|Working Group I: The Scientific Basis|
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TCR - Transient climate response
The temperature change at any time during a climate change integration depends on the competing effects of all of the processes that affect energy input, output, and storage in the ocean. In particular, the global mean temperature change which occurs at the time of CO2 doubling for the specific case of a 1%/yr increase of CO2 is termed the “transient climate response” (TCR) of the system. This temperature change, indicated in Figure 9.1, integrates all processes operating in the system, including the strength of the feedbacks and the rate of heat storage in the ocean, to give a straightforward measure of model response to a change in forcing. The range of TCR values serves to illustrate and calibrate differences in model response to the same standardised forcing. Analogous TCR measures may be used, and compared among models, for other forcing scenarios.
Equilibrium climate sensitivity
is inversely proportional to , which measures the strength of the feedback processes in the system that act to counter a change in forcing. The equilibrium climate sensitivity is a straightforward, although averaged, measure of how the system responds to a specific forcing change and may be used to compare model responses, calibrate simple climate models, and to scale temperature changes in other circumstances.
In earlier assessments, the climate sensitivity was obtained from calculations made with AGCMs coupled to mixed-layer upper ocean models (referred to as mixed-layer models). In that case there is no exchange of heat with the deep ocean and a model can be integrated to a new equilibrium in a few tens of years. For a full coupled atmosphere/ocean GCM, however, the heat exchange with the deep ocean delays equilibration and several millennia, rather than several decades, are required to attain it. This difference is illustrated in Figure 9.1 where the smooth green curve illustrates the rapid approach to a new climate equilibrium in an idealised mixed-layer case while the red curve is the result of a coupled model integration and indicates the much longer time needed to attain equilibrium when there is interaction with the deep ocean.
Effective climate sensitivity
and the effective climate sensitivity is calculated as
with units and magnitudes directly comparable to the equilibrium sensitivity. The effective sensitivity becomes the equilibrium sensitivity under equilibrium conditions with 2xCO2 forcing. The effective climate sensitivity is a measure of the strength of the feedbacks at a particular time and it may vary with forcing history and climate state.
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