in the coming centuries? The answer to these questions in large measure rests on the net amount of carbon dioxide released during the fossil fuel era, however long that may last. The discussion of Earth System Sensitivity and other considerations pertaining to very long-term climate change will be deferred to Chapter 6.

3.2
EQUILIBRIUM CLIMATE SENSITIVITY

Climate sensitivity is calculated by determining how much Earth’s surface and atmosphere need to warm in order to radiate away enough energy to space to make up for the reduction in energy loss out of the top of the atmosphere caused by the increase of CO2 or other anthropogenic greenhouse gases. In equilibrium, there is no net transfer of energy into or out of the oceans, so the equilibrium sensitivity can be treated in terms of the top-of-atmosphere energy balance.

The top-of-atmosphere balance is the rate at which energy escapes to space in the form of infrared minus the rate at which energy is absorbed in the form of sunlight, both expressed per square meter of Earth’s surface. Figure 3.1 shows schematically how the balance depends on surface temperature, for a case which is initially in equilibrium at temperature T0 (where the blue solid or dashed line crosses the horizontal axis). If CO2 is increased, leading to a reduction in outgoing radiation by an amount Δ F, Earth must warm up so as to restore balance. The amount of warming required is determined by the slope of the line describing the increase in energy imbalance with temperature. A lower slope results in a higher climate sensitivity, as illustrated by the dashed slanted lines in Figure 3.1. Climate sensitivity is often described in terms of the warming Δ T2X that would result from a standardized radiative forcing Δ F2X corresponding to a doubling of CO2 from its pre-industrial value. In the following we use the value Δ F2X = 3.7 W/m2 diagnosed from general circulation models to express the slope in terms of Δ T2X (IPCC, 2007a).

The most basic feedback affecting planetary temperature is the black-body radiation feedback, which is the tendency of a planet to lose heat to space by infrared radiation at a greater rate as the surface and atmosphere are made warmer while holding the composition and structure of the atmosphere fixed (see Table 3.2). This feedback was first identified by Fourier (1827; see Pierrehumbert, 2004). For a planet with a mean surface temperature of 14ºC (about the same as Earth’s averaged over 1951-1980) the black-body feedback alone would yield Δ T2X = 0.7ºC if the planet’s atmosphere had no greenhouse effect of any kind. Such a planet would have



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