The changing climate poses serious challenges to the transportation community, given our need to watch over transportation systems and infrastructure designed to last decades or longer. Transportation functions tied to construction, operations, maintenance, and planning need to be based on an understanding of the environment expected to support transportation facilities. Decisions therefore need to be informed by an understanding of potential future changes in climate.
Changes in the climate over the recent past have been documented by researchers, including changes in temperature, precipitation, storm activity, sea level, and wind speeds. These effects can in turn lead to impacts on transportation, such as weakened bridges and road beds, permanently flooded roads, damaged pavements, and changes in road weather that can affect safety (both positively and negatively) and economic activity. Understanding and proactively addressing the potential impacts of climate change can help avoid the potential damage, disruption in service, and safety concerns that climate change may cause.
FHWA is releasing this report in order to provide the transportation community (i.e., highway and bridge engineers, planners, NEPA practitioners, etc.) with transparent and reliable information on projected climate change effects that are most relevant to the U.S. highway system to the extent that such information was available through the summer of 2009.
This report synthesizes available information to present a picture of how the climate might change over the near term (2010-2040), by mid-century (2040-2070), and at the end of this century (2070-2100) for the country divided into nine regions, and it summarizes the current understanding of these projected effects primarily at the regional level. The nine regions match those used in the U.S. Global Change Research Program's Global Climate Change Impacts in the United States (2009) report.
There are several objectives in preparing this information. It is designed to help transportation practitioners better understand what climate change means for transportation, generally, and it is also intended to be considered where appropriate in state and metropolitan planning and project development. For example, information on changes in seasonal precipitation can inform analysis of stream flow and scour. Projections of changes in summer temperatures can inform decisions tied to infrastructure design and maintenance.
On the other hand, it represents an initial step in outlining the type and range of potential future climate change effects facing transportation in the United States, and is meant to be a starting point in understanding how climate change may affect transportation. It does not provide all the information needed. The science of projecting future climate is evolving, and in time more and better information should become available.
This report focuses on region-scale projections for temperature and precipitation, which were assembled based on the results of global climate models. The data cited were developed in support of the USGCRP document, but have not previously been published in their entirety. This information exists for all regions of the country. Additionally, the report includes the findings of a literature review in the Climate Change Effects Typology Matrix (Appendix C). The Typology collects relevant projections of climate change effects included in other reports and from peer-reviewed literature. Some of this information is also included in the regional sections contained in chapter 3. This report also includes results that were statistically downscaled from the results of climate models to the sub-regional level. This information, which is presented in several figures in Chapter 3 and Appendix B, is limited to the contiguous 48 states. The downscaling of climate data is an area of continued development, with new techniques likely to become available in the coming years.
While much of the information provided in this report applies to a region as a whole, rates of change in a given location may or may not match projections for the larger region. Thus, transportation agencies will need to work with environmental and engineering staffs to determine what these regional projections mean for their local or state level planning, programming, and maintenance efforts. The information in this report can provide insights into the range of changes and trends that may influence the state, local, or project level.
This report provides a first step in understanding the impacts that climate change may have on existing and future transportation infrastructure and operations. Additional work will be needed to understand how best to incorporate this information into project- and system-level planning, to translate this information into specific impacts on transportation, and to assess what if any modifications are necessary to the transportation network to ensure its long-term ability to provide access and mobility for people and goods. FHWA will undertake further work to develop tools, methods, and ultimately guidance that can be used to apply climate change information to decision-making. We will learn from further research (including our current work to develop preliminary tools to assess the vulnerability and risk of transportation to the effects of climate change) and the efforts of the transportation community, including State and local practitioners, on how best to apply this information. Each program area-asset management, metropolitan and statewide planning, project development, operations, safety, infrastructure, bridge, pavements, etc.-may apply this information differently based on its specific needs, local context, or other factors. As we gain more experience using this information, and the science is updated and refined, we will provide additional guidance.
This chapter briefly examines climate variability experienced through the end of the 20th century, focusing on the national level. It also summarizes a range of potential climate impacts on the highway system that can result from changes in climate, and introduces concepts such as risk and vulnerability. We have included this information early in the report-before discussing the regional climate effects-to illustrate the importance of examining the effects of climate change. Chapter 2 provides a brief overview of both the methodology used to assemble the climate data for this report and the consultation process with national and regional climate experts. (This methodology is treated in more detail in Appendix A.) Chapter 3 summarizes and discusses the available data and literature of projected climate change effects, focusing on the national and regional levels. This information serves as the foundation of this report; with the rest of the information providing context. Appendix B illustrates the Chapter 3 tables describing regional climate projections of annual and seasonal temperature and seasonal precipitation. Chapter 4 discusses needed future work, and identifies climate projections that are currently unavailable. Chapter 5 includes a glossary of terms used in the report. Chapter 6 provides the list of references.
Appendix C provides the Climate Change Effects Typology Matrix and will be posted online. It provides one-line summaries of the results of each current collected study or set of modeled data for each of the main climate effects, by region, and time frame. It was assembled while locating relevant information for this report. (Chapter 2 and Appendix A provide a description of the methodology used in developing the Climate Change Effects Typology Matrix.)
This section provides a brief review of national-scale changes in temperature, precipitation and storm activity, and sea level observed in recent decades. This information is drawn largely from the IPCC (2007a), National Science and Technology Council (2008), and USGCRP (2009) reports, and is based on satellite measurements and data from thousands of weather stations, ships, and buoys around the world carefully compiled by independent research groups.2
Over the 20th century, the Earth's annual average temperature has increased by approximately 1.3 ± 0.32°F (0.74 ± 0.18°C) (IPCC 2007a). The winter and spring seasons in the Northern Hemisphere have experienced the greatest degree of warming, with the United States experiencing a warming of near 0.58°F per decade over the past few decades (National Science and Technology Council 2008). As the frequency of heat waves has increased, the number of unusually cold days has decreased (National Science and Technology Council 2008; USGCRP 2009). However, over the past few decades, the diurnal temperature difference has not changed, with day and night temperatures rising at similar rates (USGCRP 2009).
The impact of this warming on the natural system has already been well documented. For example, the area of Arctic sea ice has shrunk at a rate of about 2.7 percent per decade, with the summer months experiencing even greater reductions of 7.4 percent per decade. This is a result of warming in the Arctic that is twice the average warming in the United States (USGCRP 2009). In the continental United States and Alaska (the middle and high latitudes), shifts in phenology-the timing of life cycles events of plants and animals-have been noted. These phenological changes include an increase in the growing season of approximately 2 weeks since 1950, and earlier annual occurrences of plant flowering and animal spring migration (USGCRP 2009).
Over the 20th century, the total average annual precipitation for the contiguous United States increased by 6 percent (National Science and Technology Council 2008). During the second half of the 20th century, some regions across the United States have experienced increases in drought severity and duration as temperatures have risen (USGCRP 2009). The United States has experienced extreme drought events in the past, such as during the mid-1930s when portions of the Great Plains became known as the "dust bowl" due to wind erosion brought on by several years of drought and compounded by the replacement of moisture-retaining natural vegetation with crops. Since the 1950s, parts of the Southeast and West have experienced an increase in drought conditions, while the Midwest and Great Plains have experienced a reduction.
During the last three decades of the 20th century, the eastern United States experienced an increase in heavy precipitation events3 (National Science and Technology Council 2008), and an increase in the proportion of total annual precipitation that falls during heavy precipitation events.
While recent research indicates there is some likelihood that the number of tropical storms and hurricanes each year in the North Atlantic has increased over the past 100 years (National Science and Technology Council 2008), the number of hurricanes that make landfall has stayed relatively constant (USGCRP 2009). The intensity of the strongest hurricanes is also likely to have increased in this region: sea surface warmth is a strong contributor to tropical storm development, and climate change is considered to have contributed to the increase of sea surface temperatures in the North Atlantic and Northwest Pacific hurricane formation regions (CCSP 2008b). However, it is unclear if and how other tropical storm development factors-such as temperature and moisture profiles, wind shear, or near-surface ocean temperature stratification-have changed. The trend is further complicated by multi-decadal variability and data-quality issues. For smaller-scale phenomena such as tornadoes, hail, lightening, and dust storms, the IPCC (2007a) concluded that there is insufficient evidence to determine the associated trends.
Over the 20th century, global average sea level has risen by 6.7 inches (0.17 meters) (IPCC 2007a). Figure 1 below demonstrates how sea-level rise varies regionally across the United States. These differences are due primarily to differences in vertical land motions (USGCRP 2009). Relative sea level is rising 0.8 to 1.2 inches per decade along most of the Atlantic and Gulf Coasts, with a few inches per decade occurring along the Louisiana Coast due to land subsidence (National Science and Technology Council 2008). Other regions of the country, such as specific coastline locations for Alaska, are experiencing land uplift and a corresponding relative sea level decline of a few inches per decade (National Science and Technology Council 2008). Sea-level rise increases the risk of impact from storm surge and waves farther inland, causing shoreline erosion and local damage.
Assessing the potential harm of climate stressors allows vulnerabilities to be addressed before they become problems. For example, knowing that a road used during emergency evacuations will be at risk for failure due to erosion allows decision makers to decide what to do before the road washes out. Transportation decision makers may decide to take measures to prevent the road from washing out or find another route. They may also decide that the costs of action are too great, and the risks are too low (or the likelihood of damage is too low), to justify any action. However, in order to make such decisions, an assessment of the climate-related risks is necessary.
Vulnerability describes how susceptible a system is to the adverse effects of climate change (IPCC 2007b).
Vulnerability Factors include the age of the infrastructure element, condition/integrity of the infrastructure element, proximity to other infrastructure elements/concentrations, and the level of service (CCSP 2008).
Exposure is the degree to which a system comes into contact with climate conditions or specific climate impacts (CIG 2007), and the probability, or likelihood, that this stress will affect transportation infrastructure (CCSP 2008a).
Risk characterizes both the probability of the event occurring and the consequence of the event (Snover et al. 2007; NZCCO 2004).
Potential Climate Impacts describes how projected climate effects may affect the highway system through current or newly introduced system exposures or sensitivities. It should be noted that the uncertainties associated with projecting the impacts continue to apply when considering risks (which also have the additional uncertainty of consequences).
Projected changes in temperature, precipitation, storm activity, wind, and sea level indicate the magnitude of the stressors to which highway infrastructure could be exposed in the future. However, these effects do not in and of themselves indicate what the ultimate consequences to the highway infrastructure will be. From a highway operations standpoint, does it actually matter that temperatures or rainfall might increase? And would these effects translate to beneficial or adverse impacts on the highway system? The answers depend on many factors, such as the severity of the climate stressor, the engineering and design characteristics of the structures, geographic and geologic characteristics, and operations and maintenance activities. It is therefore important to consider all of these factors when assessing the potential impacts of climate change.
While consideration of so many variables may seem daunting, it is important to remember that highway structures are already being exposed to many climate- or weather-related stressors. Climate change can exacerbate (or lessen) these same stressors. Highway infrastructure is already exposed to (and, to a certain extent, designed to withstand) the elements. An area that rarely experiences severe storms is unlikely to suddenly experience frequent hurricanes. Instead, an area already exposed to severe storms may experience more severe or more frequent storms. Since infrastructure is designed to withstand locally expected climate stressors of the magnitude and frequency that have historically been experienced, the risks from climate change can come from an amplification of existing stressors (NRC 2008).
In some cases, climate changes will be sufficient to push some aspect of a transportation system over a certain threshold. For example, rising sea levels can introduce erosion problems to areas that previously had limited exposure to those threats, and thereby undermine roadways that had previously been unaffected by erosion. For inland areas, falling lake levels can lead to erosion of bedrock due to wave impacts, causing roads to fail.
Many of the risks from climate change come from an increased exposure to extremes in weather and climate. One example is a projected increase in the number of days with extremely high temperatures, which cause more stress than simply an increase in the average temperature. Most infrastructures are engineered to withstand a normal or expected range of climate or weather stressors, and small changes in average climate won't have significant impacts on the structures themselves (NRC 2008). However, as the climate changes, the expected range of stressors may not accurately reflect actual exposure. For example, bridges are often designed to withstand a certain level of flooding (such as the height of a flood that normally has a 1% likelihood of occurring in a year, i.e., a 1 in 100 year flood event), a probability that is typically based on historical data. Under some climate change scenarios, a flood level that has a true 1% annual likelihood of occurring might actually be much more severe than the historical 1% return rate flood.
It is also important to note that not all climate change impacts are negative. In some areas, climate change could reduce the frequency, duration, and/or severity of some cold-weather extremes, for example. In these cases, shorter winters may lead to longer thaw seasons, or higher temperatures could allow an extension in the work season-both of which are potential benefits. But generally, it is useful to keep in mind that any deviation from the climate for which infrastructure has been designed is likely to cause negative impacts (CCSP 2008b; USGCRP 2009).
Climate change impacts on highway infrastructure can be sudden and severe, or may occur more gradually. Floods and erosion can completely and abruptly shut down a road. In contrast, an increase in the frequency and severity of extremely high temperatures can lead to pavement deterioration and rutting. These more gradual problems can be predicted and addressed through additional maintenance, but they are still costly and disruptive to traffic flow. This difference is important to remember when assessing priorities in climate adaptation. Are decision makers concerned primarily with sudden and severe highway system failures, or more generally with minimizing costs and travel disruptions?
When considering potential impacts on the highway system, it is also important to keep in mind the interconnectivity of the highway system. When looking at a specific geographic area, it can be tempting to dismiss climate stressors as not being relevant to that area. But disruptions in highway systems in one location can affect the timely delivery of goods in another location; they can also disrupt passenger travel. Additionally, areas most affected by climate change might ultimately draw larger amounts of money to repair damages and reduce future damages-diverting resources that could have been applied elsewhere.
Highway infrastructure and operations/maintenance are affected by three key categories of climate change effects: changes in temperature, changes in precipitation and storm events, and sea-level rise. This section continues by providing some example impacts that these climate effects can have on highways. The information provided in the tables below is based on a collection of existing reports and is provided for this discussion; however, the tables do not represent all plausible potential impacts. Actual impacts would depend on local conditions as well as the severity of climate change effects.
Small changes in average temperatures will not greatly affect highway systems; however, by the end of the century some regions are projected to experience significant warming that will affect highway planning, construction, and operations. Significant impacts may also occur with increases in both the intensity (how high the high temperature is) and duration of very hot periods, or decreases in the intensity and duration of periods with very cold days. Extreme temperatures can cause both structural damage to highway assets and challenges to the use and maintenance of the roads. As noted in Section 3, the number of extreme heat days is project to increase in all regions of the continental United States. Meanwhile, the shortening of the winter season could provide some benefits in terms of lengthening the construction season and reducing snow/ice removal costs, particularly in the more northern regions. However, the northern states are projected to sustain increases in freeze-thaw conditions4, potentially increasing the occurrence of frost heaves and potholes on road and bridge surfaces (USGCRP 2009). Table 1 1 provides a summary of how changes in temperature may affect highway infrastructure and operations.
|Climate Effects||Impacts on Infrastructure and Operations|
|Increases in very hot days and heat waves (higher high temperatures, increased duration of heat waves)||
|Decreases in very cold days||
|Later onset of seasonal freeze and earlier onset of seasonal thaw||
Table 1-1: Impacts of temperature on highway operations and infrastructure. Sources: NRC (2008), CCSP (2008a), CSIRO (2006), Department of Transport (U.K.) (2004)
Increased rains can cause disruptions in the use of highways (mainly due to flooding), as well as structural damage. In some areas, however, drought conditions are expected to increase, which can introduce other threats to the highway system. For example, summer precipitation is expected to decrease for most regions (see Section 3), which could lead to isolated pockets of increased drought-related impacts. Winter precipitation for most regions is projected to increase, which could result in additional snow and ice removal costs (particularly for the Northeast Great Plains and Midwest states)5. However, in many cases, those impacts are generally projected to be less in areas and at times of year that are now just below freezing but projected to warm above freezing in the future.
Storms such as tropical cyclones, thunderstorms, and extratropical cyclones can cause sudden, dramatic, and costly disruptions to the highway systems. Use of the highways can be disrupted as a result of flooding or structural failures. Severe winds and rains can also cause significant damage to structures. Table 1-2 provides a summary of how changes in severe storm intensity may affect highway infrastructure and operations.
|Climate Effects||Impacts on Infrastructure and Operations|
|Increases in intense precipitation events||
|Increases in drought conditions||
|Changes in seasonal precipitation and stream flow patterns||
|Increases in coastal storm intensity (leading to higher storm surges/wave heights, increased flooding, stronger winds)||
Table 1-2: Impacts of precipitation on highway operations and infrastructure. Sources: NRC (2008), CCSP (2008a), CSIRO (2006), personal communication with E. Robert Thieler.
Rising sea levels can permanently inundate coastal roads and cause damaging erosion. Higher sea levels can exacerbate the effects of storm surge, causing storm surges to reach greater heights and further inland, possibly inflicting additional damages on structures. Sea-level rise presents significant risks to many regions, particularly those where land is subsiding. Table 1-3 provides a summary of how changes in sea-level rise may affect highway infrastructure and operations (as changes in storm surge are a function of both storm activity and sea-level rise, the impacts provided in Table 1-3 overlap somewhat with those at the end of Table 1-2).
|Climate Effects||Impacts on Infrastructure and Operations|
|Rising sea levels (exacerbating effect of higher storm surge, increased salinity of rivers and estuaries, flooding)||
Table 1-3: Impacts of sea-level rise on operations and highway infrastructure. Sources: NRC (2008), CCSP (2008a), CSIRO (2006), ICF (2007)
2 Differences caused by changes in instruments, measurement times and locations, etc. were taken into account during data processing in the reports referenced. back
3Heavy precipitation events are defined by the referenced reports as an event with at least 2 inches of precipitation per day. For purposes of this report, heavy precipitation events constitute a storm event; however, given the limitation of the information provided, no discussion of the type of storm, the associated phenomena such as winds, nor the related stressors such as flooding can be determined. back
4Freeze-thaw conditions refer to the number of days when the maximum temperature is greater than freezing and the minimum temperature is below freezing. back
5Personal communication with Michael Wehner of the Lawrence Berkeley National Laboratory. back