Trends in US Automotive CO Emissions

The total CO2 emissions, as well as the average emission rates of all vehicles in each automaker's fleet, have continued to rise despite notable changes in factors thought to influence emissions. In particular, the past five years saw much higher gasoline prices than the period from 1990 to 1998 as well as notable developments in technology, such as the introduction of hybrid electric vehicles (HEVs). Examining aggregate emissions trends shows that annual sales of even a million HEVs—which some analysts foresee as early as 2010—would not suffice to offset even half the increase in CO2 emissions and oil consumption observed in the auto market between 1990 and 2003.

The new fleet average CO2 emission rate per vehicle is shown as the thicker line in Figure 5-1. It had been rising prior to the 1973 oil embargo,

Co2 Emission Cars Statistics
FIGURE 5-1. Average CO2 emissions rates of U.S. light-duty vehicles, new fleet and on-road stock. Source: DeCicco et al. (2005), as derived from U.S. government statistics. Notes: CO2 emissions measured by metric tons. One metric ton equals 1.102 short tons.

reaching a peak of 8.6 metric tons of CO2 per year (TCO2/yr) in 1973 and 1974. It then plummeted as fuel economy rose in response to gas lines, high fuel prices, general fears of fuel shortages, and the imposition of CAFE standards. The subsequent fuel economy decline due to the shift from cars to trucks pushed the new fleet average CO2 emissions rate slowly upward from its historical low of 4.8 TCO2/yr in 1987 and 1988. The thinner line in Figure 5-1 shows the average emissions rate of the total LDV stock, which lags that of new vehicles due to stock turnover. The stock average CO2 emissions rate continued to decline into the 1990s, but subsequently has stagnated at about 5.3 TCO2/yr.

Figure 5-2 shows the growth of U.S. automotive CO2 emissions along with a key factor behind rising emissions—namely, growth in light-duty vehicle miles traveled (VMT). As shown by the thicker line, total LDV emissions reached 317 million metric tons of carbon per year (MMTc) in 2003. This CO2 emissions level is equivalent to 8.6 million barrels of gasoline consumption per day, or 132 billion gallons per year. The 2003 level represents a net growth of 64 percent since 1970 and a 25 percent increase since 1990, a common base year for climate policy. Nevertheless, as Figure 5-2 shows, this growth in carbon emissions is much less than the 160 percent jump in VMT from 1970 to 2003. These trends illustrate how a decrease in

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FIGURE 5-2. Vehicle miles of travel and total CO2 emissions of light-duty vehicles in the United States. Source: DeCicco et al. (2005), as derived from U.S. government statistics.

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FIGURE 5-2. Vehicle miles of travel and total CO2 emissions of light-duty vehicles in the United States. Source: DeCicco et al. (2005), as derived from U.S. government statistics.

CO2 emissions rates, itself driven by the increase in fuel economy following the 1970s oil shocks, can at least partly offset the effects of increased driving.

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