The increase of global mean surface temperature (GMST) in response to a positive forcing is governed by Earth's equilibrium climate sensitivity and uptake of heat energy by the climate system. The rate of heat uptake over the latter part of the twentieth century, determined from the rate of increase of ocean heat content plus other, minor heat sinks, is found to be proportional to the increase in GMST &DeltaT, with heat uptake coefficient κ about 1.0 W m-2 K-1. This proportionality implies a linear proportionality between ΔT and forcing &DeltaT(t) = StrF(t), where Str is the transient climate sensitivity. This proportionality holds closely over the twentieth century for several published forcing data sets but with a broad range of Str, 0.19 - 0.42 K (W m-2)-1, increasing with decreasing forcing, Figure 1. The range of equilibrium climate sensitivity determined as Seq = (Str-1 - κ)-1, is even broader, 0.24 - 0.75 K (W m-2)-1 (i.e., CO2 doubling temperature 0.9 - 2.8 K) but is lower to well lower than the IPCC AR4 best estimate 3 K and associated uncertainty range. For a two-compartment model of Earth's climate system (Held et al., J. Climate, 23:2418-2427, 2010) the time constants of the upper low–heat-capacity and lower high–heat-capacity compartments are determined to be 4-9 years and about 500 years, respectively, suggesting that the sensitivity pertinent to decision making about greenhouse gas emissions is the lower, transient sensitivity. Constraining climate sensitivities requires reducing uncertainty in forcing, which is dominated by uncertainty in forcing by anthropogenic aerosols.
Figure 1. Dependence of transient (purple) and equilibrium (red) climate sensitivity on forcing between 1900 and 1998 for several published forcing data sets; uncertainties represent 1σ. Right axis gives equivalent CO2 doubling temperature evaluated with forcing 3.71 W m-2. Also shown (blue) are best estimates of equilibrium climate sensitivity and anthropogenic forcing (relative to preindustrial) and associated uncertainties given by the IPCC 2007 Assessment Report; thick uncertainty lines correspond to "likely" range; thin uncertainty lines denote "very likely" range. Modified from Schwartz (2011).
Schwartz S. E. (2011) Determination of Earth's Transient and Equilibrium Climate Sensitivities from Observations over the Twentieth Century: Strong Dependence on Assumed Forcing. Submitted
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