The climate projections provided in this report are based mostly on the results from a collection of global climate models aggregated to the multi-state regional scale. This set of data is uniformly available across all regions in the United States. Additional information of downscaled projections has been provided at the state or sub-state level as available. In the coming years, the data provided in this report will likely become outdated as model simulations and downscaling techniques that provide regional- and local-scale climate projections continue to evolve. Ideally, information downscaled to the local level will become uniformly available, a task perhaps for a National Climate Service.
Uncertainty in the projections provided in this report includes model uncertainty and emission uncertainty (i.e., how closely emissions and concentrations of GHGs in the future match the two scenarios-the B1 and A2 scenarios-used in this report), while natural variability is not directly assessed. As climate science progresses, the degree of uncertainty is likely to be reduced-particularly for regional-scale projections. For example, Hawkins and Sutton (2009) suggest that the uncertainty associated with regional projections in the near-term, dominated by model uncertainty and natural variability, could be significantly reduced through scientific progress.
Certain climate effects are particularly problematic to model, while others are simply not available at the regional or finer scale. Of the climate effects discussed by region in this report, precipitation and storm events are particularly challenging. For many regions, there is a large range of plausible values for precipitation that may swing dramatically between significant increases to significant decreases. As science hones its skills representing the water cycle and the degree of uncertainty decreases, these ranges may narrow. In addition, changes in precipitation extremes (e.g., precipitation frequency, duration, and intensity) are extremely important for planning purposes but sparsely addressed in the literature at the regional or local scale.
Continued research in building techniques for assessing projected storm growth is another area of concern for highway planners. Across the United States, storms of interest can vary between small-scale convective storms (i.e., thunderstorms) to larger-scaled cyclones (i.e., Nor'easters and other mid-latitude extratropical cyclones, tropical storms, and hurricanes). There are many factors that contribute to whether a specific storm will grow or dissipate. For example, the growth of a tropical storm into a hurricane depends on such factors as wind shear, vertical temperature structure over the ocean, and sea surface temperatures, all of which can vary differently in a changing climate system. It is a challenge for climate models to capture these various components and then project how, under a changing climate, they may affect storms.
The global estimate of sea-level rise associated with thermal expansion and ice melting is an area of intense research, with studies continually being published with new methods and refined estimates. It is likely that the global estimate of sea-level rise will continue to evolve in the coming years. This report also discusses projected local sea-level rise where global estimates are applied and adjusted through considering local factors such as vertical land motion, ocean circulation, erosion and sedimentation, and ocean density. As these projections can vary significantly between nearby locations due to local variability of these contributing factors, local-scale efforts that address these factors would provide additional data to assist highway planners. Currently, no study presented in this report provides all inclusive sea-level rise projections taking all of these factors into account. It is understood that scientists are improving their skills and methods in estimating the observed and projected local variability; the results of these efforts will provide highway planners with increased accuracy in estimating local sea-level rise.
Changes in solar radiation and humidity can affect highway systems. Though climate models do provide projections of these variables, the work is still rudimentary in using these projections for addressing climate change impacts.73 In the future, it is likely that data and projections for these variables will be available in a form useful to highway planners.
This report broadly discusses the impacts of the climate projections by treating each climate variable separately. New techniques may become available that account for the impact of multiple climate effects on a particular component of the highway system. For example, many in the health sector have replaced the use of maximum temperature with a heat index (a combination of maximum temperature and relative humidity) in determining the impact of heat events on mortality. Similar climate variable combinations may be determined that perform better in assessing the projected impacts on the highway system. For example, the assessments of projected impacts could be examined through engineering design specifications associated with worst-case scenarios. Instead of determining impact by isolating each climate effect (i.e., investigating the projected change associated with each engineering design specification), this effort would take the next step to combine the impacts associated with all the design specifications, thereby providing an approximation of impacts from multiple climate effects.