwest 34 5 10 11 8
Great Plains 21 2 6 7 5
Southwest 15 5 3 3 4
Pacific Northwest 28 11 6 3 7

Table 3-3. 1961-1979 annual and seasonal precipitation mean (inches) for the continental U.S. regions. The total seasonal amounts may not equal the annual amount provided due to rounding.

Sections 3.3 through 3.11 also provide regional summaries of studies investigating local sea-level rise. In addition, a comparison of historical trends of the region24 to the global observed trend of sea-level rise of 1.8 ± 0.5 mm yr-1 from 1961 to 2003 (IPCC 2007a) is provided to illustrate past regional difference from the global average sea-level rise.

3.3 Northeast

3.3.1 Temperature

3.3.1.1 Near-term (2010-2040)

Within the next several decades, the Northeast is likely to experience an increase in annual mean temperature of approximately 2.5°F with a likely range of 1.9 to 3.2°F (USGCRP 2009; NECIA 2006; Frumhoff et al. 2007). This projected warming is greater than the 1.5°F increase experienced over the 1993-2008 time period when compared to a 1961-1979 baseline (USGCRP 2009).25 Winter temperatures over the same time period are projected to increase even more, by approximately 3.0°F with a likely range of 1.8 to 3.8°F. Near-term summer and spring temperature increases are projected to be slightly greater than 2.0°F (USGCRP 2009). These results are consistent with projections for Pennsylvania (NECIA 2008), while the NECIA (2006) report, which uses similar but not identical emission scenarios (the high emission scenario is A1Fi), projects similar mean temperature increases for annual and summer. However, NECIA (2006) projects that the winter mean temperature is projected to experience a slightly higher increase of 3.3 to 3.4°F with a likely range of 2.5 to 4.0°F.26 These temperature increases could lead to further reduction in the thickness and duration of winter ice on lakes and rivers, more precipitation falling as rain rather than snow, and earlier spring snowmelt affecting the timing of peak river flows.

The number of extreme heat days is also projected to increase across a number of Northeast cities. Buffalo, NY is projected to experience the smallest increase (of 2 to 5 days per year) and Philadelphia, PA and Pittsburgh, PA are projected to experience the greatest increase (9 to 11 days per year) (NECIA 2006).27 Boston, MA is projected to increase experience 4 to 8 additional days per year above 90°F (USGCRP 2009; Hayhoe et al 2008).28 Other cities projected to experience 5 to 10 more days per year of extreme heat include Concord, NH; Manchester, NH; Hartford, CT; and New York City, NY (NECIA 2006). Areas of Pennsylvania could experience more than a doubling of the frequency of extreme heat days (NECIA 2008).

3.3.1.2 Mid-century (2040 - 2070)

By mid-century, the increase in annual mean temperature for the Northeast is projected to be between 3.8 to 4.8°F with a likely range of 2.8 to 5.8°F (USGCRP 2009). This range is also representative of the increase projected for the summer and fall seasons (USGCRP 2009). The Northeast is projected to experience greater warming in the winter, with temperature increases projected of 4.0 to 5.4°F with a likely range of 2.9 to 6.6°F (USGCRP 2009). The temperature increase during the spring months is slightly lower than the annual average at 3.5 to 4.1°F, with a likely range of 2.2 to 5.5°F (USGCRP 2009). The frequency of extreme heat days in Northeast cities is also likely to rise, with Boston, for example, seeing an additional 12 to 29 days over 90°F (USGCRP 2009). Seven individual Northeast cities examined are projected to experience an increase of approximately 8 to 39 extreme heat days per year (NECIA 2006). The northern cities tend to be represented by the low end of this range and the southern cities by the high end.

Table 3-4
Northeast (Δ Temperature) Near-term (°F) Mid-century (°F) End-of-century (°F)
Annual Mean 2.5 3.8 - 4.8 5.4 - 9.0
Likely 1.9 - 3.2 2.8 - 5.8 4.2 - 10.8
Very Likely 1.3 - 3.8 1.9 - 6.8 3.0 - 12.5
Winter Mean 2.8 - 3.0 4.0 - 5.4 5.9 - 9.3
Likely 1.8 - 3.8 2.9 - 6.6 4.7 - 11.0
Very Likely 0.9 - 4.7 1.8 - 7.9 3.5 - 12.8
Spring Mean 2.0 - 2.2 3.5 - 4.1 5.0 - 8.1
Likely 1.2 - 3.0 2.2 - 5.5 3.6 - 10.0
Very Likely 0.4 - 3.8 0.9 - 6.8 2.3 - 11.9
Summer Mean 2.3 - 2.5 3.7 - 4.8 5.2 - 9.4
Likely 1.8 - 3.1 2.8 - 5.8 3.9 - 11.8
Very Likely 1.3 - 3.7 1.8 - 6.9 2.7 - 14.1
Fall Mean 2.5 - 2.7 3.9 - 4.8 5.3 - 9.1
Likely 1.9 - 3.3 2.8 - 5.6 3.9 - 10.8
Very Likely 1.2 - 3.9 1.8 - 6.5 2.5 - 12.8
Table 3-4: Annual and seasonal temperature changes for the Northeast region over the near-term (2010-2029), mid-century (2040-2059) and end-of-century (2080-2098) relative to 1961-1979. The range values are from low (B1) and high (A2) emissions scenarios. Data are from the USGCRP (2009).
3.3.1.3 End-of-century (2070-2100)

By the end-of-century, the warming in the Northeast is likely to be quite significant. The projected annual mean increase is expected to be between 5.4 to 9.0°F, with a likely range of 4.2 to 10.8°F. The winter, summer, and fall changes are all relatively similar to the annual mean (USGCRP 2009). The spring months are projected to experience the lowest mean increase of all the seasons, with a mean warming of 5.0 to 8.1°F and a likely range of 3.6 to 10.0°F (USGCRP 2009). Though the NECIA (2006) study estimates similar projections, the Pennsylvania study suggests greater increases, with winter temperature projected to rise 8°F and summer temperatures projected to rise 11°F. Not only are temperatures projected to change, but also the duration of each season. Overall, under a "business as usual" scenario (A2), winters in the Northeast are projected to shorten, with the length of the winter snow season cut in half for the northern half of the region, including New York, Vermont, New Hampshire, and Maine, and reduced to a few weeks in the southern half (USGCRP 2009). Summer-like temperatures, on the other hand, are projected to persist for 6 weeks longer than usual (USGCRP 2009).

Extreme heat events29 that currently have a 5% chance of occurring each year are projected to have a 50% chance of occurring each year by late century (USGCRP 2009). Many Northeast cities are projected to experience approximately 13 to 63 more days reaching 90°F by the end of this century compared with today's observations (NECIA 2006). The northern cities in the region tend to fall in the low end of this range and the southern cities in the high end.

3.3.2 Precipitation and Storm Events

3.3.2.1 Near-term (2010-2040)

Current observations averaged over the 1993 to 2008 time period suggest that annual mean precipitation has increased by 7% relative to the 1961-1979 time period (USGCRP 2009).30 Within the next several decades, the Northeast is likely to experience wetter winter months with an average precipitation increase of about 6% and a likely range of +2 to +11% (USGCRP 2009). The fall months have the lowest mean projected increases of 1 to 2% with a likely range of -4 to +6% (USGCRP 2009). The spring and summer are projected to experience similar near-term increases in precipitation. Spring precipitation is projected to increase by 3% with a likely range of -2 to +7%, and the summer months projected to increase by 2% with a likely range of -1 to +6% (USGCRP 2009).

Understanding how precipitation intensity, duration, and frequency may change is important for planning purposes. Individual precipitation events are likely to increase in intensity by approximately 7% (NECIA 2006), so more rain may be arriving in brief pulses. Specifically, the maximum amount of precipitation to fall during any five-day period in a year is projected to increase by 9 to 12% (NECIA 2006).

Table 3-5
Northeast (Δ Precipitation) Near-term (%) Mid-century (%) End-of-century (%)
Winter Mean 6 8 - 11 11 - 17
Likely 2 - 11 2 - 18 4 - 27
Very Likely (2) - 15 (4) - 26 (4) - 36
Spring Mean 3 5 - 6 9 - 11
Likely (2) - 7 0 - 12 1 - 21
Very Likely (7) - 12 (5) - 17 (9) - 31
Summer Mean 2 1 - 2 (1) - 2
Likely (1) - 6 (6) - 7 (12) - 11
Very Likely (5) - 10 (12) - 14 (24) - 23
Fall Mean 1 - 2 3 3 - 4
Likely (4) - 6 (3) - 9 (5) - 13
Very Likely (10) - 11 (9) - 16 (15) - 23
Table 3-5: Seasonal precipitation percent changes for the Northeast region over the near-term (2010-2029), mid-century (2040-2059) and end-of-century (2080-2098) relative to 1961-1979. The range values are from low (B1) and high (A2) emissions scenarios. Values in parentheses are negative values and represent decreases in precipitation. Data are from the USGCRP (2009).
3.3.2.2 Mid-century (2040-2070)

Overall, precipitation in the Northeast is projected to increase by mid-century across all seasons, with the greatest change again projected to occur in the winter months. By mid-century, the average winter precipitation increase projected for the Northeast is 8% and 11%, with a likely range of +2 to +18% (USGCRP 2009). Another study that used a higher bounding emission scenario (A1Fi) reported an even greater increase in winter precipitation of 11 to 16% for most of this region including New England, New York, New Jersey, and Pennsylvania (NECIA 2006). Spring and fall precipitation are projected to increase more moderately at 5 to 6% with a likely range of +0 to +12%, and 3% with a likely range of -3 to +9%, respectively. Summer precipitation is projected to increase the least of all the seasons, with an increase of only 1 to 2% and a likely range of -6 to +7% (USGCRP 2009).

By mid-century, this region is projected to experience more than an 8% increase in the average amount of rain that falls on any given rainy day, with the duration of extreme rain events31 expected to increase by 1 to 1.5 days. In addition, the maximum amount of precipitation falling during any five-day period in a year is projected to increase by 8 to 13% (NECIA 2006). These increases in heavy rainfall events will increase the risk of floods for the Northeast region. NECIA (2006) suggests little change in the frequency of winter-time storms for the East Coast. However, under the "high-end" scenario (A1Fi), between 5 and 15% of these storms (an additional 1 storm per year) will move northward during late winter (Jan, Feb, March), affecting the Northeast. No change is projected for the "low-end" (B1) scenario. In addition, the impact of a higher sea level will increase the likelihood of storm damage to coastal locations.

3.3.2.3 End-of-century (2070-2100)

By the end-of-century, the Northeast is projected to experience the greatest seasonal increase in precipitation during the winter months. This increase is projected to be 11 to 17%, with a likely range of +4 to +27%. Summer will continue to be the least affected of the seasons, with average increases in total seasonal precipitation projected to be 2% under the high emissions scenario and -1% under the low emissions scenario (USGCRP 2009). By the end of the century, the intensity of any particular precipitation event is projected to increase, on average, by 12 to 13% (NECIA 2006). Additionally, the number of days in a given year with precipitation events of greater than two inches per day is projected to slightly increase by an additional 1.25 to 1.75 days per year (NECIA 2006). As air temperatures rise, the Northeast can expect a continuation of recent trends in the type of precipitation experienced during winter: less snow and more rain (NECIA 2006). Most of the Northeast could lose approximately four to 15 snow-covered days per winter month, with a 25 to 50% reduction in the length of the snow season with the onset of an earlier spring (NECIA 2006).

3.3.3 Sea-Level Rise

Global sea-level rise (SLR) of 7 to 79" (18cm to 2.0m) is projected for 2100 (see section 3.1.3. for discussion on global and local sea-level rise). SLR at the local/regional level is influenced by multiple factors, including sedimentation and erosion, ocean circulation, gravitationally induced changes, ocean density (affected by regional changes in ocean salinity and ocean temperature), and vertical motion of the land (subsidence or uplift). In the 20th century, the relative sea-level rise for the Northeast was greater than the level of global sea-level rise.32

The following discussion describes studies providing local sea-level rise projections for the Northeast. As noted in section 3.1.3 above, making local or regional projections is highly uncertain, given the incomplete understanding of some of the effects that can take place at the local level. There is no study that considers all these factors, nor is there a consistent methodology for projecting sea-level rise applied across these studies. Therefore, local SLR projections, while informative, should be considered carefully, and with a clear understanding of what factors each study includes or excludes.

Yin et al. (2009) projects that the Northeast coastline could experience a sea-level rise much greater than the global average, due to changes in ocean circulation.33 Yin et al. (2009) estimate that these ocean circulation changes will increase sea level in New York City, NY by 5.9 to 8.3" (15cm to 21cm) above what would be expected from global sea-level rise and local vertical land motions alone (this study does not consider other factors such as erosion or sedimentation). Similar amplifications are projected for Washington, DC and Boston, MA.

Sea-level rise exacerbates the impacts of strong storm events. Kirshen et al.'s (2008) analysis indicates that the storm surge elevations across the Northeast associated with a storm that has a 1% chance of occurring in 2005 are projected to increase substantially due to sea-level rise, even as storm intensity remains unchanged.34 Using the lower emission scenario (B1) as the lower bound and the Rahmstorf (2007) study as the upper bound, Kirshen et al. made the following projections for sea-level-rise-induced increases in storm surge elevation by 2100: Atlantic City, NJ is projected to experience the greatest increase of 46.9 to 74.4" (119cm to 189cm), and Woods Hole, MA is projected to experience the least at 13.2 to 28.8" (33.5 to 73.2cm). New York City, NY; Boston, MA; and New London, CT fell within the range between Woods Hole, MA and Atlantic City, NJ. This study further suggests that a flood that has a 1% chance of occurring in New York City in 2005 has about a 5% (low emission scenario) to a 50% chance (high emission scenario) of occurring in a year by 2100. Kirshen et al. (2008) did not consider other regional sea-level rise effects, such as ocean circulation or wind patterns, changes in the relative elevation of coastal land (i.e., caused by subsidence or uplift), or local changes in ocean density.

3.4 Southeast

3.4.1 Temperature

3.4.1.1 Near-term (2010-2040)

Within the next two decades, the annual mean temperature in the Southeast is projected to increase by approximately 2°F with a likely range of 1.7 to 2.7°F (USGCRP 2009; CCSP 2008a). This projected warming is greater than the 1.2°F increase already experienced over the 1993 to 2008 time period compared with a 1961-1979 baseline (USGCRP 2009).35 Projected seasonal temperatures exhibit similar increases, with both summer and fall temperatures projected to increase by slightly greater amounts, and the winter and spring by slightly less (USGCRP 2009). Not only are summer temperatures projected to increase, but so are the number of extreme heat days.36 By 2030, Houston, TX is projected under a higher emission scenario (A2) to experience a 25 to 75% probability of having 4 to 11 days above 100°F (in 2007, the probability of 4 days at 100°F was over 45% and less than 10% for 11 days) (CCSP 2008a).37

3.4.1.2 Mid-century (2040-2070)

By mid-century, the increase in annual mean temperature is projected to be approximately 3.2 to 4.0°F, with a likely range of 2.4 to 4.8°F (USGCRP 2009). The projections for temperature increases in spring and fall are relatively similar to the annual mean. The mean temperature for the summer months is projected to be the highest mean increase of all the seasons, with a mean warming of 3.5 to 4.5°F and a likely range of 2.5 to 5.6°F. The mean temperature for the winter months is projected to be slightly lower than the annual mean, with a mean warming of 2.7 to 3.6°F and a likely range of 1.6 to 4.5°F (USGCRP 2009). Extreme heat days are projected to continue to increase. By 2060, Houston, TX is projected to have a 25 to 75% probability of having 14 to more than 20 days above 100°F per year (in 2007, the probability of 14 days or more at 100°F was less than 5%) (CCSP 2008a).

3.4.1.3 End-of-century (2070-2100)

By the end-of-century, annual mean temperature in the Southeast region is projected to increase by 4.5 to 7.8°F, with a likely range of 3.4 to 9.4°F (USGCRP 2009). The projections for the spring months are similar to the annual mean. The Southeast is projected to experience the smallest warming in winter, with temperature increases of 4.0 to 6.3°F and a likely range of 2.8 to 7.9°F (USGCRP 2009). The summer months are projected to experience the greatest warming of all the seasons, with temperature increases of 4.8 to 9.0°F and a likely range of 3.5 to 11.2°F (USGCRP 2009). The fall months are projected to display a slightly higher mean than the annual average, with mean increases projected to be 4.7 to 8.3°F and a likely range of 3.5 to 9.8°F (USGCRP 2009).

Table 3-6
Southeast (Δ Temperature) Near-term (°F) Mid-century (°F) End-of-century (°F)
Annual Mean 2.1 - 2.2 3.2 - 4.0 4.5 - 7.8
Likely 1.7 - 2.7 2.4 - 4.8 3.4 - 9.4
Very Likely 1.2 - 3.2 1.6 - 5.5 2.4 - 10.9
Winter Mean 1.9 - 2.1 2.7 - 3.6 4.0 - 6.3
Likely 1.1 - 2.8 1.6 - 4.5 2.8 - 7.9
Very Likely 0.3 - 3.6 0.5 - 5.4 1.7 - 9.4
Spring Mean 1.8 - 2.0 3.1 - 3.8 4.4 - 7.5
Likely 1.3 - 2.7 2.2 - 4.6 3.2 - 9.1
Very Likely 0.6 - 3.3 1.3 - 5.4 2.0 - 10.7
Summer Mean 2.3 - 2.4 3.5 - 4.5 4.8 - 9.0
Likely 1.5 - 3.0 2.5 - 5.6 3.5 - 11.2
Very Likely 0.7 - 3.8 1.6 - 6.7 2.3 - 13.5
Fall Mean 2.3 3.4 - 4.3 4.7 - 8.3
Likely 1.8 - 2.9 2.6 - 4.9 3.5 - 9.8
Very Likely 1.2 - 3.4 1.8 - 5.6 2.4 - 11.3

Table 3-6: Annual and seasonal temperature changes for the Southeast region over the near-term (2010-2029), mid-century (2040-2059) and end-of-century (2080-2098) relative to 1961-1979. The range values are from low (B1) and high (A2) emissions scenarios. Data are from the USGCRP (2009).

By the end-of-century, studies agree that the Southeast region will experience more extreme heat events. Between 2080 and 2100, extreme heat events (a combination of temperature and humidity) that currently have a 5% chance occurring for a given year are projected to have about a 50 to 100% chance of occurring each year (USGCRP 2009). By 2099, Houston, TX is projected to experience a near 100% probability of having more than 20 days above 100°F per year (current probability of 20 days at or above 100°F is near 0%) (CCSP 2008a). Similarly, the Appalachian Mountain region is predicted to experience approximately three times more "high temperature" days38 each year by the end-of-century under a higher emission scenario (A2). (Diffenbaugh et al 2005).

3.4.2 Precipitation and Storm Events

3.4.2.1 Near-term (2010-2040)

Current observations averaged over 1993 to 2008 suggest annual mean precipitation has decreased by 1% relative to 1961-1979 (USGCRP 2009).39 In the near-term, mean precipitation in the Southeast does not exhibit a strong trend, but is generally projected to decrease in the summer and spring and increase in the fall. There is considerable disagreement between various climate models on the magnitude and, in some cases, direction of changes in precipitation in each time period.

Over the next two decades, the greatest increases in mean precipitation are projected to occur during the fall months and the greatest decreases during the spring months. The fall months are projected to increase by 1 to 2% with a likely range of -4 to +7%, while the spring months are projected to decrease by 0 to 2% with a likely range of -7 to +4% (USGCRP 2009). The winter and summer months exhibit very little change with a likely range of -6 to +5% and -8 to +8%, respectively (USGCRP 2009). In addition, if current observational trends continue, the spacing between precipitation events could continue to increase, leading to continued periods of drought (USGCRP 2009).

Table 3-7
Southeast (Δ Precipitation) Near-term (%) Mid-century (%) End-of-century (%)
Winter Mean (1) - 0 (2) - 1 (3) - 0
Likely (6) - 5 (8) - 9 (15) - 10
Very Likely (11) - 9 (15) - 16 (28) - 22
Spring Mean (2) - 0 1 - 2 (7) - 1
Likely (7) - 4 (5) - 8 (20) - 7
Very Likely (12) - 8 (11) - 14 (32) - 18
Summer Mean 0 (2) - 0 (8) - 0
Likely (8) - 8 (14) - 10 (29) - 14
Very Likely (16) - 16 (26) - 23 (50) - 35
Fall Mean 1 - 2 (2) - (1) 2 - 3
Likely (4) - 7 (9) - 5 (9) - 16
Very Likely (10) - 12 (16) - 12 (21) - 28

Table 3-7: Seasonal precipitation percent changes for the Southeast region over the near-term (2010-2029), mid-century (2040-2059) and end-of-century (2080-2098) relative to 1961-1979. The range values are from low (B1) and high (A2) emissions scenarios. Values in parentheses are negative values and represent decreases in precipitation. Data are from the USGCRP (2009).

3.4.2.2 Mid-century (2040-2070)

By mid-century, the greatest seasonal mean change in precipitation between emission scenario results is projected to occur during the winter months, with the mean ranging from an increase of 1% to a decrease of 2%, and a likely range of -8 to +9% (USGCRP 2009). The summer months exhibit a reduction in mean precipitation between 0 to 2% with a likely range of -14 to +10%. The fall months are projected to sustain a reduction in mean precipitation of 1 to 2% with a likely range of -9 to +5% (USGCRP 2009). Spring is the only season projected to have an increase in precipitation with a mean change of 1 to 2%, and a likely range of -5 to +8% (USGCRP 2009).

3.4.2.3 End-of-century (2070-2100)

At the end-of-century, the largest seasonal mean decrease of precipitation is projected to occur during the summer season, with a mean decrease of about 0 to 8% and a likely range of -29 to +14% (USGCRP 2009). The spring season mean change ranges varies from 1 to -7% with a likely range of -20 to +7% (USGCRP 2009). The mean change for the winter season varies between 0% to a reduction of 3% with a likely range of -15 to +10% (USGCRP 2009). In contrast, the mean fall precipitation is projected to increase from 2 to 3% with a likely range -9 to +16% (USGCRP 2009). The greatest degree of certainty is that precipitation in the fall will increase, while the other precipitation effects are not as certain (USGCRP 2009). Diffenbaugh et al. (2005) found that the mid-Atlantic coast would be up to 40% rainier at the end-of-century overall.

By late century, the Gulf Coast region from Galveston, TX to Mobile Bay, AL is projected to experience an intensity increase of 5% for category 1 storms and 20% for category 4 storms (CCSP 2008a). This projection assumes that changes in hurricane intensity are directly related to increases in projected sea surface temperatures. For example, sea surface temperatures in the Atlantic hurricane formation region are projected to increase from 3 to 7°F, leading to increased tropical storms (CCSP 2008a). This does not take into account changes in other contributors to tropical storm development (e.g., vertical wind shear and vertical temperature structure), and hence, higher sea surface temperatures do not necessarily translate to an increased storm intensity.

3.4.3 Sea-Level Rise

Global sea-level rise (SLR) of 7 to 79" (18 cm to 2.0 m) is projected for 2100 (see section 3.1.3. for discussion on global and local sea-level rise). SLR at the local/regional level is influenced by multiple factors, including: sedimentation and erosion, ocean circulation, gravitationally induced changes, ocean density (affected by regional changes in ocean salinity and ocean temperature), and vertical motion of the land (subsidence/uplift). Using historical records, the relative sea-level rise for the Southeast was greater than the level of global sea-level rise.40 This finding is consistent with the CCSP (2008a) study, which provides estimates of subsidence rates along the Gulf Coast that would lead to higher sea levels than the global average: Louisiana-Texas Chenier Plain at 0.19 in/yr (4.7 mm/yr), Louisiana Deltaic Plain at 0.32 in/yr (8.05 mm/yr), and Mississippi-Alabama Sound at 0.01 in/yr (0.34 mm/yr).

3.5 Midwest

3.5.1 Temperature

3.5.1.1 Near-term (2010-2040)

Within the next two decades, the annual mean temperature in the Midwest is projected to increase by approximately 2.7°F with a likely range of 1.9 to 3.3°F (USGCRP 2009; Union of Concerned Scientists 2009). This projected warming is greater than the 1.4°F increase already experienced over the 1993 to 2008 time period compared with a 1961-1979 baseline (USGCRP 2009).41 Projected summer, fall, and winter seasonal temperatures are expected to exhibit similar increases compared with the annual mean (USGCRP 2009). Spring seasonal temperatures are projected to increase slightly less than the annual mean at 2.0 to 2.4°F, with a likely range of 1.2 to 3.3°F (USGCRP 2009). These projections are similar to those provided for Chicago, IL, where temperature increases of approximately 1.8 to 3.6°F (1 to 2°C) are projected (Hellmann et al 2007).42 Observed increases in the winter season have extended the frost-free (or growing) season by a week (USGCRP 2009); the growing season is expected to continue to lengthen as temperatures continue to warm in spring and fall. In addition, higher temperatures lead to increased evaporation, reducing water levels in the Great Lakes (USGCRP 2009).

3.5.1.2 Mid-century (2040-2070)

By mid-century, the annual mean temperature increase is projected to be approximately 4.0 to 5.0°F, with a likely range of 3.0 to 6.0°F (USGCRP 2009). The projections for summer, fall, and winter temperatures are again similar to the annual mean. However, the temperature increase for the spring months is projected to be less, with a seasonal average of 3.6 to 4.2°F and a likely range of 2.2 to 5.6°F (USGCRP 2009). Chicago may experience a higher annual temperature than that suggested for the region, with an increase between 2.7 to 9°F (1.5 to 5°C) by 2070 (Hellmann et al 2007).

3.5.1.3 End-of-century (2070-2100)

By the end-of-century, annual mean temperature is projected to increase by approximately 5.6 to 9.6°F in the Midwest with a likely range of 4.3 to 11.7°F (USGCRP 2009). The projections for fall and winter temperatures continue to be similar to the annual mean. However, the spring months are projected to experience a smaller temperature increase of 5.1 to 8.4°F, with a likely range of 3.5 to 10.6°F (USGCRP 2009). The summer months display a higher annual mean temperature increase of 5.6 to 10.8°F, with a likely range of 4.2 to 14.2°F (USGCRP 2009). Annual mean temperature increases in Chicago are projected to be similar to these regional increases (Hellmann et al 2007). However, one set of studies found even higher annual mean temperature increases ranging between 6 and 14°F in Minnesota, Missouri, Indiana, Wisconsin, and Ohio, associated with a lower (B1) emission scenario and a higher (A1Fi) emission scenario (Union of Concerned Scientists 2009a-e).43

Table 3-8
Midwest (Δ Temperature) Near-term (°F) Mid-century (°F) End-of-century (°F)
Annual Mean 2.6 - 2.7 4.0 - 5.0 5.6 - 9.6
Likely 1.9 - 3.3 3.0 - 6.0 4.3 - 11.7
Very Likely 1.3 - 3.9 1.9 - 7.0 3.0 - 13.8
Winter Mean 2.6 - 3.0 4.1 - 5.3 6.0 - 9.4
Likely 1.6 - 4.0 2.9 - 8.0 4.6 - 11.5
Very Likely 0.6 - 4.9 1.7 - 7.9 3.3 - 13.5
Spring Mean 2.0 - 2.4 3.6 - 4.2 5.1 - 8.4
Likely 1.2 - 3.3 2.2 - 5.6 3.5 - 10.6
Very Likely 0.4 - 4.1 0.8 - 7.0 1.9 - 12.9
Summer Mean 2.6 - 2.8 4.1 - 5.3 5.6 - 10.8
Likely 1.9 - 3.8 2.8 - 6.8 4.2 - 14.2
Very Likely 1.0 - 4.7 1.5 - 8.3 2.7 - 17.5
Fall Mean 2.6 - 2.7 4.0 - 4.9 5.5 - 9.6
Likely 2.0 - 3.4 2.9 - 5.8 4.1 - 11.6
Very Likely 1.3 - 4.1 1.7 - 6.7 2.7 - 13.6

Table 3-8: Annual and seasonal temperature changes for the Midwest region over the near-term (2010-2029), mid-century (2040-2059) and end-of-century (2080-2098) relative to 1961-1979. The range values are from low (B1) and high (A2) emissions scenarios. Data are from the USGCRP (2009).

By the end-of-century, heat waves in the Midwest are expected to become longer, hotter, and more frequent (Ebi and Meehl 2007; USGCRP 2009). By 2100, under a higher (A2) emission scenario, a heat event that currently has a 5% chance of occurring for a given year is projected to have about a 50 to 100% chance of occurring for a given year (USGCRP 2009). In Chicago and Cincinnati, the frequency of heat waves is expected to increase 24% and 50% respectively under a business as usual scenario (Ebi and Meehl 2007).44 The average duration of these heat waves is also projected to increase by approximately 20% in both cities. Similarly, under a high (A1Fi) emission scenario, cities across the Midwest, including Des Moines, Cincinnati, and Indianapolis, are projected to experience between 65 and 85 days over 90°F each summer by the end-of-century compared with approximately 10 to 20 days averaged over 1961-1990 (Union of Concerned Scientists 2009a).45

Figure 8
Figure 8 - click for long description
Figure 8: Average number of freezing days per year in the Midwest (defined as days the minimum temperature is below 32°F). The illustrations are 16 multi-model ensemble averages for years 1961-1979, 2040-2059, and 2080-2099 for SRES A2. (USGCRP 2009)

Figure 8 illustrates the reduction of freezing days projected for the Midwest; this is particularly evident for the northern areas. For example, southern Minnesota experienced about 170 freezing days in 1961-1979 and is projected to experience about 110 freezing days in 2080-2099 under the A2 scenario.

3.5.2 Precipitation and Storm Events

3.5.2.1 Near-term (2010-2040)

Current observations averaged over 1993 to 2008 suggest that annual mean precipitation has increased by 5% relative to the 1961-1979 time period (USGCRP 2009). By far the largest seasonal increase in precipitation is projected to occur during the winter months, with an average increase of 6 to 7% and a likely range of +2 to +12% (USGCRP 2009).46 Annual mean precipitation in Chicago is projected to experience precipitation increases in line with the regional estimates (Hellmann et al. 2007). Heavy precipitation events are also projected to increase during this time, with the frequency of spring rainfall heavy downpours increasing by almost 15% in Missouri, Illinois, and Minnesota under a high emission scenario (A1Fi) compared with 1961-1990 (Union of Concerned Scientists 2009a).47 In the next two decades, heavy rains are projected to increase by 66% in St. Paul, 35% in Indianapolis, and 20% in Chicago (Union of Concerned Scientists 2009). These increases are expected to increase flooding and overload many drainage systems (USGCRP 2009).

3.5.2.2 Mid-century (2040-2070)

Precipitation increases are projected to occur in the winter and spring months. Winter precipitation, for example, is projected to increase by 8 to 9% with a likely range of +1 to +15% (USGCRP 2009). Precipitation is projected to decrease by an average of 1 to 4% in the summer months with a likely range of -15 to +8% (USGCRP 2009). However, studies disagree on the magnitude of predicted changes in precipitation. Precipitation changes in Chicago are projected to be between -2 and 10% by 2070 (Hellmann et al 2007).

Table 3-9
Midwest (Δ Precipitation) Near-term (%) Mid-century (%) End-of-century (%)
Winter Mean 6 - 7 8 - 9 10 - 14
Likely 2 - 12 1 - 15 3 - 22
Very Likely (3) - 16 (6) - 21 (3) - 30
Spring Mean 3 - 4 7 - 9 10 - 14
Likely (1) - 8 3 - 13 2 - 25
Very Likely (6) - 12 (1) - 18 (9) - 36
Summer Mean (1) (4) - (1) (9) - (2)
Likely (7) - 6 (15) - 8 (31) - 14
Very Likely (14) - 13 (26) - 19 (53) - 36
Fall Mean 1 1 - 3 2 - 3
Likely (5) - 7 (6) - 11 (10) - 17
Very Likely (11) - 13 (14) - 18 (23) - 30

Table 3-9: Seasonal precipitation percent changes for the Midwest region over the near-term (2010-2029), mid-century (2040-2059) and end-of-century (2080-2098) relative to 1961-1979. The range values are from low (B1) and high (A2) emissions scenarios. Values in parentheses are negative values and represent decreases in precipitation. Data are from the USGCRP (2009).

3.5.2.3 End-of-century (2070-2100)

Again, the majority of the increase in precipitation will occur during the winter and spring months, in which precipitation is projected to increase by an average of 10 to 14% with a likely range of +3 to +22%, and 10 to 14% with a likely range of +2 to +25%, respectively (USGCRP 2009). Average summer precipitation is projected to decrease by 2 to 9% with a likely range of -31 to +14% (USGCRP 2009). USGCRP (2009) projects the likelihood of summer-time drought increasing. Annual mean precipitation in Chicago is projected to change by -1 to +19% by the end-of-century (Hellmann et al. 2007). There was some disagreement between studies on the magnitude of seasonal mean precipitation. A study by the Union of Concerned Scientists (2009) found higher overall increases in winter precipitation, with 20-50% increases in Missouri, Minnesota, and Michigan.48 The same study found 10-20% less rain in summer precipitation for most of the Midwest region by the end-of-century. Heavy spring downpours (defined as two inches of rain in one day) are expected to become more frequent during this time frame, with approximately 30% increases projected for Iowa, Ohio, Illinois, and Wisconsin (Union of Concerned Scientists 2009a-e).

3.6 Great Plains

3.6.1 Temperature

3.6.1.1 Near-term (2010-2040)

Within the next two decades, the annual mean temperature of the Great Plains is projected to increase by approximately 2.5°F, with a likely range of 1.8 to 3.1°F (USGCRP 2009). This projected warming is greater than the 1.3°F increase already experienced over the 1993 to 2008 time period relative to a 1961-1979 baseline (USGCRP 2009).49 Fall temperature increases are projected to be similar to the projected annual mean warming. Summer temperature increases are projected to be slightly higher than the annual mean warming, at 2.7 to 2.9°F, with a likely range of 1.8 to 3.7°F (USGCRP 2009). Spring temperature increases are expected to be slightly lower than the annual mean increase, at 1.9 to 2.2°F, with a likely range of 1.2 to 3.0°F (USGCRP 2009); likewise, winter temperature increases are also expected to be slightly lower than the annual mean increase, at 2.2 to 2.5°F, with a likely range of 1.4 to 3.4°F (USGCRP 2009).

Great Plains (Δ Temperature) Near-term (°F) Mid-century (°F) End-of-century (°F)
Annual Mean 2.4-2.5 3.8-4.7 5.4-9.2
Likely 1.8-3.1 2.7-5.8 3.9-11.2
Very Likely 1.1-3.8 1.6-6.9 2.5-13.2
Winter Mean 2.2-2.5 3.6-4.3 5.3-8.3
Likely 1.4-3.4 2.4-5.6 3.8-10.4
Very Likely 0.6-4.2 1.2-6.9 2.2-12.5
Spring Mean 1.9-2.2 3.4-4.0 4.8-8.0
Likely 1.2-3.0 2.1-5.5 3.1-10.3
Very Likely 0.5-3.9 0.8-6.9 1.3-12.7
Summer Mean 2.7-2.9 4.3-5.6 5.8-10.6
Likely 1.8-3.7 3.0-7.1 4.1-13.6
Very Likely 0.8-4.6 1.7-8.7 2.4-16.6
Fall Mean 2.4-2.5 3.8-4.7 5.5-9.6
Likely 1.8-3.3 2.7-5.7 4.0-11.5
Very Likely 1.1-4.0 1.6-6.7 2.4-13.5

Table 3 10: Annual and seasonal temperature changes for the Great Plains region over the near-term (2010-2029), mid-century (2040-2059) and end-of-century (2080-2098) relative to 1961-1979. The range values are from low (B1) and high (A2) emissions scenarios. Data are from the USGCRP (2009).

3.6.1.2 Mid-century (2040-2070)

By mid-century, annual mean temperature is projected to increase by 3.8 to 4.7°F with a likely range of 2.7 to 5.8°F (USGCRP 2009). Fall temperatures are expected to increase similarly to the annual mean. Summer months are projected to have a greater temperature increase of 4.3 to 5.6°F, with a likely range of 3.0 to 7.1°F (USGCRP 2009). Winter warming is projected to be lower than the annual mean increase. Warming in the spring is projected to be the least of all seasons, with an increase of 3.4 to 4.0°F and a likely range of 2.1 to 5.5°F (USGCRP 2009).

3.6.1.3 End-of-century (2070-2100)

By the end-of-century, annual mean temperature is projected to have increased by 5.4°F to 9.2°F with a likely range of 3.9 to 11.2°F (USGCRP 2009). Seasonal warming trends are projected to continue, with slightly greater or similar warming in the summer and fall months, and less warming in the winter and spring months. Summer mean temperatures are expected to increase 5.8 to 10.6°F, with a likely range of 4.1 to 13.6°F (USGCRP 2009). The smallest seasonal temperature increase is projected to occur in the spring, with an increase of 4.8 to 8.0°F and a likely range of 3.1 to 10.3°F (USGCRP 2009).

Lenihan et al. (2008) made projections of changes in maximum temperatures for the Great Plains region (the USGCRP projections reported above are for mean temperature).50 They projected that the late century increase in maximum temperatures would be greatest in the central and northern areas of the Great Plains. The authors found that average monthly maximum temperatures would increase by 7 to 13°F (4 to 7°C) across Oklahoma, Kansas, Nebraska, and North and South Dakota under lower emission (B2) and higher emission (A2) scenarios. Texas displayed smaller increases ranging from 5 to 9°F (3 to 5°C).

3.6.2 Precipitation and Storm Events

3.6.2.1 Near-term (2010-2040)

Averaging the 1993 to 2008 time period of observations across the Great Plains region suggests that annual mean precipitation has increased by 4% relative to the 1961-1979 time period (USGCRP 2009).51 In the near-term, mean precipitation is generally projected to increase in the winter and spring, and to decrease in the summer. It is unclear whether fall precipitation will increase or decrease. There is considerable disagreement between various climate models on the magnitude and direction of changes in precipitation in that season.

Over the next two decades, mean precipitation is projected to increase by 3% in the winter, with a likely range of -2 to +7%. The spring increase is projected to be between 1 to 2%, with a likely range of -3 to +6% (USGCRP 2009). Precipitation may decrease in the summer by 2 to 3%, with a likely range of -9 to +4%. The likely range for precipitation in the fall is between -5 and +6% (USGCRP 2009).

3.6.2.2 Mid-century (2040-2070)

Similar trends in precipitation are projected for the Great Plains through the middle of the century. The winter months are projected to experience a mean precipitation increase of 4 to 5%, with a likely range of -1 to +9%. An increase of roughly 3% is projected for the spring, with a likely range of -3 to +8% (USGCRP 2009). Precipitation is projected to decline by 3 to 5% in the summer with a likely range of -18 to +7%. The likely range of precipitation change in the fall is -9 to +7% (USGCRP 2009).

Great Plains (Δ Precipitation) Near-term (%) Mid-century (%) End-of-century (%)
Winter

Mean

3 4-5 5-8

Likely

(2)-7 (1)-9 (1)-17

Very Likely

(6)-11 (6)-14 (9)-25
Spring

Mean

1-2 3 3-4

Likely

(3)-6 (3)-8 (7)-12

Very Likely

(7)-9 (9)-14 (16)-21
Summer

Mean

(3)-(2) (5)-(3) (9)-(3)

Likely

(9)-4 (18)-7 (29)-11

Very Likely

(15)-11 (30)-20 (49)-31
Fall

Mean

0 (1) (1)-2

Likely

(5)-6 (9)-7 (17)-12

Very Likely

(11)-12 (17)-14 (31)-26

Table 3 11: Seasonal precipitation percent changes for the Great Plains region over the near-term (2010-2029), mid-century (2040-2059) and end-of-century (2080-2098) relative to 1961-1979. The range values are from low (B1) and high (A2) emissions scenarios. Values in parentheses are negative values and represent decreases in precipitation. Data are from the USGCRP (2009).

3.6.2.3 End-of-century (2070-2100)

By late century, projections indicate that the Great Plains are expected to continue to experience wetter winters and springs, and drier summers. Mean precipitation is projected to increase by 5 to 8% in the winter, with a likely range of -1 to +17%. Spring precipitation is projected to increase by 3 to 4%, with a likely range of -7 to +12% (USGCRP 2009). The summer months are projected to have 3 to 9% lower mean precipitation, with a likely range of -29 to +11%. Projections of fall precipitation range from a 1% decrease to a 2% increase, with a likely range of -17 to +12% (USGCRP 2009).

3.7 Southwest

3.7.1 Temperature

3.7.1.1 Near-term (2010-2040)

Within the next several decades, the Southwest can expect to see increases in both annual average and seasonal average temperatures, with the greatest warming expected in the summer months. The projection for the annual mean temperature increase is approximately 2.4°F, with a likely range of 1.7 to 3.0°F; the fall months are projected to warm similarly (USGCRP 2009). This projected warming is greater than the 1.6°F increase already experienced over the 1993 to 2008 time period relative to a 1961-1979 baseline (USGCRP 2009).52 The summer months are projected to experience the greatest warming, with an increase of approximately 2.7°F and a likely range of 1.8 to 3.4°F (USGCRP 2009). The spring and winter months are projected to have the smallest mean temperature increase, approximately 2.2°F, and a likely range of 1.3 to 3.1°F (USGCRP 2009).

3.7.1.2 Mid-century (2040-2070)

By mid-century, the ranges in mean temperature projections widen as a result of the widening of plausible scenarios. The annual mean temperature increase for the Southwest is projected to be 3.6 to 4.5°F with a likely range of 2.6 to 5.5°F (USGCRP 2009). Spring and fall season averages are projected to change similarly to the annual mean. Winter is expected to see less substantial increases than the other seasons, with projections of 3.2 to 3.9°F and a likely range of 2.0 to 5.1°F (USGCRP 2009). Conversely, mid-century summer temperatures are projected to continue to warm more than other seasons, with mean temperature increases of 4.1 to 5.3°F and a likely range of 3.1 to 6.5 °F (USGCRP 2009). Cayan et al. (2009) project annual mean temperature increases for the state of California in 2050 to be between 1.8 to 5.4°F (1 to 3°C) under the lower (B1) and higher (A2) emission scenarios, a much greater range than those provided for the Southwest.53

3.7.1.3 End-of-century (2070-2100)

By the end-of-century, the annual mean temperature in the Southwest region is projected to increase considerably compared with the earlier time horizons. The annual mean temperature increase for the region is projected to be 5.2 to 8.7°F, with a likely range of 3.8 to 10.2°F (USGCRP 2009). As with previous periods, summer is projected to experience the greater warming, with temperature increases of 5.6 to 9.7°F and a likely range of 4.2 to 11.6°F. Winter is projected to see the smallest seasonal temperature increase of 4.8 to 7.6°F, with a likely range of 3.3 to 9.4°F (USGCRP 2009). Projections from Cayan et al. (2009) for California are qualitatively consistent with the USGCRP (2009) results. They project the annual mean temperature increase for the state of California to be roughly 4 to 9°F (2 to 5°C) under the lower (B1) and higher (A2) emission scenarios, with the smallest projected warming in the winter season of 1.8 to 7.2°F (1 to 4°C) and the greatest projected warming in the summer season of 2.7 to 10.8°F (1.5 to 6°C).

Table 3-12
Southwest (Δ Temperature) Near-term (°F) Mid-century (°F) End-of-century (°F)
Annual Mean 2.3-2.4 3.6-4.5 5.2-8.7
Likely 1.7-3.0 2.6-5.5 3.8-10.2
Very Likely 1.0-3.7 1.6-6.4 2.5-11.8
Winter Mean 2.1-2.2 3.2-3.9 4.8-7.6
Likely 1.4-3.0 2.0-5.1 3.3-9.4
Very Likely 0.6-3.8 0.8-6.2 1.8-11.3
Spring Mean 2.1-2.2 3.5-4.1 4.9-8.0
Likely 1.3-3.1 2.1-5.2 3.3-9.9
Very Likely 0.4-4.0 0.8-6.3 1.7-11.8
Summer Mean 2.6-2.7 4.1-5.3 5.6-9.7
Likely 1.8-3.4 3.1-6.5 4.2-11.6
Very Likely 1.1-4.2 2.1-7.7 2.8-13.5
Fall Mean 2.3-2.4 3.7-4.6 5.3-9.2
Likely 1.7-3.0 2.8-5.4 4.0-10.6
Very Likely 1.1-3.6 2.0-6.2 2.8-12.0

Table 3-12: Annual and seasonal temperature changes for the Southwest region over the near-term (2010-2029), mid-century (2040-2059) and end-of-century (2080-2098) relative to 1961-1979. The range values are from low (B1) and high (A2) emissions scenarios. Data are from the USGCRP (2009).

Diffenbaugh et al. (2005) project that the Southwest region could see up to 100 "high temperature" days54 per year under a higher (A2) emission scenario. The frequency in high temperature days for California and Utah is projected to increase by four times today's numbers. Additionally, the USGCRP (2009) describes heat events (a combination of heat and humidity) that currently have a 5% chance of occurring for a given year are projected to increase in frequency, with about a 50 to 100% chance of occurring for a given year under a higher (A2) emission scenario. Figure 9 illustrates the geographic variability of experiencing heat days today across the Southwest and further demonstrates how this variability and frequency will increase by the end of the century.

Figure 9
Figure 9 - click for long description
Figure 9: Average number of heat days per year (defined as days the maximum temperature is above 90°F). The illustrations are 16 multi-model ensemble averages for years 1961-1979, and 2060-2099 for SRES A2. (USGCRP 2009)

3.7.2 Precipitation and Storm Events

3.7.2.1 Near-term (2010-2040)

Observations in the Southwest region averaged over 1993 to 2008 suggest that annual mean precipitation has increased by less than 1% relative to the 1961-1979 time period (USGCRP 2009).55 Within the next several decades, winter precipitation is expected to increase while precipitation in the other three seasons is projected to decrease. Winter precipitation is projected to increase by 2 to 4%, with a likely range of -6 to +14% (USGCRP 2009). The greatest seasonal decrease in precipitation of 4 to 5% is projected for the summer season, with an associated likely range of -14 to +4% (USGCRP 2009).

3.7.2.2 Mid-century (2040-2070)

The general precipitation trends projected for the next several decades are expected to continue through the middle of the century: winter will likely see increases in precipitation while the other seasons can expect decreases. Winter precipitation is projected to increase by 1 to 5%, with a likely range of -6 to +16% (USGCRP 2009). The summer and fall seasons are projected to experience decreases of 5 to 8% with a likely range of -22 to +7%; and 2 to 3% with a likely range of -11 to +5%, respectively (USGCRP 2009). The greatest decrease in precipitation of 6 to 10% is projected for spring, with a likely range of -20 to 0% (USGCRP 2009).

For transportation planning purposes, the type of precipitation is similar in importance to the amount. In the Southwest, the type of precipitation (rain vs. snow) will be affected by the earlier onset of spring following warmer winters. Leung et al. (2004) project that the Sierra Nevada Mountains will see a 60 to 70% decline in snowpack by mid-century averaged over 2040 to 2060 under a business as usual scenario.56 During the same timeframe, more precipitation is likely to fall as rain than snow in the Colorado River Basin, with a 10 to 20% reduction in snow, and a more than 30% reduction in the Sacramento-San Joaquin River basin (Leung et al. 2004). Averaged over 2035 to 2064, the amount of water stored as snow in the Sierra Nevada Mountains as of April 1 is projected to decrease by 12 to 42% at all elevations under a lower (B1) and higher (A2) emission scenario (Cayan et al. 2008).57

Table 3-13
Southwest (Δ Precipitation) Near-term (%) Mid-century (%) End-of-century (%)
Winter Mean 2-4 1-5 2-5
Likely (6)-14 (6)-16 (13)-23
Very Likely (16)-24 (17)-27 (30)-40
Spring Mean (5)-(4) (10)-(6) (19)-(7)
Likely (10)-2 (20)-0 (32)-1
Very Likely (16)-8 (31)-10 (45)-9
Summer Mean (5)-(4) (8)-(5) (5)-(3)
Likely (14)-4 (22)-7 (24)-13
Very Likely (23)-13 (36)-21 (43)-32
Fall Mean (1)-0 (3)-(2) (1)-0
Likely (6)-6 (11)-5 (15)-12
Very Likely (12)-11 (20)-13 (28)-26

Table 3-13: Seasonal precipitation percent changes for the Southwest region over the near-term (2010-2029), mid-century (2040-2059) and end-of-century (2080-2098) relative to 1961-1979. The range values are from low (B1) and high (A2) emissions scenarios. Values in parentheses are negative values and represent decreases in precipitation. Data are from the USGCRP (2009).

3.7.2.3 End-of-century (2070-2100)

By the end-of-century, winter precipitation is projected to increase by 2 to 5%, with a likely range of -13 to +23%, while the remaining seasons are projected to continue to decline in seasonal average precipitation (USGCRP 2009). The spring season is projected to experience the greatest decrease in precipitation of 7 to 19%, with a likely range of -32 to +1% (USGCRP 2009). It is expected that the shift in precipitation type from snow to rain will continue (USGCRP 2009). The amount of water stored as snow on April 1 is projected to decrease by 32 to 79% under a lower (B1) and higher (A2) emission scenario (Cayan et al. 2008). In sum, this region is expected to endure a greater likelihood of drought while, conversely, having an increased risk of flooding (USGCRP 2009).

3.7.3 Sea-Level Rise

Global sea-level rise (SLR) of 7 to 79" (18cm to 2.0m) is projected for 2100 (see section 3.1.3 for discussion on global and local sea-level rise). SLR at the local/regional level is influenced by multiple factors, including sedimentation and erosion, ocean circulation, gravitationally induced changes, ocean density (affected by regional changes in ocean salinity and ocean temperature), and vertical motion of the land (subsidence or uplift). In the 20th century, the relative sea-level rise for the Southwest region is generally similar to the level of global sea-level rise.58

The following discussion describes studies providing local sea-level rise projections for the Southwest region. As noted in section 3.1.3 above, making local or regional projections is highly uncertain, given the incomplete understanding of some of the effects that can take place at the local level. There is no study that considers all these factors, nor is there a consistent methodology applied across these studies. Therefore, local SLR projections, while informative, should be considered carefully, and with a clear understanding of what factors each study includes or excludes.

Projections of local sea-level rise along the California coast have been developed that take into account tides, weather, and monthly and interannual sea level fluctuations from El Niño/Southern Oscillation (Cayan et al. 2008). These projections are not provided here as they do not consider other effects such as the changes in elevation of coastal land (i.e., caused by subsidence or uplift), local changes in ocean density, or erosion and sedimentation on local sea-level rise. These projections are driven by conservative IPCC global sea-level rise estimates that have been enhanced with accelerated estimates of projected ice melt.,5960 The Climate Change Effects Typology Matrix provides the results of this study and further finds that if the sea-level rise realized was within a moderate estimate of the Table 3-1 projections, extreme events and their duration would increase substantially.

3.8 Pacific Northwest

3.8.1 Temperature

3.8.1.1 Near-term (2010-2040)

Within the next two decades, the annual mean temperature is projected to increase for the Pacific Northwest by approximately 2°F, with a likely range of 1°F to 3°F (USGCRP 2009; Mote et al. 2005; Mote and Salathe 2009). This projected warming is greater than the 1°F increase already experienced over the 1993 to 2008 time period relative to a 1961-1979 baseline (USGCRP 2009).61 Winter and fall changes are expected to be relatively similar to the annual mean. Summer months are projected to experience the greatest warming of 2.5 to 2.8°F and a likely range of 1.6 to 3.7°F (USGCRP 2009), whereas spring months are projected to experience the smallest warming of 1.7 to 1.9°F and a likely range of 0.8 to 3.0°F (USGCRP 2009).

3.8.1.2 Mid-century (2040-2070)

By mid-century, the annual mean temperature increase is projected to be approximately 3.6 to 4.3°F, with a likely range of 2.6 to 5.4°F (USGCRP 2009). For 2040, Mote et al. (2005) projected a slightly lower annual mean temperature increase of 3.0°F (1.6°C). The projections of warming during winter and fall are relatively similar to the annual mean. The smallest warming of seasonal mean temperature is projected to occur during the spring months, at 3.1 to 3.4°F and a likely range of 1.7 to 4.7°F (USGCRP 2009). The largest warming of seasonal mean temperature is expected to be in the summer months, with a projected increase of 4.1 to 5.5°F and a likely range of 3.0 to 6.9°F (USGCRP 2009). This increase in temperatures is likely to cause more precipitation to fall as rain rather than snow, particularly at lower altitudes (USGCRP 2009). Heat events are projected to increase in the Pacific Northwest, particularly in south-central Washington and the western lowlands. By mid-century south-central Washington could experience an additional one to three heat waves annually, with other locations experiencing up to one additional heat wave each year under a moderate (A1B) emission scenario (Salathe et al. 2009).62 The frequency of warm nights63 is also expected to increase by roughly 7 to 20% across Washington (Salathe et al. 2009).

3.8.1.3 End-of-century (2070-2100)

By the end-of-century, annual mean temperature is projected to have increased by 5.1 to 8.3°F, with a likely range of 3.7 to 10.0°F (USGCRP 2009). Mote and Salathe (2009) projected a smaller annual mean temperature increase of 4.8 to 6.8°F based on a low (B1) emission scenarios and a moderate (A1B) emission scenario.64 The projections for winter and fall months are again similar to the annual mean. The mean temperature for the spring months is projected to increase by just 4.4 to 6.6°F, with a likely range of 2.5 to 8.9°F (USGCRP 2009). The summer months are projected to experience the greatest warming at 5.8 to 10.5°F, with a likely range of 4.2 to 13.1°F (USGCRP 2009).

Table 3-14
Pacific Northwest (Δ Temperature) Near-term (°F) Mid-century (°F) End-of-century (°F)
Annual Mean 2.2 3.6-4.3 5.1-8.3
Likely 1.4-2.9 2.6-5.4 3.7-10.0
Very Likely 0.7-3.7 1.6-6.4 2.3-11.8
Winter Mean 2.1-2.2 3.5-3.9 5.1-7.6
Likely 1.4-3.0 2.3-5.2 3.5-9.5
Very Likely 0.6-3.8 1.1-6.5 1.8-11.4
Spring Mean 1.7-1.9 3.1-3.4 4.4-6.6
Likely 0.8-3.0 1.7-4.7 2.5-8.9
Very Likely (0.2)-4.1 0.3-6.1 0.6-11.2
Summer Mean 2.5-2.8 4.1-5.5 5.8-10.5
Likely 1.6-3.7 3.0-6.9 4.2-13.1
Very Likely 0.7-4.6 1.8-8.4 2.5-15.7
Fall Mean 2.0-2.2 3.4-4.2 4.8-8.4
Likely 1.4-3.0 2.5-5.3 3.5-10.2
Very Likely 0.7-3.6 1.5-6.4 2.1-11.9

Table 3-14: Annual and seasonal temperature changes for the Pacific Northwest region over the near-term (2010-2029), mid-century (2040-2059) and end-of-century (2080-2098) relative to 1961-1979. The range values are from low (B1) and high (A2) emissions scenarios. Data are from the USGCRP (2009).

3.8.2 Precipitation and Storm Events

3.8.2.1 Near-term (2010-2040)

Observations across the Pacific Northwest region averaged over 1993 to 2008 suggest that annual mean precipitation has increased by less than 1% relative to the 1961-1979 time period (USGCRP 2009).65 Mean precipitation in the Pacific Northwest is generally projected to increase in the winter, spring, and fall, while summer precipitation is projected to decrease. There is considerable disagreement across various climate models on the magnitude and direction of changes in precipitation.

Over the next two decades, mean precipitation is projected to increase by roughly 3 to 5% for winter and fall seasons, with a likely range of approximately -3 to +12% (USGCRP 2009). The spring seasons are estimated to experience a slightly lower increase of 3%, with a likely range of -1 to +7% (USGCRP 2009). Precipitation in the summer months, on the other hand, is projected to decrease by more than 6% with a likely range of -17 to +3% (USGCRP 2009).

3.8.2.2 Mid-century (2040-2070)

By mid-century, winter mean precipitation is projected to increase by 5 to 7%, with a likely range of -3 to +17% (USGCRP 2009). Spring and fall are also projected to undergo precipitation increases of 3 to 5% with a likely range of -3 to +10%; and 5% with a likely range of -3 to +13%, respectively (USGCRP 2009). Summer, on the other hand, is projected to undergo decreases in precipitation of 8 to 17%, with a likely range of -28 to +1% (USGCRP 2009). Springtime snowpack is projected to decrease by mid-century in response to the increased wintertime temperature and greater occurrence of rain (versus snow). Higher average temperatures in the fall and winter will cause more precipitation to fall as rain rather than snow, and the snowpack will begin to melt earlier in the season. By the 2040s, April 1st snowpack is projected to decline by as much as 40% in the Cascade mountains (Payne et al. 2004, as cited in USGCRP 2009), and Leung et al. (2004) projected a 10 to 20% decline in snowfall over fall, winter, and spring in the Columbia River Basin by mid-century under a business as usual scenario. In addition, warm-season runoff is projected to decrease by 30% or more on the western slopes of the Cascade Mountains and by 10% in the Rocky Mountains (USGCRP 2009). The effect of the reduction in summer precipitation will be magnified by the increased evaporation as summer temperatures warm.

Table 3-15
Pacific Northwest (Δ Precipitation) Near-term (%) Mid-century(%) End-of-century (%)
Winter Mean 3-5 5-7 8-15
Likely (3)-12 (3)-17 (1)-29
Very Likely (11)-20 (12)-27 (14)-43
Spring Mean 3 3-5 5-7
Likely (1)-7 (3)-10 (2)-15
Very Likely (6)-11 (8)-15 (10)-23
Summer Mean (7)-(6) (17)-(8) (22)-(11)
Likely (17)-3 (28)-1 (42)-(1)
Very Likely (27)-12 (40)-10 (62)-18
Fall Mean 4 5 7-9
Likely (3)-11 (3)-13 (7)-24
Very Likely (10)-18 (11)-21 (22)-39

Table 3-15: Seasonal precipitation percent changes for the Pacific Northwest region over the near-term (2010-2029), mid-century (2040-2059) and end-of-century (2080-2098) relative to 1961-1979. The range values are from low (B1) and high (A2) emissions scenarios. Values in parentheses are negative values and represent decreases in precipitation. Data are from the USGCRP (2009).

By mid-century, Salathe et al. (2009) projected that precipitation intensity, defined in this study as annual total precipitation divided by the number of "wet" days where precipitation exceeds 1 millimeter, would increase slightly across much of Washington, with substantial increases occurring only in the northwest portion of the state under a moderate (A1B) emission scenario. The fraction of precipitation that falls on days where precipitation exceeds the 95th percentile was projected to increase in the eastern and western parts of the state, with decreases in the central portion along the leeward side of the Cascade Mountains (Salathe et al. 2009).

3.8.2.3 End-of-century (2070-2100)

By the end-of-century, seasonal mean precipitation is projected to have increased substantially in the winter months by 8 to 15%, with a likely range of -1 to +29% (USGCRP 2009). Spring and fall seasonal mean precipitation totals are projected to increase by 5 to 7% with a likely range of -2 to +15%; and 7 to 9% with a likely range of -7 to +24%, respectively (USGCRP 2009). The summer season is projected to continue to experience substantial declines in precipitation of 11 to 22%, with a likely range of +1 to +42% (USGCRP 2009). Diffenbaugh (2005) projected an increase of up to 10 extreme precipitation events per year in the Pacific Northwest (up to a 140% increase) under a higher (A2) emission scenario with some variation depending on location within the region.66

3.8.3 Sea-Level Rise

Global sea-level rise (SLR) of 7 to 79" (18cm to 2.0m) is projected for 2100 (see section 3.1.3 for discussion on global and local sea-level rise). SLR at the local/regional level is influenced by multiple factors, including sedimentation and erosion, ocean circulation, gravitationally induced changes, ocean density (affected by regional changes in ocean salinity and ocean temperature), and vertical motion of the land (subsidence or uplift). In the 20th century, the relative sea-level rise for the Pacific Northwest exhibits large variability, with locations across the region exhibiting both greater and lesser rise than the global trend.67 For the Northwest region, sea-level rise is compounded by increased beach erosion and increased winter rainfall, which could saturate soils in the coastal bluffs leading to landslides (USGCRP 2009).

The following discussion describes studies providing local sea-level rise projections for this region. As noted in section 3.1.3 above, making local or regional projections is highly uncertain, given the incomplete understanding of some of the effects that can take place at the local level. There is no study that considers all these factors, nor is there a consistent methodology applied across these studies. Therefore, local SLR projections, while informative, should be considered carefully, and with a clear understanding of what factors each study includes or excludes.

Mote et al. (2008) estimated local sea-level rise at three different locations along the Washington state coast: Northwest Olympic Peninsula, the central and southern coast, and Puget Sound. For advisory purposes only, this study provides projections of local sea-level rise at these locations, allowing for changes in: (i) global sea-level rise, (ii) coastal elevation from the vertical movement of land at the different locations, and (iii) local wind patterns that push water toward or away from the coast. Mid-century and end-of-century estimates of the impact of vertical land motion on sea-level rise are provided for each location, with the end-of-century estimate of local vertical uplift being 15.7" (40 cm) for the Northeast Olympic Peninsula, 3.9" (10 cm) for the Central and the Southern Coast, and no change for Puget Sound. Assuming these rates remain constant in the future, studies that project global average sea-level rise should be lessened by these amounts to obtain estimates of local sea-level change.

Sea level along the Washington coastline can undergo considerable seasonal variability, with mean sea levels being 20" (50 cm) higher during the winter months compared with the summer months. The seasonal variability is explained by shifts in atmospheric circulation (winds) directly affecting the ocean elevation; that is, a northward wind can push water towards the shore increasing ocean elevation. Based on the same premise, an El Niño event can further increase sea level by 12" (30 cm). Based on an analysis of 30 scenarios, this study finds the changes in projected wintertime northward wind to range from minimal (suggesting a less than 1" reduction in mean sea level for 2050 and 2100) to increased strength (suggesting as much as a 6" (15 cm) increase in mean sea level for 2050-2099 compared with 1950-1999).68

3.9 Alaska

3.9.1 Temperature

3.9.1.1 Near-term (2010-2040)

Within the next several decades, Alaska may experience an increase in annual average temperature of about 2.4 to 2.6°F, with a likely range of 1.5 to 3.6°F (USGCRP 2009). Fall and spring seasonal temperature projections are similar to the annual average. The greatest warming is projected to occur in the winter, when temperatures are projected to increase 3.1 to 4.0°F and a likely range of 1.0 to 5.9°F (USGCRP 2009). Summers are projected to warm on average by 1.3°F, with a likely range of 0.6 to 2.0°F, although summer is likely to be the least affected of all seasons (USGCRP 2009). The CCSP (2008) projects a slightly higher average annual temperature increase of 3.6°F in Alaska by 2030. It is interesting to note that while Alaska has warmed at more than twice the rate of the national average (USGCRP 2009), the magnitude of the projections for the annual mean average in Alaska is consistent with the projected annual mean averages of other U.S. regions. The seasonal averages are also consistent with other northern regions, with the winter months demonstrating the greatest potential increase. The higher temperatures are reducing sea ice and glacier mass or glacier extent, leading to an earlier spring snowmelt, and affecting permafrost (USGCRP 2009). Permafrost warming leads to land subsidence, directly affecting highway systems (Larsen et al. 2008). These higher temperatures will also lead to greater evaporation rates, potentially reducing soil moisture and leading to reductions in the water levels in closed-basin lakes (USGCRP 2009).

3.9.1.2 Mid-century (2040-2070)

By mid-century, the average projected increase in annual average temperature is 4.3°F with a likely range of 3.6 to 5.0°F (USGCRP 2009). Fall and spring are expected to continue to experience similar seasonal mean temperature changes compared with the annual average, though with some differences. Seasonal mean temperatures for fall are projected to increase slightly more than the annual average, by 4.4 to 4.9°F, with a likely range of 3.5 to 5.6°F (USGCRP 2009). Mean spring temperature is projected to increase by 3.9 to 4.0°F, with a likely range of 3.6 to 11.5°F-slightly less than the annual average. As with the earlier projected period, the largest increase in average temperatures occurs during the winter months, with the seasonal mean temperature increase of 6.2 to 6.4°F and a likely range of 5.1 to 7.7°F (USGCRP 2009). Summer is expected to continue to be the least affected season, with a projected increase of 2.1 to 2.5°F and a likely range of 0.7 to 3.5°F (USGCRP 2009).

Table 3-16
Alaska (Δ Temperature) Near-term (°F) Mid-century (°F) End-of-century (°F)
Annual Mean 2.4-2.6 4.3 6.7-9.9
Likely 1.5-3.6 3.6-5.0 4.6-11.7
Very Likely 0.4-4.7 2.9-5.7 2.4-13.5
Winter Mean 3.1-4.0 6.2-6.4 9.9-14.5
Likely 1.0-5.9 5.1-7.7 7.5-17.4
Very Likely (1.1)-7.9 3.8-9.0 5.1-20.2
Spring Mean 2.3-2.6 3.9-4.0 6.2-9.1
Likely 0.6-4.7 2.5-5.5 3.6-11.5
Very Likely (1.5)-6.8 1.1-7.0 (0.9)-14.0
Summer Mean 1.3 2.1-2.5 3.9-5.9
Likely 0.6-2.0 0.7-3.5 1.5-8.7
Very Likely (0.1)-2.8 (0.8)-5.0 (1.0)-13.9
Fall Mean 2.3-2.8 4.4-4.9 7.0-10.0
Likely 1.6-3.1 3.5-5.6 4.9-11.5
Very Likely 0.8-3.7 2.6-6.3 2.8-13.0

Table 3-16: Annual and seasonal temperature changes for Alaska over the near-term (2010-2029), mid-century (2040-2059) and end-of-century (2080-2098) relative to 1961-1979. The range values are from low (B1) and high (A2) emissions scenarios. Parentheses represent negative projections. Data are from the USGCRP (2009) and are based on the five climate models identified as the top performers for Alaska by Walsh et al. (2008). The results listed in the typology matrix for Alaska are provided for the suite of models consistent with the other U.S. regions.

3.9.1.3 End-of-century (2070-2100)

By late century, Alaska will likely see an increase of 6.7 to 9.9°F in average annual temperature, with a likely range of 4.6 to 11.7°F (USGCRP 2009). Spring and fall seasonal mean temperature projections continue to follow the annual trends. Winter will continue to be the most affected of the seasons, with an average seasonal temperature increase of 9.9 to 14.5°F and a likely range of 7.5 to 17.4°F (USGCRP 2009). Summer is likely to remain the least affected of the seasons, with projected increases of 3.9°F to 5.9°F and a likely range of 1.5 to 8.7°F (USGCRP 2009). By the end of the 21st century, northern Alaska could see surface temperatures increase by more than 9.0°F (5°C) (IPCC 2007a) and permafrost temperatures on the Seward Peninsula could increase by 0 to 5.8°F (Busey et al. 2008) under a moderate (A1B) emission scenario.69 Additionally, by the 2080 to 2100 period, an extreme heat event that currently has a 5% chance of occurring per year is likely to have a 10% chance of occurring within a given year under a higher (A2) emission scenario (USGCRP 2009).

3.9.2 Precipitation and Storm Events

3.9.2.1 Near-term (2010-2040)

The winter months are projected to have the most noticeable change in seasonal precipitation. Within the next several decades, Alaska is likely to experience an increase in winter mean precipitation of 6 to 9% with a likely range of +1 to +16% (USGCRP 2009). Similar to projected temperatures, summer will likely be the least affected of the seasons in terms of precipitation. Summer is expected to experience an increase in mean precipitation of approximately 6% with a likely range of +3 to +9% (USGCRP 2009). Spring and fall are estimated to receive additional precipitation of 5 to 8% with a likely range of +0 to +15%; and 7 to 8% with a likely range of +4 to +11%, respectively (USGCRP 2009).

3.9.2.2 Mid-century (2040-2070)

By mid-century, winters in Alaska may continue to experience the largest seasonal increase in mean precipitation, 15 to 17%, with a likely range of 10 to 20% (USGCRP 2009). Spring is likely to be the least affected of the seasons, with changes in mean precipitation estimated to by an increase of 9 to 12% with a likely range of +3 to +15%. The average projected increases for fall and summer mean precipitation are moderate compared with the other seasons at 10 to 14%, with a likely range of +8 to +16%; and 11 to 13%, with a likely range of +7 to +17%, respectively (USGCRP 2009).

Table 3-17
Alaska (Δ Precipitation) Near-term (%) Mid-century(%) End-of-century (%)
Winter Mean 6-9 15-17 23-37
Likely 1-16 10-20 18-48
Very Likely (5)-23 4-25 13-59
Spring Mean 5-8 9-12 18-30
Likely 0-15 3-15 12-34
Very Likely (7)-22 (3)-21 7-38
Summer Mean 6 11-13 17-23
Likely 3-9 7-17 14-29
Very Likely (1)-13 3-20 11-36
Fall Mean 7-8 10-14 17-30
Likely 4-11 8-16 9-39
Very Likely 1-13 6-19 1-46

Table 3-17: Seasonal precipitation percent changes for Alaska over the near-term (2010-2029), mid-century (2040-2059) and end-of-century (2080-2098) relative to 1961-1979. The range values are from low (B1) and high (A2) emissions scenarios. Values in parentheses are negative values and represent decreases in precipitation. Data are from the USGCRP (2009) and are based on the five climate models identified as the top performers for Alaska by Walsh et al. (2008). The results listed in the typology matrix for Alaska are provided for the suite of models consistent with the other U.S. regions.

3.9.2.3 End-of-century (2070-2100)

By the end-of-century, winter will likely continue to see the greatest increases in total seasonal precipitation. Winters in Alaska could see an 23 to 37% increase in mean precipitation, with a likely range of +18 to +48% (USGCRP 2009). The fall season is likely to experience the smallest increases of 17 to 30%, with a likely range of +9 to +39% (USGCRP 2009). Spring and summer continue to demonstrate moderate increases compared with the other seasons, at 18 to 30%, with a likely range of +12 to +34%; and 17 to 23%, with a likely range of +14 to +29%, respectively (USGCRP 2009).

Storm activity may increase as the Pacific storm track moves northward and sea surface temperatures increase (USGCRP 2009). These storms are expected to be situated over oceans with less ice cover (both in magnitude and seasonal duration), and may increase in frequency and intensity as the warmer ocean may supply these storm with more heat and moisture (USGCRP 2009).

3.9.3 Sea-Level Rise

Global sea-level rise (SLR) of 7 to 79" (18cm to 2.0m) is projected for 2100 (see section 3.1.3 for discussion on global and local sea-level rise). SLR at the local/regional level is influenced by multiple factors, including sedimentation and erosion, ocean circulation, gravitationally induced changes, ocean density (affected by regional changes in ocean salinity, and ocean temperature), and vertical motion of the land (subsidence or uplift). In the 20th century, the relative sea-level rise for Alaska exhibits large variability, with some locations demonstrating slightly greater rise compared to observed global sea-level rise, and with other stations demonstrating considerably lower.70 Parts of Alaska are undergoing uplift in response to glacial ice loss and active regional tectonic deformation at a rate considered to keep pace with rising global sea levels. In fact, relative sea level in the southeast and south central Gulf of Alaska coastal area may actually decrease substantially (Larsen et al. 2004; Kelly et al. 2007). It should be noted that under high sea-level rise projections, it is possible that sea-level rise rates may approach or exceed uplift rates.

3.10 Hawaii

3.10.1 Temperature

Over the next few decades, annual mean temperatures in Hawaii are projected to increase by about 1.8°F, with a likely range of 1.0 to 2.5°F (USGCRP 2009). The seasonal mean temperatures and ranges are very similar to the annual mean, with an even consistency of warming throughout the year. This relationship continues through to the end-of-century. By mid-century, the annual mean temperatures may increase by 2.7 to 3.3°F with a likely range of 2.0 to 4.0°F (USGCRP 2009). By the end-of-century, the annual mean temperature is projected to have increased by 3.9 to 6.7°F with a likely range of 2.8 to 7.8°F (USGCRP 2009). Some small seasonal variations from the annual mean are projected for the end-of-century.

Table 3-18
Hawaii (Δ Temperature) Near-term (°F) Mid-century (°F) End-of-century (°F)
Annual Mean 1.7 - 1.8 2.7 - 3.3 3.9 - 6.7
Likely 1.0 - 2.5 2.0 - 4.0 2.8 - 7.8
Very Likely 0.3 - 3.2 1.2 - 4.6 1.8 - 8.9
Winter Mean 1.7 2.7 - 3.2 3.8 - 6.4
Likely 1.0 - 2.4 1.9 - 3.9 2.7 - 7.5
Very Likely 0.3 - 3.0 1.1 - 4.6 1.6 - 8.5
Spring Mean 1.6 - 1.8 2.7 - 3.2 3.8 - 6.3
Likely 0.9 - 2.6 1.9 - 3.7 2.8 - 7.2
Very Likely 0.1 - 3.4 1.1 - 4.3 1.8 - 8.1
Summer Mean 1.8 2.7 - 3.4 3.9 - 6.7
Likely 1.0 - 2.6 1.9 - 4.0 2.8 - 7.9
Very Likely 0.1 - 3.4 1.1 - 4.7 1.7 - 9.2
Fall Mean 1.8 2.8 - 3.5 4.0 - 7.2
Likely 1.0 - 2.6 2.0 - 4.4 2.8 - 8.7
Very Likely 0.3 - 3.3 1.2 - 5.2 1.6 - 10.3

Table 3-18: Annual and seasonal temperature changes for Hawaii over the near-term (2010-2029), mid-century (2040-2059) and end-of-century (2080-2098) relative to 1961-1979. The range values are from low (B1) and high (A2) emissions scenarios. Data are from the USGCRP (2009).

3.10.2 Precipitation and Storm Events

3.10.2.1 Near-term (2010-2040)

While the temperature is not expected to have much seasonal variation, precipitation is projected to vary seasonally. Over the next few decades, the greatest decrease in precipitation is projected to occur during the summer months, with declines of 3 to 5% and a likely range of -18 to +8% (USGCRP 2009). Winter mean precipitation is also projected to decrease by approximately 2% and a likely range of -15 to +10% (USGCRP 2009). Fall mean precipitation is projected to change between -1 to +2%, with a likely range of -11 to +11% (USGCRP 2009). Spring mean precipitation is projected to change by 0 to +1%, with a likely range of -10 to +11% (USGCRP 2009).

Table 3-19
Hawaii (Δ Precipitation) Near-term (°F) Mid-century (°F) End-of-century (°F)
Winter Mean (2) (3) - (2) (4) - (1)
Likely (15) - 10 (21) - 18 (25) - 22
Very Likely (27) - 22 (40) - 37 (47) - 46
Spring Mean 0 - 1 (2) (5) - 6
Likely (10) - 11 (13) - 9 (20) - 9
Very Likely (20) - 21 (24) - 20 (34) - 22
Summer Mean (5) - (3) (3) -(1) (1) - 5
Likely (18) - 8 (23) - 21 (42) - 51
Very Likely (30) - 20 (45) - 43 (88) - 98
Fall Mean (1) - 2 3 - 6 1 - 19
Likely (11) - 11 (15) - 27 (38) - 75
Very Likely (21) - 21 (36) - 47 (95) - 132

Table 3-19: Seasonal precipitation percent changes for the Hawaii over the near-term (2010-2029), mid-century (2040-2059) and end-of-century (2080-2098) relative to 1961-1979. The range values are from low (B1) and high (A2) emissions scenarios. Values in parentheses are negative values and represent decreases in precipitation. Data are from the USGCRP (2009).

3.10.2.2 Mid-century (2040-2070)

By mid-century, only the fall months are projected to increase in precipitation, by 3 to 6% and a likely range of -15 to +27% (USGCRP 2009). The other three seasons are projected to decrease in precipitation. The winter and spring months are similar in projected mean change but not when comparing the likely range. The winter months continue to project a decrease in mean precipitation by 2 to 3% with a likely range of -21 to +18% (USGCRP 2009). The spring months may have a decrease in mean precipitation by 2%, with a likely range of -13 to +9% (USGCRP 2009). The summer months have a larger variability of decreased mean precipitation of 1 to 3%, with a likely range of -23 to +21% (USGCRP 2009).

3.10.2.3 End-of-century (2070-2100)

By the end-of-century, the winter months continue to demonstrate a decrease in mean precipitation of 1 to 4%, with a likely range of -25 to +22% (USGCRP 2009). The spring and summer months have a mean change in precipitation that crosses direction. The changes in spring mean precipitation vary between a decrease of 5% to an increase of 6%, with a likely range of -20 to +9% (USGCRP 2009). The changes in summer mean precipitation vary between a decrease of 1% to an increase of 5%, with a large likely range of -42 to +51% (USGCRP 2009). The fall months exhibit an increase in the mean change for precipitation, of 1 to 19%, with a likely range of -38 to +75% (USGCRP 2009).

3.10.3 Sea-Level Rise

Global sea-level rise (SLR) of 7 to 79" (18cm to 2.0m) is projected for 2100 (see section 3.1.3 for discussion on global and local sea-level rise). SLR at the local/regional level is influenced by multiple factors, including sedimentation and erosion, ocean circulation, gravitationally induced changes, ocean density (affected by regional changes in ocean salinity and ocean temperature), and vertical motion of the land (subsidence or uplift). In the 20th century, the relative sea-level rise for Hawaii is generally similar to that of observed global sea-level rise.71 Some islands are particularly vulnerable to sea-level rise and storm surge, with resulting shoreline erosion. For example, the Northwestern Hawaiian Islands are low-lying islands particularly vulnerable to sea-level rise (USGCRP 2009).

3.11 Puerto Rico

The information provided below represents the Caribbean region.

3.11.1 Temperature

Over the next few decades, the annual mean temperatures are projected to increase by about 1.7°F, with a likely range of 1.2 to 2.1°F (USGCRP 2009). The magnitude of the warming is expected to be roughly similar in all four seasons through to the end-of- century. By mid-century, the annual mean temperatures may increase by 2.5 to 3.1°F, with a likely range of 2.0 to 3.5°F (USGCRP 2009). By the end-of-century, the annual mean temperature is projected to have increased by 3.6 to 6.1°F, with a likely range of 2.7 to 6.8°F (USGCRP 2009).

Table 3-20
Caribbean (Δ Temperature) Near-term (°F) Mid-century (°F) End-of-century (°F)
Annual Mean 1.6 - 1.7 2.5 - 3.1 3.6 - 6.1
Likely 1.2 - 2.1 2.0 - 3.5 2.7 - 6.8
Very Likely 0.8 - 2.5 1.4 - 3.9 1.9 - 7.5
Winter Mean 1.6 2.4 - 3.0 3.5 - 5.8
Likely 1.1 - 2.0 1.9 - 3.5 2.6 - 6.6
Very Likely 0.8 - 2.5 1.3 - 3.9 1.7 - 7.5
Spring Mean 1.5 - 1.6 2.5 - 3.0 3.5 - 5.8
Likely 1.1 - 2.0 1.9 - 3.4 2.7 - 6.5
Very Likely 0.6 - 2.5 1.3 - 3.7 1.8 - 7.2
Summer Mean 1.7 - 1.8 2.6 - 3.2 3.7 - 6.2
Likely 1.2 - 2.1 2.1 - 3.6 2.8 - 6.9
Very Likely 0.8 - 2.6 1.5 - 4.0 1.9 - 7.6
Fall Mean 1.7 - 1.8 2.7 - 3.3 3.7 - 6.4
Likely 1.3 - 2.1 2.1 - 3.7 2.8 - 7.1
Very Likely 0.9 - 2.5 1.5 - 4.1 1.9 - 7.9

Table 3-20: Annual and seasonal temperature changes for the Caribbean over the near-term (2010-2029), mid-century (2040-2059) and end-of-century (2080-2098) relative to 1961-1979. The range values are from low (B1) and high (A2) emissions scenarios. Data are from the USGCRP (2009).

3.11.2 Precipitation and Storm Events

Overall, the Caribbean is expected to experience significant reductions in precipitation for almost all seasons and time horizons. The greatest reduction of precipitation is projected for the summer and spring months, and the least reduction in the winter and fall months.

3.11.2.1 Near-term (2010-2040)

Over the next few decades, the largest reduction in mean precipitation is expected in the summer months of 10 to 7%, with a likely range of -16 to +1% (USGCRP 2009). Winter mean precipitation is also projected to decrease by approximately 1 to 3% and a likely range of -9 to +5% (USGCRP 2009). Fall mean precipitation is projected to decrease by 1 to 2%, with a likely range of -9 to +6% (USGCRP 2009). Spring mean precipitation is projected to decrease by 6 to 7%, with a likely range of -15 to +2% (USGCRP 2009).

Table 3-21
Caribbean (Δ Precipitation) Near-term (°F) Mid-century (°F) End-of-century (°F)
Winter Mean (3) - (1) (5) - (3) (8) - (2)
Likely (9) - 5 (14) - 5 (22) - 6
Very Likely (15) - 11 (22) - 12 (35) - 19
Spring Mean (7) - (6) (16) - (8) (28) - (9)
Likely (15) - 2 (25) - 0 (39) - 2
Very Likely (24) - 11 (33) - 9 (51) - 13
Summer Mean (10) - (7) (18) - (12) (36) - (14)
Likely (16) - 1 (31) - (1) (52) - 0
Very Likely (23) - 8 (44) - 11 (68) - 14
Fall Mean (2) - (1) (4) - (3) (9) - (4)
Likely (9) - 6 (15) - 7 (28) - 10
Very Likely (16) - 12 (26) - 18 (47) - 29

Table 3-21: Seasonal precipitation percent changes for the Caribbean over the near-term (2010-2029), mid-century (2040-2059) and end-of-century (2080-2098) relative to 1961-1979. The range values are from low (B1) and high (A2) emissions scenarios. Values in parentheses are negative values and represent decreases in precipitation. Data are from the USGCRP (2009).

3.11.2.2 Mid-century (2040-2070)

Mid-century precipitation is expected to continue to have declined in all seasons. The winter and fall months are projected to have similar mean changes; but their likely ranges are noticeably different. The fall months are projected to decrease in precipitation by 3 to 4%, with a likely range of -15 to +7% (USGCRP 2009). The winter months are projected to decrease in mean precipitation by 3 to 5%, with a likely range of -14 to +5% (USGCRP 2009). The spring months are projected to have a substantial decrease in mean precipitation by 8 to16%, with a likely range of -25 to 0% (USGCRP 2009). The summer months are expected to have a larger decrease in mean precipitation of 12 to 18%,with a likely range of -31 to -1% (USGCRP 2009).

3.11.2.3 End-of-century (2070-2100)

Precipitation totals are expected to continue to decline through the end-of-century. The winter months are projected to continue to experience a decrease in mean precipitation of 2 to 8%, with a likely range of -22 to +6% (USGCRP 2009). The spring mean precipitation is projected to decrease by 9 to 28%, with a likely range of -39 to +2% (USGCRP 2009). The summer mean precipitation is projected to decrease by 14 to 36%, with a large likely range of -52 to 0% (USGCRP 2009). The fall months are projected to experience a decrease in mean precipitation of 4 to 9%, with a likely range of -28 to +10% (USGCRP 2009).

3.11.3 Sea-level rise

Global sea-level rise (SLR) of 7 to 79" (18cm to 2.0m) is projected for 2100 (see section 3.1.3 for discussion on global and local sea-level rise). SLR at the local/regional level is influenced by multiple factors, including sedimentation and erosion, ocean circulation, gravitationally induced changes, ocean density (affected by regional changes in ocean salinity and ocean temperature), and vertical motion of the land (subsidence or uplift). In the 20th century, the relative sea-level rise for the Caribbean is generally slightly less than that of global sea-level rise.72


11It should be noted that the time ranges used in the tables and maps (and the USGCRP data) are slightly different than those defined for this report. While the USGCRP data are representative of the data included under the time periods as defined in this report, the time periods are not an exact match.

12These ranges correspond to the mean averages as defined in this report, demonstrating differences between climate model results based on a given emission scenario. back
13The IPCC definition for "likely range" is -40% to +60% around the mean for each SRES. The full likely range provided in this report is from the lowest point and highest point of all the likely ranges. back
14These figures were provided by personal communication with Katharine Hayhoe, Texas Tech University and produced for USGCRP (2009). back
15It should be noted that 2 inches per day of precipitation may not be considered a threat to highway infrastructure; however, a few studies cited in this report provide projections for the increased frequency of the 95th percentile of precipitation. back
16A type of extratropical storm that often initially develops as a cold-core low pressure system near the Gulf of Mexico, gathering warmth and moisture, then travels northward, developing along the East Coast. back
17Pineapple Express occurs when humid subtropical air originating near Hawaii travels to California causing great rains and floods. back
18A tropical storm, by definition, becomes a hurricane when sustained winds reach 74 miles per hour. A tropical depression, tropical storm, and a hurricane are all types of tropical cyclones. back
19"Steric" sea-level change refers to changes in sea level due to thermal expansion and salinity. "Eustatic" sea-level rise refers to the changes in sea level in response to the melting of small glaciers and ice sheets. "Isostatic" sea-level change is due to the shifting of land masses through adjustment to glacial loading or unloading, thermal buoyancy, or plate tectonics. "Dynamic" sea-level change is due to changes in ocean circulation. back
20In this case, an extreme heat event is defined as a day where the maximum temperature exceeds 90°F. back
21Statistically downscaled data from the CMIP3 database are used for this figure. back
22This information was provided by personal communication with Jay H. Lawrimore of the National Climatic Data Center. back
23Seasons are defined as follows: Winter (December, January, February), Spring (March, April, May), Summer (June, July, August), and Fall (September, October, November). back
24The National Water Level Observation provides historic sea level trends for 128 stations along the U.S. coastline. These measurements are provided by NOAA and include stations that provide sea level trends over a 30 year span or longer (NOAA 2010). back
25This information was provided by personal communication with Jay H. Lawrimore of the National Climatic Data Center.back
26The NECIA (2008) and NECIA (2006) study uses statistical downscaling of the results of three climate models: CM2.1, HadCM3, and PCM, relative to a 1961-1990 baseline.back
27Extreme heat day is defined by this study as the number of days with temperature above 90 °F.back
28Hayhoe et al. (2008) provides results based on the A1Fi (high) and B1 (low) emission scenarios; the climate model results of CM2.1, HadCM3, and PCM are statistically downscaled.back
29Extreme heat event is based on apparent temperature, a combination of high temperature and humidity.back
30This information was provided by personal communication with Jay H. Lawrimore of the National Climatic Data Center. Annual precipitation provides some indication of regional change, but is not an adequate indicator when determining impacts on transportation as it masks much of the seasonal variability.back
31Extreme rain event is defined as more than 2 inches per day.back
32The National Water Level Observation provides historic sea level trends for 28 stations along the Northeast U.S. coastline. These measurements are provided by NOAA and include stations that provide sea level trends over a 30-year span or, in most cases, much longer (NOAA 2010). The Northeast trend is compared against the global observed trend of sea-level rise of 1.8 ± 0.5 mm yr-1 from 1961 to 2003 (IPCC 2007a).back
33Dynamic sea-level rise at New York City at end-of-century (i.e., 2091-2100) is projected using the GFDL CM2.1 climate model under B1 (low emission) and A2 (high emission) scenarios relative to 1981-2000 mean sea level (Yin et al. 2009).back
34Storm surge elevation along the Northeast coast at end-of-century (i.e., 2100) is projected using a long-term average of highest daily tides at each location, a mid-range of global sea-level rise predicted by IPCC under the B1 (low emissions) scenario, and high-range associated with the mid-range projection provided by Rahmstorf (2007), relative to the North American Vertical Datum (NAVD) in 1988. Tide measurements, due to the location of the tide gauges, measure both storm surge and increased river flow during coastal flooding events. (Kirshen et al. 2008).back
35This information was provided by personal communication with Jay H. Lawrimore of the National Climatic Data Center. back
36Extreme heat day defined here as a daily maximum temperature above 100 °F. back
37CCSP 2008a study draws from results of 17 climate models represented in the CMIP3 dataset for IPCC AR4. back
38High temperature days are defined as defined as being at or above the 95th percentile among current daily temperature records. This study uses RegCM3 and a baseline period of 1961-1985. back
39This information was provided by personal communication with Jay H. Lawrimore of the National Climatic Data Center. Annual precipitation provides some indication of regional change but is not an adequate indicator when determining impacts on transportation as it masks much of the seasonal variability. back
40The National Water Level Observation provides historic sea level trends for 37 stations along the Southeast U.S. coastline. These measurements are provided by NOAA and include stations that provide sea level trends over a 30-year span or, in most cases, much longer (NOAA 2010). The Southeast trend is compared against the global observed trend of sea-level rise of 1.8 ± 0.5 mm yr-1 from 1961 to 2003 (IPCC 2007a). back
41This information was provided by personal communication with Jay H. Lawrimore of National Climatic Data Center. back
42Hellman et al (2007) study projects changes in annual average temperature relative to 1961-1990 drawing from 21 IPCC AR4 models using a higher (A1Fi) emission scenario and lower (B1) emission scenario. The projection provided is for 2010-2039. back
43This study provides projections based on statistically downscaled data of three climate models (CM2.1, HadCM3, and PCM) which represent the spectrum of climate sensitivity; the baseline period is 1961-1990. back
44This study defines a heat wave as the maximum temp exceeding the 97.5 percentile for at least 3 days, the average minimum temperature above the 97.5 percentile for at least 3 days, and the maximum temperature above the 81st percentile for the entire period. This study used a 'business as usual' scenario and 1961-1990 as relative baseline. back
45This study uses three state-of-the-art global climate models (CM 2.1, HadCM3, and PCM) that together adequately capture climate sensitivity. back
46This information was provided by personal communication with Jay H. Lawrimore of the National Climatic Data Center. Annual precipitation provides some indication of regional change but is not an adequate indicator when determining impacts on transportation as it masks much of the seasonal variability. back
47This study defines a heavy downpour as more than 2" of rain per day. back
48This study uses three state-of-the-art global climate models (CM 2.1, HadCM3, and PCM) that together adequately capture climate sensitivity, and the B1 emission scenario (low) and A1Fi emission scenario (high). back
61This information was provided by personal communication with Jay H. Lawrimore of the National Climatic Data Center. back
62This study define heat waves as three or more days where daily heat index exceeds 90 °F. Two climate models (CCSM3 and ECHAM5) are used to force the Weather Research and Forecasting Regional Model. back
63Warm nights are defined as nights with a minimum temperature above the 90th percentile. back
64Nineteen climate models were statistically downscaled for this study, with the 5th to 95th percentile range being 4 to 9.7 degrees Fahrenheit. The reference period is 1970-1999. back
65This information was provided by personal communication with Jay Lawrimore of the National Climatic Data Center. Annual precipitation provides some indication of regional change; however, it is not an adequate indicator when determining impacts on transportation as it masks much of the seasonal variability. back
66This study defines extreme precipitation events as the number of days where precipitation exceeds the 95th percentile. A regional model (RegCM3) was used and projections for 2071-2095 are compared against a 1961 to 1985 baseline. back
67The National Water Level Observation provides historic sea level trends for approximately 11 stations for the Pacific Northwest U.S. coastline. These measurements are provided by NOAA and include stations that provide sea-level trends over a 30 year span or, in most cases, much longer (NOAA 2010). The Pacific Northwest trend is compared against the global observed trend of sea-level rise of 1.8 ± 0.5 mm yr-1 from 1961 to 2003 (IPCC 2007a). back
68Local sea-level rise at three locations along the Northwest coast mid-century (i.e., 2050) and at end of century (i.e., 2100), projected using estimates of regional vertical land motion, atmospheric dynamics, and global sea-level rise projected by IPCC (2007a) under B1 (low emissions) and A1F1 (high emissions) scenarios. Sea-level rise is given relative to 1980-1999 mean sea level. The study caveats the results to be used for advisory purposes only. See Climate Change Effects Typology Matrix for projected results. (Mote et al. 2008) back
69This study uses the TOPP numerical model and compares 2090-2100 projections relative to 2001-2004 temperatures. Largest change is on the coast with some high elevation areas becoming slightly colder. back
70The National Water Level Observation provides historic sea level trends for approximately 15 stations for the Alaska U.S. coastline. These measurements are provided by NOAA and include stations that provide sea level trends over a 30-year span or, in most cases, much longer (NOAA 2010). This trend is compared against the global observed trend of sea-level rise of 1.8 ± 0.5 mm/yr from 1961 to 2003 (IPCC 2007a). back
71The National Water Level Observation provides historical sea-level trends for 5 stations in Hawaii. These measurements are provided by NOAA and include stations that provide sea-level trends over a 30-year span or, in most cases, much longer (NOAA 2010). This trend is compared against the global observed trend of sea-level rise of 1.8 ± 0.5 mm/yr from 1961 to 2003 (IPCC 2007a). back
72The National Water Level Observation provides historical sea-level trends for 2 stations in Puerto Rico. These measurements are provided by NOAA and include stations that provide sea-level trends over a 30 year span or, in most cases, much longer (NOAA 2010). This trend is compared against the global observed trend of sea-level rise of 1.8 ± 0.5 mm yr-1 from 1961 to 2003 (IPCC 2007a). back

Updated: 03/27/2014
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