Figure 2.7 shows the CO2-equivalent concentrations for these two scenarios at three points in time. Notice that in the case of the tighter radiative forcing goal, there is some “overshoot”. That is, the target is exceeded in the middle part of the century and then gradually approached. This is due to the assumption that there will be a “negative” emitting technology, Bioenergy with Carbon Capture and Sequestration (BECS). Otherwise a faster decline rate of the capital stock would be required.

2.3
SHORT-LIVED RADIATIVE FORCING AGENTS: PEAK TRIMMING VERSUS BUYING TIME

The role of CO2 emissions in Earth’s climate future is unique among the major radiative forcing agents, because the impact of the CO2 emitted into the atmosphere will continue to alter Earth’s energy budget for millennia to come. As noted above, Earth’s energy budget is also subject to the influence of a number of short-lived radiative forcing agents whose radiative effect would decay to zero on a time scale of weeks to decades if their sources where shut off. These agents include aerosols, black carbon on snow or ice, and methane.

Aerosols are produced by burning biomass and fossil fuels, but unlike CO2 they are not an inevitable by-product of combustion. Some aerosols reflect energy to space, but other aerosols such as black carbon absorb sunlight. Reflecting aerosols unambiguously lead to cooling of the surface. Their effect has offset a portion of the radiative forcing from the anthropogenic increase of greenhouse gases so far, and any action that reduces reflecting aerosol emissions will lead to a nearly immediate warming. The effect of airborne absorbing aerosols is more subtle, because they primarily act to shift the absorption of solar radiation from the surface to the interior of the atmosphere, leaving the top-of-atmosphere energy budget largely unchanged (Randles and Ramaswamy, 2008). The global mean effect of surface black carbon is difficult to quantify but is unambiguously a warming, amplified further by the albedo feedback of melting snow and ice (Flanner et al., 2007; Hansen and Nazarenko, 2004; McConnell et al., 2007). Aerosol effects can include direct damage to human health and agriculture, implying that they should be cleaned up for reasons independent of climate (Agrawal et al., 2008; Ramanathan et al., 2008). A key question is whether the effort to do so will help or hurt other efforts to keep warming in check. Although the discussion of aerosol effects will be based primarily on temperature changes, it should be kept in mind that the spatially inhomogeneous radiative forcing from aerosols can lead to regional effects such as changes in clouds and the



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