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2 Understanding Climate Change T his chapter begins with an overview of current understanding of the role greenhouse gases (GHGs) play in the atmosphere and evidence for how they are already influencing the earth’s climate in both general and specific ways. The discussion includes a review of the climate change projections of global climate models and some of the evidence that has led recent national and international scientific assessments—including those of the Intergovernmental Panel on Climate Change (IPCC) (2007), the National Research Council (2001), and the Climate Change Science Program (CCSP) Synthesis and Assessment Report 1.1 (Karl et al. 2006)—to link the rise in temperature, particularly since the 1970s, to increases in GHGs. Next is a discussion of the projected climate changes for North America most relevant for U.S. transportation. For each climate variable, past projections and key uncertainties are also discussed. The chapter ends with a series of findings. OVERVIEW OF GLOBAL CLIMATE CHANGE The Greenhouse Effect and Atmospheric Composition The natural “greenhouse” effect is real and is an essential component of the planet’s climatic processes. A small proportion (roughly 2 percent) of the atmosphere is, and long has been, composed of GHGs (water vapor, car- bon dioxide, ozone, and methane). These gases effectively prevent part of the heat radiated by the earth’s surface from otherwise escaping to space. The response of the global system to this trapped heat is a climate that is warmer than it would be without the presence of these gases; in their 36

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Understanding Climate Change 37 absence, the earth’s temperature would be too low to support life as we know it. Among the GHGs, water vapor is by far the most dominant, but other gases augment its effect through greater trapping of heat in certain portions of the electromagnetic (light) spectrum. In addition to the natural greenhouse effect outlined above, a change is under way in the greenhouse radiation balance. Some GHGs are prolif- erating in the atmosphere because of human activities and increasingly trapping more heat. Direct atmospheric measurements made over the past 50 years have documented steady growth in the atmospheric abundance of carbon dioxide (CO2). In addition to these direct, real-time measurements, ice cores have revealed the atmospheric CO2 concentrations of the distant past. Measurements using air bubbles trapped within layers of accumulat- ing snow show that atmospheric CO2 has increased by nearly 35 percent over the Industrial Era (since 1750), compared with its relatively constant abundance over at least the preceding 10,000 years (see Figure 2-1). The predominant causes of this increase in CO2 are the combustion of fossil fuels and deforestation. Further, the abundance of methane has doubled over the Industrial Era, although its increase has slowed during the past decade for reasons not clearly understood. Other heat-trapping gases are also increasing as a result of human activities. Scientists are unable to state with certainty the rate at which these GHGs will continue to increase because of uncertainties in future emissions, as well as in how these emis- sions will be taken up by the atmosphere, land, and oceans. They are certain, however, that once in the atmosphere, these gases have a relatively long residence time, on the order of a century (IPCC 2001). This means they become well mixed across the globe. There is no doubt that the composition of the atmosphere is affected by human activities. Today GHGs are the largest human influence on atmospheric composition. The increase in GHG concentrations in the atmosphere implies a positive radiative forcing (i.e., a tendency to warm the climate system). Increases in heat-trapping GHGs are projected to be amplified by feed- back effects, such as changes in water vapor, snow cover, and sea ice. As atmospheric concentrations of CO2 and other GHGs increase, the resulting rise in surface temperature leads to less sea ice and snow cover, causing the planet to absorb more of the sun’s energy rather than reflecting it back to space, thereby raising temperatures even further. Present evidence also suggests that as GHGs lead to rising temperatures, evaporation

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38 Potential Impacts of Climate Change on U.S. Transportation FIGURE 2-1 Atmospheric concentrations of carbon dioxide, methane, and nitrous oxide over the past 10,000 years (large panels) and since 1750 (inset panels). Measurements are from a combination of ice cores (going back 10,000 years) and atmospheric samples in the 20th century. (Source: IPCC 2007, Figure SPM-1, p. 15. Reprinted with permission of the IPCC Secretariat, Geneva, Switzerland.)

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Understanding Climate Change 39 increases, leading to more atmospheric water vapor (Soden et al. 2005; Trenberth et al. 2005). Additional water vapor, the dominant GHG, acts as a very important feedback to increase temperature further. The most uncer- tain feedback is related to clouds, specifically changes in cloud frequency, location, and height. The range of uncertainty spans from a significant pos- itive feedback to no feedback, or even a slightly negative feedback. Present understanding suggests that these feedback effects account for at least half of the climate’s warming (IPCC 2001; Karl and Trenberth 2003). The exact magnitude of these effects remains a significant source of uncertainty in understanding the impact of increasing GHGs. Increases in evaporation and water vapor affect global climate in other ways besides causing rising temperatures, such as increasing rainfall and snowfall rates and accelerating drying during droughts. Particles suspended in the atmosphere (aerosols) resulting from human activities can also affect climate. Aerosols vary considerably by region. Some aerosol types (e.g., sulfate) act in a way opposite to the GHGs by reflecting more solar radiation back to space than the heat they absorb, and thereby causing a negative radiative forcing or cooling of the climate system. Other aerosols (e.g., soot) act in the same way as GHGs and warm the climate. In contrast to the long-lived nature of CO2, aerosols are short-lived and removed from the lower atmosphere within a few days. Therefore, human- generated aerosols exert a long-term forcing on climate only because their emissions continue each day of the year. The effects of aerosols on climate can be manifested directly by their ability to reflect and trap heat, but also indirectly by changes in the lifetime of clouds and in the clouds’ reflectiv- ity to sunshine. The magnitude of the negative forcing of the indirect effects of aerosols is highly uncertain, but it may be larger than that of their direct effects (IPCC 2001). Emissions of GHGs and aerosols continue to alter the atmosphere by influencing the planet’s natural energy flows (see Box 2-1), which can cause changes in temperature and precipitation extremes, reductions in snow cover and sea ice, changes in storm tracks, and increased intensity of hur- ricanes (IPCC 2007). There are also natural factors that exert a forcing effect on climate [e.g., changes in the sun’s energy output and short-lived (a few years) aerosols in the stratosphere following episodic and explosive volcanic eruptions]. If all the possible influences of natural and human cli- mate forcings over the past several decades are considered, increases in GHGs have had a larger influence on the planet’s radiation flow than all the

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40 Potential Impacts of Climate Change on U.S. Transportation BOX 2-1 What Warms and Cools the Earth? The sun is the earth’s main energy source. Its output appears nearly constant, but small changes during an extended period of time can lead to climate changes. In addition, slow changes in the earth’s orbit affect how the sun’s energy is distributed across the earth, creating another variable that must be considered. Greenhouse gases warm the earth: Water vapor (H2O), supplied from oceans and the natural biosphere, accounts for two-thirds of the total greenhouse effect but acts primarily as a feedback. In contrast to other greenhouse gases, the amount of water vapor in the atmosphere generally cannot be controlled by humans. Water vapor introduced directly into the atmosphere from agricultural or other activities does not remain there very long and is overwhelmed by natural sources; thus it has little warming effect. Carbon dioxide (CO2) has natural and human sources. CO2 levels are increasing as a result of the burning of fossil fuels. Methane (CH4) has both human and natural sources and has risen signif- icantly since preindustrial times as the result of an increase in several human activities, including raising of livestock; growing of rice; use of landfills; and extraction, handling, and transport of natural gas. Ozone (O3) has natural sources, especially in the stratosphere, where changes caused by ozone-depleting chemicals have been important; ozone also is produced in the troposphere (the lower part of the atmo- sphere) when hydrocarbons and nitrogen oxide pollutants react. Nitrous oxide (N2O) has been increasing from agricultural and industrial sources. Halocarbons continue to be used as substitutes for chlorofluorocarbons (CFCs) as refrigerant fluids, and CFCs from pre–Montreal Protocol usage as refrigerants and as aerosol-package propellants remain in the atmosphere. Scientists have a high level of understanding of the human contributions to climate forcing by carbon dioxide, methane, nitrous oxide, and CFCs and a medium level of understanding of the human contributions to climate forcing by ozone (Forster et al. 2007). (continued)

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Understanding Climate Change 41 Some aerosols (airborne particles and droplets) warm the earth: Black carbon particles, or “soot,” produced when fossil fuels or vegetation is burned, generally have a warming effect by absorbing solar radiation. Some aerosols cool the earth: Sulfate (SO4) aerosols from burning of fossil fuels reflect sunlight back to space. Volcanic eruptions emit gaseous sulfur dioxide (SO2), which, once in the at- mosphere, forms SO4 aerosols and ash. Both reflect sunlight back to space. Scientists currently have a low level of understanding of the human contri- butions to climate forcing by aerosols (Forster et al. 2007). Changes in land cover, ice extent, and cloud cover can warm or cool the earth: Deforestation produces land areas that reflect more sunlight back to space; replacement of tundra by coniferous trees that create dark patches in the snow cover may increase absorption of sunlight. Sea ice reflects sunlight back to space; reduction in the extent of sea ice allows more sunlight to be absorbed into the dark ocean, causing warming. Clouds reflect sunlight back to space but can also act like a greenhouse gas by absorbing heat leaving the earth’s surface; the net effect depends on how the cloud cover changes. Source: Adapted from Staudt et al. 2006, p. 7. other forcings, one that continues to grow disproportionately larger (IPCC 2007; Karl and Trenberth 2003). Human activities also have a large-scale impact on the earth’s land sur- face. Changes in land use due to urbanization and agricultural practices, although not global, are often most pronounced where people live, work, and grow food and are part of the human impact on climate. Land use changes affect, for example, how much of the sun’s energy is absorbed or reflected and how much precipitation evaporates back into the atmo- sphere. Large-scale deforestation and desertification in Amazonia and the Sahel, respectively, are two instances in which evidence suggests the likeli-

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42 Potential Impacts of Climate Change on U.S. Transportation hood of a human influence on regional climate (Andreae et al. 2004; Chagnon and Bras 2005). In general, city climates differ from those in sur- rounding rural green areas, causing an “urban heat island” due to greater heat retention of urban surfaces, such as concrete and asphalt, as well as the waste generated from anthropogenic activities1 (Bornstein and Lin 2000; Changnon et al. 1981; Jones et al. 1990; Karl et al. 1988; Landsberg 1983; Peterson 2003). What Is a Climate Model and Why Is It Useful? Many of the scientific laws governing climate change and the processes involved can be quantified and linked by mathematical equations. Fig- ure 2-2 shows schematically the kinds of processes that can be included in climate models. Among them are many earth system components, such as atmospheric chemistry, ocean circulation, sea ice, land surface hydrology, biogeochemistry,2 and atmospheric circulation. The physics of many, though not all, of the processes governing climate change are well understood and may be described by mathematical equations. Linking these equations creates mathematical models of climate that may be run on computers or supercomputers. Coupled climate models can include mathematical equations describing physical, chemical, and biogeochem- ical processes and are used because the climate system is composed of different interacting components. Coupled climate models are the preferred approach to climate modeling, but they cannot at present include all details of the climate system. One rea- son is that not all details of the climate system are understood, even though the major governing processes are known well enough to allow models to reproduce observed features, including trends, of global climate. Another reason is the prohibitive complexity and run-time requirements of models that might incorporate all known information about the climate system. Decisions on how to build any given climate model include trade-offs among the complexity of the model and the number of earth system components included, the model’s horizontal and spatial resolution, and the number of The global effects of these urban heat islands have been analyzed extensively and assessed to 1 ensure that they do not bias measurements of global temperature. 2 Biogeochemistry refers to the biological chemistry of the earth system, such as the uptake of atmospheric carbon by land and ocean vegetation.

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Understanding Climate Change 43 FIGURE 2-2 Components of the climate system and their interactions, including the human component. All these components must be modeled as a coupled system that includes the oceans, atmosphere, land, cryosphere, and biosphere. GCM = General Circulation Model. (Source: Karl and Trenberth 2003, Figure 3. Reprinted from Science, Vol. 302, No. 5651, with permission.)

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44 Potential Impacts of Climate Change on U.S. Transportation years of simulations the model can produce per day of computer time. Consequently, there is a hierarchy of models of varying complexity, often based on the degree to which approximations are required for each model or component processes omitted. Approximations in climate models represent aspects of the models that require parameter choices and “tuning.” As a simple example, imagine rep- resenting a single cumulus cloud in a global climate model. The cloud may encompass only a few hundred meters in vertical and horizontal space—a much finer resolution than can be run on today’s coupled atmosphere and ocean climate models. As a result, if such clouds are to be incorporated into the climate model, some approximations must be made regarding the clouds’ statistical properties within, say, an area 100 or 1,000 times larger than the cloud itself. This is referred to as model parameterization, and the process of selecting the most appropriate parameters to best simulate observed conditions is called model tuning. Similar methods are also required in today’s state-of-the-science weather forecasting models. An important difference between weather forecasting models and climate models is that the former are initialized with a specific set of observations representing today’s weather to predict the weather precisely x days or hours into the future. By contrast, the initial conditions of climate models are much less important. Also, climate models are not intended to predict specific future weather events. Rather, they are used to simulate many years of “weather” into the future with the intent of understand- ing the change in the collection of weather events at some point in the future compared with some point in the past (often the climate of the past 30 years or so). Scientists are thus interested in properties of climate, such as average rainfall and temperature and the degree of fluc- tuation about that average. This comparison enables scientists to study the output of climate model simulations to understand the effect of various modifications of those aspects of the climate system that might cause the climate to change. A key challenge in climate modeling is to isolate and identify cause and effect. Doing so requires knowledge about the changes and variations in the external forcings controlling climate and a compre- hensive understanding of climate feedbacks (such as a change in the earth’s reflectivity because of a change in the amount of sea ice or clouds) and nat- ural climate variability. A related key challenge in climate modeling is the representation of sub-grid-scale processes, such as in some storms, and land-terrain effects.

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Understanding Climate Change 45 Model simulations of climate over specified periods can be verified and validated against the observational record. Likewise, model parame- terization schemes for particular processes of interest can be tested by comparison with observations and with higher-resolution, smaller-scale models. Models that describe climate variability and change well can be used as a tool to increase understanding of the climate system. Once eval- uated and validated, climate models can then be used for predictive purposes. Given specific forcing scenarios, the models can provide viable projections of future climate. In fact, climate models have become the pri- mary means of projecting climate change, although ultimately, future projections are likely to be determined through a variety of means, includ- ing the observed rate of global climate change. How Do We Know the Global Air Temperature Is Increasing? A comprehensive analysis of changes in temperatures near the earth’s sur- face and throughout much of the atmosphere is presented in the April 2006 CCSP Synthesis and Assessment Report 1.1 (Karl et al. 2006). This report addresses the nagging issue of differences in the rate of warming between measurements derived near the surface (typically 2 m above the surface) and those taken from higher in the atmosphere (i.e., the lower troposphere, or the atmosphere below roughly 12 km). The surface air temperatures are derived from several different analysis teams, using various combinations of ocean ships and buoys, land observations from weather reporting sta- tions, and satellite data. Atmospheric data sets have been derived by using satellites, weather balloons, and a combination of the two. Considering all the latest satellite, balloon, and surface records, the CCSP report concludes that there is no significant discrepancy between the rates of global temperature change over the past several decades at the surface compared with those higher in the atmosphere. The report does acknowledge, however, that there are still uncertainties in the tropics, related primarily to the data obtained from weather balloons. Many devel- oping countries are struggling to launch weather balloons routinely and process their measurements, and it is unclear whether scientists have been able to adjust adequately for known biases and errors in the data. Globally, data indicate that rates of temperature change have been similar throughout the atmosphere since 1979, when satellite data were first available, and that the rates of change have been slightly greater in the

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46 Potential Impacts of Climate Change on U.S. Transportation troposphere than on the earth’s surface since 1958 (when weather balloons first had adequate spatial coverage for global calculations). The global sur- face temperature time series shown in Figure 2-3 indicates warming on even longer time scales, with acceleration since 1976. Instrumental temperature measurements are not the only evidence for increasing global temperatures. The observed increased melting of glaciers can be used to estimate the rate of temperature increase since the late 19th century. Estimates of near-surface temperature based on glacial melting are very similar to estimates based on instrumental temperature data. A 15 to 20 percent reduction in Arctic sea ice since the 1970s, a 10 percent decrease in snow cover since the 1970s, and shortened periods of lake and river ice cover (about 2 weeks shorter since the 19th century) have been observed. Also, ocean heat content has significantly increased over the past several decades (IPCC 2007). 0.6 400 380 Global Temperature Anomaly (ºC) (bars) 0.4 CO2 Concentration (ppmv) (curve) 360 0.2 340 0.0 320 –0.2 300 –0.4 280 260 –0.6 1880 1900 1920 1940 1960 1980 2000 FIGURE 2-3 Globally averaged surface air temperature and carbon dioxide (CO2) concentration [parts per million by volume (ppmv)] since 1880. Note that the shaded bars refer to global temperature anomalies and the solid line to CO2 concentrations. (Source: Updated from Karl and Trenberth 2003.)

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68 Potential Impacts of Climate Change on U.S. Transportation directly driven by temperature change and to some extent by atmospheric and oceanic circulation; (b) storminess as related to wave height and storm surges; (c) precipitation and related snow and ice cover; and (d) sea level as related to land ice, ocean temperature, and movement of the land relative to the ocean due to geologic features and glacial rebound of the land as land ice melts. Generally, the extent of sea ice is important because the ice dampens the energy of ocean waves. Wave energy is dependent on the distance traveled by the wind over open water. Less extensive sea ice exposes the coastline to more frequent and potentially higher ocean waves and swells. Temperature drives the extent of sea ice, but changes in atmospheric and ocean circulation also play an important role in multiyear variations in the extent and location of sea ice. Changes in the type, amount, and intensity of precipitation, as well as the extent of snow and ice cover, can also con- tribute to coastal erosion from stream flow and overland runoff to the sea. Loss of permafrost along coasts can lead to subsidence of the land, which occurs when ice beneath the sea and along the shoreline melts. Alaska has considerable permafrost along its northern and western coasts. The height of the sea relative to the land is the ultimate long-term driver of coastal ero- sion, but Alaskan sea level rise is complicated by both climatic factors and geologic forces, affecting local and regional changes in the height of the land relative to the ocean. Atmospheric Temperature Temperatures in Alaska have increased. Observational data indicate that Alaskan spring and summer surface temperatures have increased by about 2°C to 3°C (about 4°F to 5°F) in the past few decades. However, there are no discernible trends in temperature during autumn, and changes in win- ter temperature are more complex. There were two 5-year periods in the first half of the 20th century when temperatures were nearly as warm as today, but record-breaking high temperatures have become more common during recent decades. Most climate model projections for temperature change during the 21st century suggest that Alaska, and the Arctic as a whole, will warm at least twice as much as the rest of the world. The warming is expected to be greatest during the cold half of the year. The observed lack of warming dur- ing the autumn and the relatively large increases during other times of the year are not entirely consistent with model projections; they do not depict this asymmetry.

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Understanding Climate Change 69 As temperatures increase and sea ice continues to melt, a natural cli- mate feedback occurs as a result of less reflection of sunlight by the ocean formerly covered by sea ice. This feedback can lead to accelerated warm- ing and additional sea ice melting. At present, the rate of loss of Northern Hemisphere sea ice is exceeding climate model projections, and at the present rate of loss, summer sea ice will be absent before the middle of this century. Climate models do project an acceleration of sea ice retreat over the 21st century, with periods of extensive melting lasting progres- sively further into spring and fall. All climate models project this trend to continue regardless of the emission scenario used and the sensitivity of the model. Large portions of Northern Hemisphere sea ice form during the cold seasons and melt during the warm seasons. Considerable sea ice persists through the melt season, but because of ocean circulation and the resultant ice movement, multiyear sea ice makes up only a fraction of the total ice extent. Records indicate that the formation of new sea ice each year cannot keep pace with the rate of melting, which is con- sistent with observed surface warming. Northern Hemisphere sea ice has been decreasing steadily since the 1950s, measured largely through continuous coverage provided by NOAA polar orbiting satellites begin- ning in the 1970s. Prior to that time, assessment of the extent of Northern Hemisphere sea ice during the first half of the 20th century was limited to reports from land stations and ocean surface observations. Scientists have less confidence in the data for the first part of the century, but independent anecdotal evidence, such as interviews with native peoples of Alaska, also suggests substantially greater extent of sea ice earlier in the century. It is important to understand trends in the extent of coastal sea ice because it is an important determinant of wave energy affecting coastlines. As the storms that create wave energy also exhibit strong seasonal varia- tion, it is important to know how sea ice is changing by season. Since the 1950s, the extent of sea ice during winter and autumn has decreased from 15 million square kilometers (km2) to 14 million km2 and from 12 million km2 to 11 million km2, respectively. Since the 1950s, decreases in spring and summer have been substantially greater, down from an average of 15 mil- lion km2 to 12 million km2 and 11 million km2 to 8 million km2, respectively. This is equivalent to more than 10 percent of the North American land mass and is an area larger than the state of Alaska.

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70 Potential Impacts of Climate Change on U.S. Transportation Extratropical Storms The climatology of Pacific Ocean storms favors the development of the strongest storms (extratropical cyclones) from autumn to spring. Although there are remaining uncertainties about the quality of the data, analyses of Pacific Ocean extratropical cyclones over the past 50 years indicate little change in the total number but a significant increase in the number of intense storms (those with low central pressure and resultant high winds and waves). The increase in extratropical storms is punctuated by consid- erable year-to-year variability. Both observational evidence and modeling projections support the notion that as the world warms, the intensity of cyclones in the northern Pacific (and the northern Atlantic) will increase (e.g., Lambert and Fyfe 2006; Wang et al. 2006). Even without an increase in storm intensity, the greater expanse of open water due to less extensive sea ice means that ocean waves, with resultant coastal erosion, can occur more frequently and with greater impact. Precipitation and Extent of Snow Cover One of the most difficult quantities to measure across the state of Alaska is precipitation. This is the case because of the variable nature of precipita- tion in general, the relatively low number of observing stations across the state, and the difficulty of providing high-quality data in the harsh Arctic environment. The large uncertainty in estimated precipitation trends is also due to the difficulty of measuring wind-blown solid precipitation. On the basis of existing records, however, there is evidence to indicate that during the past 40 years, as temperatures have warmed, more pre- cipitation has been falling in liquid form (rain) as opposed to solid form (snow, ice). The quantity of precipitation also increased during the 20th century, with much of that increase occurring during the recent period of warming over the past 40 years. The increase is estimated to be between 10 and 20 percent, with most of it occurring during the summer and winter rather than during the transition seasons. Because of greater overall precipitation in the summer, the percent increase in summer equates to a greater quantity of precipitation compared with winter. Analyses of changes in intense precipitation events have been conducted for areas south of 62°N latitude. They show that the frequency of intense precipitation events has increased substantially (30 to 40 percent) during the past several decades. Thus, a disproportionate amount of the precipita- tion increase is attributable to the most intense precipitation events.

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Understanding Climate Change 71 Climate models project that precipitation will increase by a greater proportion in the high latitudes compared with the rest of the world. This result is consistent from model to model, as is the fact that this increase is expected to be disproportionately larger in the more intense precipitation events. Both of these phenomena can lead to increased erosion. NOAA’s polar-orbiting environmental satellite data and surface-based observations have also revealed major changes in the extent of snow cover. The extent of North American snow cover has decreased by about 1 mil- lion km2, and this trend is expected to continue or accelerate. Surface observers also report a 1- to 2-week reduction in the number of days with snow on the ground across the state. In addition, in the Arctic, the lake and river ice season is now estimated to be 12 days shorter than in the 19th century. The increase in total and liquid precipitation, especially when falling on less extensive snow cover, can affect soil erosion. However, the complex effects of changes in precipitation type and intensity, earlier breakup of winter ice, and less extensive snow cover have not been well evaluated with respect to potential impacts on coastal erosion and flooding. It will be nec- essary to know which factor dominates in order to understand whether coastal erosion and flooding will be enhanced or ameliorated as a result of changes in the extent of precipitation and snow cover. Permafrost Thawing of the permafrost, especially along the northern coasts, is expected to continue. Long-term measurements of temperatures within the per- mafrost are rare, but it is clear that as air and ocean temperatures have warmed, permafrost has been melting. As permafrost melts along the coastlines, the effect on coastal erosion can be compounded by the retreat of sea ice. The thaw causes the land to subside along the shore, expos- ing more land to the action of the waves. The thaw also causes slumping and landslides in the interior, undermining structures built on or near permafrost. Sea Level A general increase in sea level would expose more land to coastal erosion through wave energy and storm surges. However, it is important to rec- ognize that there are many local and regional variations in sea level rise, and Alaska is no exception in this regard. Complications arise because of

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72 Potential Impacts of Climate Change on U.S. Transportation geologic forces; the rebound of the land as glaciers melt; and in some areas, local engineering projects. For certain areas in Alaska (e.g., parts of southeast Alaska), sea level is actually falling as a result of natural geologic and glacial rebound effects, but this is generally not the case in much of the state. It is clear, however, that changes in Alaskan climate are among the greatest in the world. They have likely played an important role in determining the extent of coastal erosion and flooding in the state and are likely to continue to do so in the future. Accelerated coastal erosion and flooding linked to sea level rise in Alaska cannot be ruled out. FINDINGS The state of the science continues to indicate that modern climate change is affected by human influences, primarily human-induced changes in atmospheric composition that are warming the climate. These changes result mainly from emissions of GHGs associated with energy use, but on local and regional scales, urbanization and land use changes are also important contributors to climate change. Once in the atmosphere, GHG concentrations have a long residence time—on the order of a century. Thus, they would continue to affect climate conditions even if GHG emis- sions were eliminated today, and they demand a response. Substantial progress has been made in monitoring and understanding the causes of climate change, but scientific, technical, and institutional challenges to improving projections of future climate change remain. For example, considerable uncertainty persists about the rates of climate change that can be expected during the 21st century. Nevertheless, it is clear that climate change will be increasingly manifested in important and tan- gible ways, such as changes in extremes of temperature and precipitation, decreases in seasonal and perennial snow and ice extent, and rising sea lev- els. In addition, climate models project an increase in the intensity of strong hurricanes, with an increase in related storm rainfall rates, in the 21st century. Thus, as human-induced climate changes are superimposed on the natural variability of the climate, the future will include new classes of weather and climate extremes not experienced in modern times. Climate changes will affect transportation largely through these extremes. The U.S. transportation system was built for the typical weather and climate experienced locally, including a reasonable range of extremes. If projected climate changes push environmental conditions outside the range for which the system was designed—and the scientific evidence sug-

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Understanding Climate Change 73 gests that this will be the case—the impacts will be significant. They will vary by mode of transportation and region of the country, and some will be positive; in general, however, the impacts will be widespread and costly in both human and economic terms and require significant changes in the planning, design, construction, operation, and maintenance of transporta- tion systems. In the next chapter, the likely impacts of projected climate changes on transportation are examined in detail. REFERENCES Abbreviations CCSP U.S. Climate Change Science Program IPCC Intergovernmental Panel on Climate Change NOAA National Oceanic and Atmospheric Administration NRC National Research Council Alexander, L. V., X. Zhang, T. C. Peterson, J. Caesar, B. Gleason, A. M. G. Klein Tank, M. Haylock, D. Collins, B. Trewin, F. Rahimzadeh, A. Tagipour, P. Ambenje, K. Rupa Kumar, J. Revadekar, and G. Griffiths. 2006. Global Observed Changes in Daily Climate Extremes of Temperature and Precipitation. Journal of Geophysical Research. Vol. 111, No. D5, Mar. 15. Allen, M. R. 2005. The Spectre of Liability: Part 1—Attribution. In The Finance of Climate Change: A Guide for Governments, Corporations, and Investors (K. Tang, ed.), Chapter 29, Risk Books, Haymarket, London. Alpert, P., T. Ben-Gai, A. Baharad, Y. Benjamini, D. Yekutieli, M. Colacino, L. Diodato, C. Ramis, V. Homar, R. Romero, S. Michaelides, and A. Manes. 2002. The Paradoxical Increase of Mediterranean Extreme Daily Rainfall in Spite of Decrease in Total Values. Geophysical Research Letters, Vol. 29, No. 11, p. 1536. Andreae, M. O., D. Rosenfeld, P. Artaxo, A. A. Costa, G. P. Frank, K. M. Longo, and M. A. Silva-Dias. 2004. Smoking Rain Clouds over the Amazon. Science, Vol. 303, No. 5662, pp. 1337–1342. Bachelet, D., R. P. Neilson, J. M. Lenihan, and R. J. Drapek. 2001. Climate Change Effects on Vegetation Distribution and Carbon Budget in the United States. Ecosystems, Vol. 4, pp. 164–185. Bengtsson, L., K. I. Hodges, M. Esch, N. Keenlyside, L. Kornblueh, J.-J. Luo, and T. Yamagata. 2007. How May Tropical Cyclones Change in a Warmer Climate. Tellus A, Vol. 59, No. 4, pp. 539–561. Bornstein, R., and Q. Lin. 2000. Urban Heat Islands and Summertime Convective Thunderstorms in Atlanta: Three Case Studies. Atmospheric Environment, Vol. 34, No. 3, pp. 507–516. Brooks, H. E., and N. Dotzek. 2008. The Spatial Distribution of Severe Convective Storms and Analysis of Their Secular Changes. In Climate Extremes and Society (H. Diaz and R. Murnane, eds.), Cambridge University Press.

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