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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 earthâs 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 assessmentsâincluding 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)âto 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 âgreenhouseâ effect is real and is an essential component of the planetâs 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 earthâs 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
Understanding Climate Change 37 absence, the earthâs 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 sunâs 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
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.)
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 climateâs 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 cloudsâ 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 planetâs 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 sunâs 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 planetâs radiation flow than all the
40 Potential Impacts of Climate Change on U.S. Transportation BOX 2-1 What Warms and Cools the Earth? The sun is the earthâs 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 earthâs orbit affect how the sunâs 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 preâMontreal 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)
Understanding Climate Change 41 Some aerosols (airborne particles and droplets) warm the earth: Black carbon particles, or âsoot,â 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 earthâs 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 earthâs 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 sunâs 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-
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 âurban heat islandâ 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 modelâs horizontal and spatial resolution, and the number of 1 The global effects of these urban heat islands have been analyzed extensively and assessed to 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.
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.)
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 âtuning.â 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 spaceâa much finer resolution than can be run on todayâs 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 cloudsâ 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 todayâs 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 todayâs 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 âweatherâ 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 earthâs 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.
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 earthâs 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
46 Potential Impacts of Climate Change on U.S. Transportation troposphere than on the earthâs 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 (ÂºC) (bars) 0.4 CO2 Concentration (ppmv) (curve) 360 0.2 340 0.0 320 â0.2 300 â0.4 280 â0.6 260 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.)
Understanding Climate Change 47 Why Do Scientists Think Humans Are Influencing the Earthâs Climate? Since the 1980s, the scientific community has been actively working on detecting climate change and determining how much of the change is attributable to human activities. As described above, one set of tools often used for detection and attribution is mathematical computer models of the climate. Outstanding issues in modeling include specifying forcing mecha- nisms (e.g., the causes of climate variability and change) within the climate system; addressing complex GHG feedback processes (e.g., methane and carbon) and properly dealing with indirect aerosol forcings and complex physical feedback processes (e.g., energy and water sources); and improv- ing simulations of regional weather, especially extreme events. Todayâs inadequate or incomplete measurements of the various forcing mecha- nisms, with the exception of well-mixed GHGs, add uncertainty when one is trying to simulate past and present climate. Confidence in predicting future climate depends on using climate models to attribute past and pres- ent climate changes to specific causes. Despite these issues, a substantial and growing body of evidence (IPCC 2007) shows that climate models are useful tools for understanding the factors leading to climate change. Recent CO2 emission trends are upward, with increases of 0.5 to 1 per- cent per year over the past few decades. Concentrations of both reflective and nonreflective aerosols are also estimated to be increasing. Net positive radiative forcings3 from GHGs dominate the net cooling forcings from aerosols, and the global temperature change over the past 25 to 30 years has exceeded the bounds of natural variability estimated from climate simula- tions with no human-caused changes. This has been the case since about 1980. As an example of how models are used to detect human influence on the climate system, Figure 2-4 shows that, without including all the known forcing mechanisms (natural and human or anthropogenic), the models cannot replicate observed global temperature changes. Moreover, many aspects of the climate system other than global surface temperatures have been tested for human influences. 3 Radiative forcing can be thought of as the change in heat flow [expressed in watts per square meter (W/m2)] at the tropopause due to an internal change or a change in the external forcing of the climate system, such as a change in the concentration of CO2 or the output of the sun. The tropopause is the boundary between the troposphere and the stratosphere, represented by a rather abrupt change from decreasing to increasing temperature with height.
48 Potential Impacts of Climate Change on U.S. Transportation Models using only natural forcings Models using both natural and Temperature Anomaly (ÂºC) 1.0 anthropogenic forcings Observations 0.5 0.0 1900 1950 2000 Year FIGURE 2-4 Comparison of observed global change in surface temperature with simulations by climate models using natural and anthropogenic forcings. Decadal averages of observations are shown for 1906 to 2005 (black line) plotted against the center of the decade and relative to the corresponding average for 1901â1950. Solid shading shows the 5 to 95 percent range for 19 simulations from five climate models using only the natural forcings due to solar activity and volcanoes. Banded shading shows the 5 to 95 percent range for 58 simulations from 14 climate models using both natural and anthropogenic forcings. (Source: IPCC 2007, Figure SPM-4, p. 18. Reprinted with permission of the IPCC Secretariat, Geneva, Switzerland.) Today there is convincing evidence from a variety of climate change detection and attribution studies pointing to human influences on climate. These studies include continental and subcontinental analyses of changes in temperature; the paleoclimatic4 temperature record; three-dimensional analyses of changes in atmospheric temperature, in free atmospheric tem- 4 Climate during periods prior to the development of measuring instruments includes historical and geological time for which only proxy climate indicators are available. A proxy climate indicator is a local record that is interpreted, on the basis of physical and biophysical principles, to represent some combination of climate-related variations back in time. Climate-related data derived in this way are referred to as proxy data. Examples of proxies are tree ring records, characteristics of corals, and various data derived from ice cores.
Understanding Climate Change 49 perature, in sea ice extent and other components of the cryosphere, and in ocean heat content; and new studies on extreme weather and climate events. Thus, there is high confidence that the observed warming, espe- cially during the period since the 1970s, is due mainly to human-caused increases in GHGs (Allen 2005; Gillett et al. 2002; Hegerl et al. 2001; IPCC 2007; Karl et al. 2006; Karoly and Wu 2005; Stone and Allen 2005; Stott et al. 2001; Tett et al. 2002; Zhang et al. 2006; Zwiers and Zhang 2003). How climate warming will be manifested over the next 50 to 100 years and which factors will have the greatest potential impact on transportation are discussed in the following section. CLIMATE CHANGES RELEVANT TO U.S. TRANSPORTATION Climate variability and change impact transportation mainly through changes in weather extremes, such as very hot days, very cold days, or severe storms; changes in climate extremes,5 such as increases in the probability of intense precipitation events and extended droughts; and sea level rise. The U.S. transportation system was built for the typical weather and climate experienced locally, including a reasonable range of extremes, such as flood- ing events occurring as rarely as once in 100 years. Moderate changes in the mean climate have little impact on transportation infrastructure or opera- tions because the system is designed to accommodate changing weather conditions. However, changes in weather and climate extremes can have a considerable impact on transportation, especially if they push environ- mental conditions outside the range for which the system was designed. Weather and climate extremes of relevance to transportation have been changing over the past several decades and are projected to continue to change in the future, with both negative and positive effects on the trans- portation system. Table 2-1 lists the potential climate changes of greatest relevance for transportation, including the level of uncertainty associated with each. The following subsections address these changes in turn, largely summa- rizing the findings of a paper commissioned for this study (by Peterson 5The exact threshold for what is classified as an extreme varies from one analysis to another, but an extreme event would normally be as rare as, or rarer than, the top or bottom 10 percent of all occurrences (CCSP 2007). For the purposes of this report, all tornadoes and hurricanes are considered extreme.
50 Potential Impacts of Climate Change on U.S. Transportation TABLE 2-1 Level of Uncertainty Associated with Potential Climate Changes of Greatest Relevance to Transportation Potential Climate Change of Relevance to U.S. Transportation Level of Uncertainty Temperature Increases in very hot days and heat waves Very likely Decreases in very cold days Virtually certain Increases in Arctic temperatures Virtually certain Later onset of seasonal freeze and earlier onset of seasonal thaw Virtually certain Sea level rise Virtually certain Precipitation Increases in intense precipitation events Very likely Increases in drought conditions for some regions Likely Changes in seasonal precipitation and flooding patterns Likely Storms Increases in hurricane intensity Likely Increased intensity of cold-season storms, with increases in Likely winds and in waves and storm surges Note: Italicized uncertainty designations are those identified by IPCC (2007). Others reflect the committeeâs judgment, based on the available literature. IPCC (2007, 3) Working Group I established the following terminology to describe uncertainty, that is, probability of occurrence: virtually certain, â¥99 percent; extremely likely, â¥95 percent; very likely, â¥90 percent; likely, â¥66 percent; more likely than not, â¥50 percent; unlikely, â¤33 percent; very unlikely, â¤10 percent; extremely unlikely, â¤5 percent. et al. 2006; see Appendix C). Each subsection highlights past trends, future projections, and key uncertainties. (The reader is referred to the paper by Peterson et al. 2006 for more detail and additional figures to support the discussion.) Note that the discussion generally progresses from those climate changes about which there is most certainty to those about which there is less. Changes in Temperature An increase in air temperature allows more water vapor in the atmosphere, which defines the upper bounds of the amount of precipitation that can occur during short-term (e.g., hourly to 1-day) extreme precipitation events. Surface moisture, if available (as it always is over the oceans), effectively acts as the âair conditionerâ of the surface, as heat used for evaporation moistens rather than warms the air. Therefore, another consequence of global heat- ing of the lower troposphere is accelerated land-surface drying and more
Understanding Climate Change 51 atmospheric water vapor (the dominant GHG). Human-induced warming has been linked to the water vapor increases in both surface observations (Willett et al. 2007) and satellite observations over the oceans (Santer et al. 2007). Without an increase in precipitation, accelerated drying increases the incidence and severity of droughts (Dai et al. 2004), whereas additional atmospheric water vapor increases the risk of heavy precipitation events (Trenberth et al. 2003). Increases in global temperature also cause sea surface temperatures to rise, one of several important factors affecting hurricane intensity. Changes in Temperature Including Extremes U.S. temperatures have been rising over the past century, with more rapid increases since 1970 than earlier, as shown in Figure 2-5. It is unlikely that North American temperature changes since 1950 are due to natural cli- mate variability alone (Karoly et al. 2003). The warming has not been uniform across the continent. In general, the western portion of the 56.0 13.0 55.0 54.0 12.0 53.0 ÂºC ÂºF 52.0 11.0 51.0 Yearly Values 50.0 10.0 Filtered Values Long-Term Mean 49.0 1900 1920 1940 1960 1980 2000 Year FIGURE 2-5 Area-averaged mean temperature time series for the contiguous United States. (Source: National Oceanic and Atmospheric Administration, National Climatic Data Center.)
52 Potential Impacts of Climate Change on U.S. Transportation contiguous United States has warmed more than the eastern portion. Alaska has warmed the most rapidly, with temperatures in some regions increasing by more than 0.6Â°C (1.1Â°F) per decade since 1970. These warming trends are projected to continue over the next cen- tury on the basis of reasonable scenarios for future GHG emissions. Figure 2-6 shows the temperatures projected for the eastern United States for three different scenarios, each scenario having been run by multiple models. Other areas of the United States show similar warm- ing trends [see Figure 6 in the commissioned paper by Peterson et al. 2006 (Appendix C)]. It is interesting to note that for the next 30 years, the uncertainties are primarily model related and not due to different emis- sions scenarios. Even if atmospheric concentrations remained at current levels, the models would still project similar warming over the next couple of decades (IPCC 2007). ÂºC ÂºC Year FIGURE 2-6 Annual surface air temperature anomaly, from the 1990â1999 average, for the eastern United States and for three different emissions scenarios (SRES = Special Report on Emission Scenarios). (Source: Peterson et al. 2006, Figure 5.)
Understanding Climate Change 53 Increases in Very Hot Days and Heat Waves The next century is likely to bring more very hot days and heat waves [see Figure 9 in the paper by Peterson et al. 2006 (Appendix C)]. The number of days with temperature above 32.2Â°C (90Â°F) and 37.7Â°C (100Â°F) has been increasing since 1970, but it is not quite as high today as during the early 1950s, when several areas, particularly the south-central United States, had severe droughts. By 2090â2099, it is expected that the average temper- ature on the hottest day of the year will be 2.5Â°C to 4.5Â°C (4.5Â°F to 8.1Â°F) warmer than the hottest day of the year in the 1990s. Not only will there be hotter and more very hot days, but it is likely that the continental United States will have significantly more heat waves with sustained high temperatures for 5 consecutive days or longer. There are several ways to conceptualize the change in very hot days. For example, the 20-year return value for the hottest day of the year in 2090â2099 can be compared with the same value for the 1990s. The 20-year return value is the temperature that is reached or exceeded on average once every 20 years over a long period of time. Such temperatures are truly rare events because they are expected to be reached only three or four times during the course of a human lifetime. Over most of the con- tinental United States, the present-day 20-year return value temperatures would be reached or exceeded seven times or more in a 20-year interval by the end of the 21st century. Hence, the rare high-temperature event be- comes commonplace in this scenario. Figure 2-7 depicts another way of considering the change in very hot days expected in the next century, with Dallas, Texas, as an example. The figure shows the probability of having 1 to 20 days during the summer when the temperature exceeds 43.3Â°C (110Â°F). The probability increases substantially 25, 50, and 90 years in the future. Similar plots are presented for Minneapolis, Minnesota, and Honolulu in the paper by Peterson et al. 2006 (see Appendix C). Decreases in Very Cold Days The number of very cold days has been decreasing in the United States since about 1970 (see Figure 2-8). This trend is also expected to continue into the future across the continent. For example, in the Washington, D.C., area, there is currently a 75 percent chance that 3 days each winter will have maximum temperatures at or below freezing. By the end of the cen- tury, this probability is projected to drop to 20 percent.
54 Potential Impacts of Climate Change on U.S. Transportation 100 Current Year: 2007 + 25 Years: 2032 80 + 50 Years: 2057 + 90 Years: 2097 60 Probability 40 20 0 0 2 4 6 8 10 12 14 16 18 20 Number of Days FIGURE 2-7 Current and future probability of having 1 to 20 days during the summer at or above 43.3ØC (110ØF) in Dallas, Texas. (Source: Peterson et al. 2006, Figure 10b.) Later Onset of Seasonal Freeze and Earlier Onset of Seasonal Thaw It is not just extremes of temperature that can have an impact on trans- portation. In particular, the number of days from the last freeze in the spring to the first freeze in the fall is expected to increase. Figure 2-9 shows a corresponding periodâthe length of time between the first day in the year that the maximum daily temperature reaches 21.1Â°C (70Â°F) and the last day of the year when this occurs. This interval has been increasing since 1970 and can be expected to increase further in the future. While there is considerable year-to-year variability in the number of freezeâthaw days (i.e., days when an observation stationâs maximum temperature is above freezing and its minimum temperature below freezing), no distinct trend has been observed in this quantity. Changes in Sea Level Sea level is projected to rise over the next century, but there is significant uncertainty as to how much and how fast. The IPCC Third Assessment Report includes a range of estimates that sea levels will rise 0.1 to 0.9 m above 1990 levels by 2100 (IPCC 2001). To put this in context, the IPCC
10 Maximum Temperature Anomaly of Number of Very Cold Days Minimum Temperature 8 6 4 2 0 -2 -4 -6 -8 1950 1960 1970 1980 1990 2000 Year FIGURE 2-8 U.S. nationally averaged anomaly of the number of days at or below the coldest 10 percent of January maximum and minimum temperatures at each station (percentiles were calculated on a 1961â1990 base period). (Source: Peterson et al. 2006, Figure 14.) 30 20 Number of Days 10 0 -10 -20 1950 1960 1970 1980 1990 2000 Year FIGURE 2-9 U.S. area-averaged anomaly of the length of time between the first day above 21.1ØC (70ØF) in the spring and the last day above 21.1ØC in the fall. (Source: Peterson et al. 2006, Figure 19.)
56 Potential Impacts of Climate Change on U.S. Transportation estimates that during the past 6,000 years, global average sea level varia- tions on time scales of a few hundred years and longer are likely to have been less than 0.3 to 0.5 m. Observed sea level changes from tide gauges and satellite altimeters indicate that the 1993â2005 rate of sea level rise was 3 mm per year (Church and White 2006). If this linear trend contin- ues, sea level will rise by about 0.3 m by the end of the 21st century. Several analyses have identified a number of factors that as yet have uncertain likelihoods but could easily contribute to nonlinear and abrupt rises in sea level (IPCC 2007; Schoof 2007; Vaughan et al. 2007). Such extrapolations are tentative, however, because the extent to which the trends of the past decade are due to natural variability in the climate sys- tem is unknown. Global warming affects sea level through two mechanisms: thermal expansion of seawater and melting of ice present on land surfaces. Other factors also play a role in sea level, such as the amount of water held back by human-made land reservoirs, leading to sea level falls, but these factors are less important. There are still problems in reconciling the observed changes of the past century with the estimated contribu- tions from these different sources (Munk 2002). Most of the projected sea level rise is due to thermal expansion, but should the melting of the polar ice caps accelerate, sea level would rise much higher. The rapid melting of Greenland, which would have a very large impact, is possi- ble, but too little is known to assess its likelihood (IPCC 2007). Current model projections of sea level rise are based on the observed rate of melting during 1993â2003, but these rates could increase or decrease in the future. More important to transportation than the global change in sea level is the local apparent change in sea level (Burkett 2002; Titus 2002). Estimates of local apparent sea level rise take into account the vertical movement of land and coastal erosion. Coastal erosion, in turn, is driven by sea level rise. To estimate local sea level rise, land subsidence in the Gulf Coast and uplift along the New England coast are important factors (NRC 1987). Figure 2-10 illustrates that because of these factors, different regions can have quite different local sea level rise. Impacts of Sea Level Rise on Shoreline Location Predicting rates of shoreline retreat and land loss is critical to planning future coastal infrastructure. According to the Bruun rule, shorelines
Understanding Climate Change 57 Galveston, TX 40 Scale (cm) New York, NY 20 0 Baltimore, MD Key West, FL San Francisco, CA Sitka, AK 1900 1920 1940 1960 1980 2000 Year FIGURE 2-10 Trends in sea level from global changes in seawater volume and local changes in land surface elevation for representative locations in the United States. (Source: NOAA 2001, p. 4.)
58 Potential Impacts of Climate Change on U.S. Transportation retreat so as to maintain a constant slope, and by some estimates, move inland roughly 150 m for every meter rise in sea level (Bruun 1962; Leatherman et al. 2000). Thus, for a 0.5-m worldwide sea level rise, sandy shores could retreat 75 m. Although the Bruun rule is useful as a conceptual model, rigorous application of coastal geology and climatol- ogy models is necessary for risk analysis at specific locations. Exacerbation of Storm Surge by Sea Level Rise Storm surge is the abnormal rise in sea level accompanying a hurricane or other intense storm, above the level of the normal or astronomic tide. Storm surge can be exacerbated by tidal piling, a phenomenon of abnor- mally high water levels from successive incoming tides that do not completely drain because of strong winds or waves persisting through successive tide cycles. Flooding due to coastal storms results from a com- bination of storm surge and intense precipitation. Storm surge is of great concern to port operations, mooring facilities, and moored vessels, as well as to coastal infrastructure that is vulnerable to flooding. Storm surge has been estimated or modeled by using the United States Army Corps of Engineersâ Waterways Experiment Station model; the National Weather Serviceâs Sea, Lake, and Overland Surge from Hur- ricanes model; and more recently the Advanced Circulation Model (ADCIRC) (Westerink et al. 1994). These models use wind fields from past storms as input; these historical input data are updated infrequently. When updated, these models show wider areas of 100-year floodplains. For example, a recent analysis with the ADCIRC model using input data through the 2005 hurricane season showed greater storm surge and higher flooding. The magnitude of the 100-year storm surge flood (previously established using data for 1900â1956) would now recur at an interval of 75 years on the basis of data for 1900â2005 (Levinson 2006). Changes in Precipitation Changes in the Intensity of Heavy and Extreme Precipitation Basic theory and climate model simulations as well as empirical evidence (see Figure 2-11) confirm that warmer climates, owing to increased water vapor, lead to more intense precipitation events even when total precipi- tation remains constant, with prospects for even stronger events when precipitation amounts increase. Figure 2-12 depicts the aggregate land-
Understanding Climate Change 59 FIGURE 2-11 Diagram showing that warmer climates have a higher percentage of total rainfall coming from heavy and very heavy events. The data are based on a worldwide distribution of observing stations, each with the same seasonal mean precipitation amount of 230 (Â±5) mm. In cool climates, there are more daily precipitation events than in warmer climates (adapted from Karl and Trenberth 2003). The various cloud and rain symbols reflect the different daily precipitation rates and are categorized in the top panel of the figure to reflect the approximate proportion of the different rates for cool, moderate, and warm climates across the globe. surface worldwide changes in intense precipitation events over the last half of the 20th century, with an associated geographic depiction of where changes in intense precipitation have occurred; most areas show increases. Worldwide, an increase of a few percent in intense precipitation events is evident since the middle of the 20th century, particularly in the middle and
60 Potential Impacts of Climate Change on U.S. Transportation 90N 45N 0 45S 90W 0 90E 180 â4 â2 0 2 4 (a) 3 2 Percent 1 0 â1 â2 1950 1960 1970 1980 1990 2000 Percent Change per Decade (b) FIGURE 2-12 Trends in the contribution to total annual precipitation from very wet days (95th percentile) in percent per decade: (a) regional changes, with stippled areas not reporting; and (b) worldwide changes in areas with adequate data. Percentiles were calculated on the basis of 1961â1990 data. [Source: Alexander et al. 2006. (Copyright 2006 by American Geophysical Union. Reproduced with permission of American Geophysical Union in the format Other book via Copyright Clearance Center.)]
Understanding Climate Change 61 high latitudes. This leads to more frequent events that are currently rare. For example, by the end of the 21st century, a conservative projection of climate change has the recurrence period (or average expected waiting time) for the current 1-in-20-year, heaviest daily precipitation event reduc- ing to every 6 to 8 years over much of North America (Kharin et al. 2007; Wehner 2005). The practical implications of addressing these changes are seen in the National Oceanic and Atmospheric Administrationâs (NOAAâs) recent update of the Ohio River Basinâs 100-year daily precipitation return period. These data are used to help set engineering design standards related to excessive rainfall. Over the past several decades, increases in the amount of precipitation occurring during the heaviest daily precipitation events have been observed in many areas of the central and eastern United States (Groisman et al. 2004; Groisman et al. 2005; Karl and Knight 1998). In fact, over the 20th century, annual precipitation averaged across the United States increased by about 7 percent, but very intense precipitation events (above the 95th percentile) increased by nearly three times that rate (20 percent). The observed behavior supports one of the most con- fident projections that scientists can make about future precipitation. Considerable analysis has shown that because water vapor has increased in the atmosphere and will continue to do so with added anthropogenic GHG emissions, the intensity of precipitation will continue to increase in much of the United States (and elsewhere). In many regions of the world, increases in extreme precipitation are occurring even when total precipi- tation is relatively constant (Alpert et al. 2002; Groisman et al. 2003; Groisman et al. 2005). In areas where overall precipitation increases, the increase in the intensity of very heavy precipitation events will be even greater. There are several different ways to think about how the increase in the intensity of heavy and extreme precipitation events might be manifested. One option is to consider changes in a 20-year return event. In the A1b emissions scenario, the present-day 20-year precipitation event would take place 2 to 4 times as frequently by the end of the 21st century [see Figure 27c and text in Peterson et al. 2006 (Appendix C) for an explanation of the emis- sions scenarios]. Another useful measure is the Simple Daily Intensity Index, which equals the total annual precipitation divided by the number of days with precipitation in that year. Figure 2-13a shows that this quantity has increased over the United States, indicating that on days that precipitation
62 Potential Impacts of Climate Change on U.S. Transportation 0.6 0.4 0.2 Anomaly in mm 0 -0.2 -0.4 1950 1960 1970 1980 1990 2000 Year (a) SRES A2 SRES A1B 0.5 SRES B1 0.5 0.4 0.4 mm/day 0.3 0.3 0.2 0.2 0.1 0.1 0 0 2000 2020 2040 2060 2080 2100 Year (b) FIGURE 2-13 (a) Upward trend in the Simple Daily Intensity Index (i.e., total precipitation per year divided by the number of days with precipitation) indicating that, on a U.S. area-averaged basis, when precipitation does occur, it tends to be heavier. (b) Median model-projected changes in the Simple Daily Intensity Index for the continental United States. (Source: Peterson et al. 2006, Figures 28a, 28b.)
Understanding Climate Change 63 does occur, the amount is becoming greater. Median model projections for the future over the continental United States (Figure 2-13b) indicate that the Simple Daily Intensity Index is expected to continue to increase over the next century. Changes in the Severity and Frequency of Drought Drought is a recurring feature of the climate system; major droughts have occurred in the past and are expected in the future. At any given time, at least part of the United States is in drought, with proportions ranging from 5 to 80 percent of the nationâs total land area. U.S. droughts show pro- nounced multiyear to multidecadal variability, but there is no convincing evidence for systematic long-term trends toward more or fewer events. Drought calculations have shown that over the United States, the increase in temperatures that may have led to increased evaporation has been com- pensated by a general increase in precipitation during the past few decades (Dai et al. 2004), with the result that there has been no general trend in drought intensity nationwide (Figure 2-14). Over the United States, climate model projections of precipitation change by the end of the 21st century show a tendency for increasing winter precipitation and decreasing sum- mer precipitation as global temperatures increase. Locations that do experience decreased precipitation in addition to the continuing increase in temperatures, such as the recently observed record-high JanuaryâJune of 2006 (NOAA 2006), could have greater drought severity and frequency, especially during periods of dry weather due to increases in evaporation. Long-term warming trends have already led to changes in the timing of snowmelt and stream flows, especially in the West, resulting in earlier peak stream flows and diminished summertime flows. For the continental United States, the most extensive drought in the modern observational record occurred from 1933 to 1938. In July 1934, 80 percent of the United States was gripped by moderate or greater drought (see Figure 2-14), and 63 percent was experiencing severe to extreme drought. During 1953â1957, severe drought covered up to 50 percent of the country. Paleoclimatic data (e.g., tree ring measurements) have been used to reconstruct drought patterns for the period prior to the modern instru- mental record (Cook et al. 1999; Cook et al. 2004). These reconstructions show that during most of the past two millennia, the climate of the west- ern United States has been more arid than at present. The recent intense western drought from 1999 to 2004 that strongly affected the Colorado
64 Potential Impacts of Climate Change on U.S. Transportation 1930s Dust Bowl 1950s Mini-Dust Bowl Moderate - Extreme Drought 60 Percent Area Dry 40 20 0 1900 1920 1940 1960 1980 2000 Year FIGURE 2-14 Percentage of the contiguous U.S. land area in moderate to severe drought, January 1900âMarch 2006, based on the Palmer Drought Index. (Source: NOAA, National Climatic Data Center.) River basin was exceeded in severity as recently as the 19th century. Within the past millennium, severe droughts in both the western United States and the Midwest have occurred that have lasted for multiple decades. One of the more robust findings of the IPCC (2007) relates to recent agreement among virtually all climate model simulations of the 21st cen- tury that a drying of the southwestern United States is evident. Seager et al. (2007) provide the details and indicate that this drying is attribut- able to both an increase in evapotranspiration and reduced precipitation. Droughts in this part of the country that occur naturally, such as those of the past two millennia, would be expected to be enhanced as a result of greenhouse forcing of the climate. Increased temperatures will lead to increased drying during periods of dry weather, leading to more intense droughts in much of the United States. For the southwestern United States, reduced precipitation will add to this effect.
Understanding Climate Change 65 Changes in Storms Changes in Hurricane Intensity and Frequency Tropical storms, particularly hurricanes, are an important issue of concern for the United States. The record-breaking hurricane season of 2005, espe- cially the havoc created by Katrina, raised public awareness of the dangers of hurricanes to new heights. Hurricanes respond to a number of environ- mental factors, including ocean temperatures, atmospheric stability, wind changes, El NiÃ±o, and others. One important question is whether hurricane activity has changed over the past 100 years. Since 1995, Atlantic hurricane activity has increased substantially, with more and more intense hurri- canes, compared with the previous two decades, and this increased level of activity is also reflected in those hurricanes striking the United States (see Figure 2-15). Earlier periods, however, such as 1945â1970, were nearly as active. Number of Landfalling Hurricanes Pentad FIGURE 2-15 Number of hurricanes striking the United States, 1901â2005, summed by 5-year periods (e.g., 1901â1905, 1906â1910). The black bars represent the number of major hurricanes (Category 3â5) and the gray bars the number of weaker Category 1 and 2 hurricanes per 5-year period (pentad). (Source: NOAA, National Climatic Data Center.)
66 Potential Impacts of Climate Change on U.S. Transportation An important consideration with regard to hurricane intensity is the trend toward warmer sea surface temperatures, particularly in the tropical Atlantic and the Gulf of Mexico, indicating that climate change may play some role in increased hurricane intensity (Emanuel 2005; Webster et al. 2005). Another factor is a slow cycle of natural fluctuations in atmo- spheric conditions and ocean temperatures in the North Atlantic, referred to as the Atlantic Multidecadal Oscillation, which is currently in a warm ocean temperature phase. What does the future hold for hurricane activity? In the near term, it is expected that favorable conditions for Atlantic hurricanes will persist for the next decade or so on the basis of previously active periods. For the longer term, climate models project an increase in the intensity of strong hurricanes in the 21st century (Bengtsson et al. 2007; McDonald et al. 2005; Oouchi et al. 2006; Sugi et al. 2002). Specifically, this translates to increases in wind speed and about a half-category increase in intensity on the com- monly used Saffir-Simpson Hurricane Intensity Scale as tropical sea surface temperatures increase by nearly 2Â°C. Given these conditions (stronger hurricanes and warmer tropical sea surface temperatures), climate mod- els also predict an increase in storm rainfall rates of about 20 percent (T. R. Knutson, personal communication, 2006). No robust projections concerning the annual global number of tropical storms have yet emerged from modeling studies, but more detailed analyses focused on the Atlantic Ocean suggest no significant increases in the annual number of Atlantic tropical storms (CCSP 2007). Many relevant factors, such as future changes in wind field patterns, remain very difficult to predict. Extratropical Storms The IPCC AR4 models projected a reduction in the total number of mid- latitude cyclones and an increase in the number of intense storms. This is a robust result, yielded by essentially all the models (Lambert and Fyfe 2006). Associated with these changes is an increase in ocean wave height in the northeastern Atlantic and the northern Pacific (Wang et al. 2004). Analysis of the conditions that cause thunderstorm systems in the United States to produce hail results in a time series fairly similar to the U.S. tem- perature time series shown in Figure 2-5, decreasing from 1950 to the 1970s and then increasing (Brooks and Dotzek 2008). Projecting these conditions into the future is difficult because contemporary climate mod- els lack sufficient resolution to simulate these storms directly.
Understanding Climate Change 67 Visibility Some of the climate changes discussed earlier in this chapter may have sec- ondary effects. Visibility is one such effect. For example, if the number of intense extratropical storms increases, they may be accompanied by more time with low visibility during heavy snowfall events (Rasmussen et al. 1999). Projections of drying in the interior of continents would imply the possibility of increases in blowing dust. The risk of forest wildfires in the American West is strongly associated with increased spring and summer temperatures and an earlier spring melt (Westerling et al. 2006) as well as possible biomass increases from increased precipitation (e.g., Bachelet et al. 2001; Lenihan et al. 2003). These are exactly the conditions being projected by models for the future. Therefore, wildfire-induced decreases in visibility are likely to become more frequent. It is uncertain from a theoretical stand- point how the occurrence of fog might change in a warming climate. Therefore, while the number of low-visibility events associated with intense storms and fires might be projected to increase, it is uncertain whether the total number of occurrences of low visibility would increase, decrease, or remain the same in the projected climate of the future. Transportation is significantly affected when visibility drops to less than about 400 m (0.25 mi). Times with such low visibility are associated primarily with fog, heavy precipitation, blowing sand or snow, or smoke from wildfires. Observations of past trends in visibility and models of changes projected for the future are not currently available. While visibil- ity is observed at airports throughout the United States, changes in observing practices over the past decades make it inappropriate to exam- ine long-term changes in low visibility without a major effort to assess the dataâs homogeneity. Climate Considerations Related to Alaska The Alaskan Arctic and sub-Arctic are recognized as the area of the world where changes to the climate are likely to be among the greatest, leading to significant impacts. In addition, the area has always experienced great natu- ral climatic variability. Because the climate changes in Alaska are so distinct from those in the rest of the United States, this section is devoted to an examination of these expected changes as they relate to transportation. Climate variables of particular relevance to the transportation sector in Alaska include (a) the extent of sea ice, snow cover, and permafrost, all
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 2Â°C to 3Â°C (about 4Â°F to 5Â°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.
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.
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 62Â°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.
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. NOAAâs 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
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 timeâon 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 designedâand the scientific evidence sug-
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