FIGURE 1.1 Illustrative CO₂ emission profiles (A) and corresponding concentrations (B) derived from Wigley et al. (1996) and given in CCTP (2006). The equilibrium surface temperature change associated with steady-state concentrations is shown in red in (B). The surface warming estimates adopt the IPCC (2007a)-recommended climate sensitivity of 3°C warming due to a doubling of CO₂. In addition, they assume that aerosols from air pollution are eliminated and that other greenhouse gases are fixed at 2005 values. SOURCE: Modified from CCTP (2006).

about 440 ppm by 2030 (Figure 1.1), committing the planet to additional warming. These projections are based on estimates that CO₂ emissions in China increased at an annual rate of about 3 to 4 percent during the past 10 years (IPCC, 2007a; IEA, 2007), but a subsequent province-based inventory concluded that emissions actually increased at a higher rate of about 10 to 11 percent (Auffhammer and Carson, 2008). For comparison, total fossil fuel emissions from the United States increased by about 11 percent over the entire 10-year period.¹ Emissions from a number of other developed countries were also higher than agreed-to targets. These disparities between projected and actual emissions underscore the large uncertainties inherent in projecting CO₂ and other greenhouse gas emissions, particularly beyond a decade.

The Intergovernmental Panel on Climate Change (IPCC) projections may have been too conservative in other cases as well. For example, observed increases in surface temperatures and sea level from 1990 to 2007 were in the upper range of IPCC model predic-