Earth's equilibrium climate sensitivity, Seq, the change in global mean surface temperature (GMST) that would result from a given forcing F, normalized to that forcing, is a widely used metric for comparing climate models or for assessing the amount of carbon dioxide that might be added to the atmosphere consistent with a given allowable increase in global temperature. However, because of the large heat capacity of the deep ocean the time scale for reaching the equilibrium temperature change following imposition of a forcing is about 500 yr, during which time there is net heat flow from the upper compartment of the climate system that is radiatively coupled to space to the deep ocean; this heat flow diminishes the response of the upper compartment to imposed forcings. Observationally the rate of increase in ocean heat content over the past 50 years has been proportional to the increase in GMST, with proportionality constant κ of about 1.0 W m-2 K-1. This proportionality leads to a relation between transient and equilibrium climate sensitivity Seq = (Seq-1 - κ)-1, where the transient sensitivity Str is the constant of proportionality between GMST and forcing. The increase in GMST observed over the twentieth century is found to be proportional to F for several published forcing data sets but with Str depending strongly on the forcing data set employed, ranging from 0.19 to 0.42 K (W m-2)-1, increasing with decreasing forcing. The corresponding equilibrium sensitivity ranges from 0.24 to 0.75 K (W m-2)-1. These values are lower to well lower than the IPCC (2007) estimated range for equilibrium sensitivity. The long time scale for reaching equilibrium sensitivity suggests that the measure of climate sensitivity relevant to policy considerations is the lower, transient sensitivity.
This page was last updated 2011-10-30.
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