In sharp contrast, some greenhouse gases have biogeochemical properties that lead to atmospheric retention times (lifetimes) of centuries or even millennia. These gases can accumulate in the atmosphere whenever emissions exceed the slow rate of their loss, and concentrations would remain elevated (and influence climate) for time scales of many years even in the complete absence of further emission. Like the water in a bathtub, concentrations of carbon dioxide are building up because the anthropogenic source substantially exceeds the natural net sink. Even if human emissions were to be kept constant at current levels, concentrations would still increase, just as the water in a bathtub does when the water comes in faster than it can flow out the drain. The removal of anthropogenic carbon dioxide from the atmosphere involves multiple loss mechanisms, spanning the biosphere and ocean (see Section 2.4), and carbon dioxide removal cannot be characterized by any single lifetime. Although some carbon dioxide would be lost rapidly to the terrestrial biosphere and to the shallow ocean if human emissions cease, some of the enhanced anthropogenic carbon will remain in the atmosphere for more than 1,000 years, influencing global climate (Archer and Brovkin, 2008). The warming induced by added carbon dioxide is expected to be nearly irreversible for at least 1,000 years (Matthews and Caldeira, 2008; Solomon et al., 2009), see Section 3.4.

Figure 2.1 shows that carbon dioxide is the largest driver of current anthropogenic climate change. Other gases such as methane, nitrous oxide, and halocarbons also make significant contributions to the current total CO2-equivalent concentration, while aerosols (see Section 2.3) exert an important cooling effect that offsets some of the warming. The best estimate of net total CO2 equivalent concentration of the sum across these forcing agents in the year 2005 is about 390 ppmv (with a very likely range from 305 to 430 ppmv). Global carbon dioxide emissions have been increasing at a rate of several percent per year (Raupach et al., 2007). If there were to be no efforts to mitigate its emission growth rate, scenario studies suggest that carbon dioxide could top 1,000 ppmv by the end of the 21st century. Carbon dioxide alone accounts for about 55% of the current total CO2-equivalent concentration of the sum of all greenhouse gases, and it will increase to between 75 and 85% by the end of this century based on a range of future emission scenarios (see Section 2.2). Thus carbon dioxide is the main forcing agent in all of the stabilization targets discussed here, but the contributions of other gases and aerosols to the total CO2-equivalent remain significant, motivating their consideration in analysis of stabilization issues.

How large a reduction of emissions is required to stabilize carbon dioxide concentrations, and does it depend upon when it is done or on the



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