ditional climate risks that depend on how the climate system responds to concentrations above 450 ppm CO2-eq. For instance, this could send the climate system over critical thresholds (e.g., irreversible drying of the subtropics, melting large glaciers, and raising sea levels); once such thresholds are crossed, reducing CO2-eq concentrations may be ineffective for bringing the climate system back to a particular state (Solomon et al., 2009). Finally, allowing emissions to follow an overshoot pathway in the near term leaves open the possibility that, once the concentration target is exceeded, the necessarily steeper emissions declines later in the century may never materialize.

Radiative forcing and CO2-eq concentrations. Radiative forcing is a measure of impact on the Earth’s radiative balance from changes in concentrations of key substances such as GHGs and aerosols. It is generally expressed in terms of watts per meters squared (W/m2) but can also be expressed in terms of CO2-equivalent(CO2 -eq) concentrations, that is, the concentrations of CO2 only that would lead to the same impact on the Earth’s radiative balance (Ramaswamy et al., 2001). Radiative forcing agents in the atmosphere that are most relevant to considerations of future climate change include the “Kyoto” gases (those included in the Kyoto Protocol: CO2, CH4, N2O, HFCs, PFCs, and SF6); CFCs and other ozone-depleting substances covered by the Montreal Protocol; tropospheric ozone; different types of aerosols including sulfates, black carbon, and organic carbon; and land use changes that affect the reflectivity of the Earth’s surface.

Many global emissions scenarios consider only CO2 or only the Kyoto gases. The results of analyses that include only Kyoto gas forcing differ from those that include “full forcing,” primarily due to the impact of aerosols. The aerosol influence is complex; some types such as black carbon exert positive forcing (warming), while other types such as sulfates exert negative forcing (cooling). It is estimated that, overall, aerosol-related cooling influences currently lower total forcing by roughly an equivalent of 50 ppm CO2 (Forster and Ramaswamy, 2007). Many studies indicate that the aerosol influence will attenuate over the coming century, however, particularly if strong climate change limiting policies are enacted. This is because aerosol emissions from fossil fuel combustion are expected to be significantly reduced, both as an indirect result of GHG mitigation efforts and as a direct result of concerns over health impacts. For example, in scenarios from the EMF22 models that most comprehensively considered full forcing, it was found that, by the end of the century, Kyoto-only CO2-eq concentrations were roughly equal to full-forcing concentrations.

delays in global participation, no models could produce the scenario that met 450 ppm CO2-eq by 2100.

The EMF22 results indicate that atmospheric GHG concentrations can be kept below 450 ppm CO2-eq only if the United States and other high-income countries, along with China, India, and many other low- and middle-income countries around the world, take aggressive actions to reduce emissions starting within the next few years. This would represent a dramatic change from recent trends across the globe. If the major

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