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7 CHAPTER two OUR CHANGING CLIMATE WHAT IS CLIMATE CHANGE? prevalent in the atmosphere. Other types of GHGs are more potent, though less common, than CO2. These include meth- Climate models predict that the global climate will shift ane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFCs), in a number of ways over the next century. By 2100, we perfluorocarbons (PFCs), and sulfur hexafluoride (SF6). are likely to see global average sea levels higher by 7 to 23 inches. Global average temperatures are expected to Climate scientists predict that global GHG emissions will rise by between 3.2°F and 7.2°F (2). Rainfall patterns are have to be reduced by 50% to 80% below 1990 levels by the likely to change, with some parts of the world becoming year 2050 to avoid the most catastrophic impacts associated wetter, especially during the winter months, and other parts with global temperature rise (3). An increasing number of becoming hotter and drier. The frequency and severity of nongovernmental organizations and U.S. states are now call- heat waves and storms may increase. Rising temperatures ing for this scale of reduction in emissions. and higher sea levels, the result of warming oceans and melting ice caps, are already observable in some areas over the last century. During the 20th century, global sea levels GREENHOUSE GAS EMISSIONS FROM PASSENGER TRAVEL rose about 5 to 9 in., and global average temperatures rose by about 1.4°F (2). In the United States, transportation is a leading source of the These phenomena are collectively known as climate GHG emissions that contribute to climate change. Figures change. Most climate scientists now agree that increases 2 and 3 show the relationship of transportation GHG emis- in global concentrations of GHGs, largely attributable to sions to other emissions sources. On-road transportation humans, are the predominant cause of climate change. accounts for more than a quarter of the United States' 7,150 Human activities, such as driving cars, producing and con- million metric tons of CO2 equivalent (MMtCO2e) annual suming energy, and clearing forests, are significant con- GHG emissions. Passenger travel in light-duty vehicles [cars, tributors to GHG emissions. The principal source of GHG sports utility vehicles (SUVs), and pickup trucks] accounts emissions from human activities is the combustion of fossil- for nearly two-thirds of U.S. transportation emissions. The based fuels, including oil, coal, and natural gas. remaining transportation emissions come from freight trucks and transportation by air and other modes. Public Climate change may have potentially catastrophic effects transportation also produces GHG emissions from buses, on both the natural and human environments as it dis- trains, and other transit vehicles, but these modes account rupts ecosystems and threatens buildings, infrastructure, for less than 1% of total emissions from the U.S. transporta- and human health. Expected shifts in climate may reduce tion sector, as calculated from 2005 U.S. transit emissions as crop yields, increase the risk of invasive species, exacer- estimated in Davis and Hale (4). [Total U.S. transportation bate drought conditions, and threaten endangered species. CO2 emissions in 2005 were 1,882 MMtCO2e, as reported The built environment is also at risk. Human settlements in by the EPA (5).] Transportation is also the fastest growing coastal and low-lying areas are particularly vulnerable to source of GHG emissions in the United States. From 1990 changes in sea level and to storm and precipitation events. to 2006, transportation emissions grew by 25%, although These areas will almost certainly be at higher risk from emissions have declined slightly since 2005. flooding as the climate changes. Transportation infrastruc- ture in particular will be threatened by shifts in the global Passenger travel in cars, SUVs, and pickup trucks alone climate. Changes in temperatures, precipitation, and water accounts for about 18% of total U.S. GHG emissions. Ameri- levels threaten to strain asphalt roadways, railroads, air- cans use cars for the majority of trips to work, school, shop- ports, and shipping lanes beyond the design conditions they ping, and entertainment destinations, often driving alone. were built to withstand. Public transportation provides a lower-emitting alternative to car-based travel. GHG emissions per passenger mile are A number of GHGs contribute to global climate change. Of often substantially lower for public transportation than for these, carbon dioxide (CO2) is the most important and the most private vehicles.
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8 FIGURE 2 U.S. GHG emissions by source, 2007. (Source : U.S. Environmental Protection Agency, Inventory of Greenhouse Gas Emissions and Sinks: 19902007, Apr. 2009). Note : "Other" includes rail, ships and boats, pipelines, and lubricants. FIGURE 3 U.S. GHG emissions by economic sector, 19902007 (with electricity distributed to end-use sectors) (Source : U.S. Environmental Protection Agency, Inventory of Greenhouse Gas Emissions and Sinks: 19902007, Apr. 2009). Figure 4 shows average emissions per passenger mile of U.S. transit services versus a single-occupancy vehicle (SOV). Transit emissions from each mode are lower than SOV emissions, because transit vehicles carry multiple pas- sengers at once. The relative GHG efficiency of transit vehi- cles is based on transit's higher occupancy rates. CO2 accounts for the vast majority of GHG emissions from transportation, making up approximately 95% of all GHG emissions from on-road and off-road vehicles. CO2 emitted from the tailpipes of vehicles is directly proportional to the amount of gasoline or diesel fuel consumed. These petro- leum-based fuels contain large amounts of carbon, which, when combusted, combines with oxygen in the atmosphere FIGURE 4 National average GHG emissions per passenger to form CO2. Vehicles also emit small amounts of CH4 and mile by mode (Source : Hodges, Public Transportation's Role in Responding to Climate Change, Federal Transit Administration, N2O from their tailpipes. Emissions of these gases depend on U.S. Department of Transportation, Jan. 2009.) the specific fuel and vehicle technologies, and on operating
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9 conditions. Vehicles can also emit trace amounts of other CO2-equivalents (CO2e). CO2 equivalent measures of other GHGs, including HFCs and PFCs from air conditioning and greenhouse gases take into account the potency, or global refrigerated units and SF6 from electrical equipment. warming potential (GWP) of each gas. Table 10 in chapter five lists the potency of each gas. Emissions reported in this CO2 emissions are also the most easily estimated of GHGs. synthesis are provided in tons of CO2 or, if other gases are Discussion and analyses of GHG emissions from transporta- included in the figure, tons of CO2e. The term "GHG emis- tion are often limited to CO2 emissions. When other gases sions" refers to any or all GHGs. are included in calculations, they can be represented by