Potential Impacts of Climate Change on U.S. Transportation Special Report 290 (2008) / Chapter Skim
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2 Understanding Climate Change
2 Understanding Climate Change
Pages 36-78
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From page 36... ...
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)
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. Increases in heat-trapping GHGs are projected to be amplified by feedback effects, such as changes in water vapor, snow cover, and sea ice.
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. Measurements are from a combination of ice cores (going back 10,000 years)
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, which can cause changes in temperature and precipitation extremes, reductions in snow cover and sea ice, changes in storm tracks, and increased intensity of hurricanes (IPCC 2007)
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, 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.
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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.
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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.
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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.
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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.
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Models that describe climate variability and change well can be used as a tool to increase understanding of the climate system. Once evaluated and validated, climate models can then be used for predictive purposes.
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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)
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Confidence in predicting future climate depends on using climate models to attribute past and present climate changes to specific causes. Despite these issues, a substantial and growing body of evidence (IPCC 2007)
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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)
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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.
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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)
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2004) , whereas additional atmospheric water vapor increases the risk of heavy precipitation events (Trenberth et al.
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. ºC ºC Year FIGURE 2-6 Annual surface air temperature anomaly, from the 1990–1999 average, for the eastern United States and for three different emissions scenarios (SRES = Special Report on Emission Scenarios)
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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.
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While there is considerable year-to-year variability in the number of freeze–thaw days (i.e., days when an observation station's maximum temperature is above freezing and its minimum temperature below freezing) , no distinct trend has been observed in this quantity.
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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 1961–1990 base period)
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. Estimates of local apparent sea level rise take into account the vertical movement of land and coastal erosion.
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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.
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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
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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
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3 2 Percent 1 0 –1 –2 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)
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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.
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(b) Median model-projected changes in the Simple Daily Intensity Index for the continental United States.
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. 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 summer precipitation as global temperatures increase.
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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.
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. (Source: NOAA, National Climatic Data Center.)
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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.
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the extent of sea ice, snow cover, and permafrost, all
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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.
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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 progressively further into spring and fall.
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. 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.
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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 necessary 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.
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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.
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In the next chapter, the likely impacts of projected climate changes on transportation are examined in detail. REFERENCES Abbreviations CCSP U.S.
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In The Potential Impacts of Climate Change on Transportation, Summary and Discussion Papers. Federal Research Partnership Workshop, Brookings Institution, Washington, D.C., Oct.
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In Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (S.
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2005. Tropical Storms: Representation and Diagnosis in Climate Models and the Impacts of Climate Change.
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In The Potential Impacts of Climate Change on Transportation, Summary and Discussion Papers, Federal Research Partnership Workshop, Brookings Institution, Washington, D.C., Oct.
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2007. Attribution of Observed Surface Humidity Changes to Human Influence.
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