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TCR - Transient climate response

Figure 9.1: Global mean temperature change for 1%/yr CO2 increase with subsequent stabilisation at 2xCO2 and 4cCO2. The red curves are from a coupled AOGCM simulation (GFDL_R15_a) while the green curves are from a simple illustrative model with no exchange of energy with the deep ocean. The “transient climate response”, TCR, is the temperature change at the time of CO2 doubling and the “equilibrium climate sensitivity”, T2x, is the temperature change after the system has reached a new equilibrium for doubled CO2, i.e., after the “additional warming commitment” has been realised.

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
The “equilibrium climate sensitivity” (IPCC 1990, 1996) is defined as the change in global mean temperature, T_2x, that results when the climate system, or a climate model, attains a new equilibrium with the forcing change F_2x resulting from a doubling of the atmospheric CO₂ concentration. For this new equilibrium dH/dt = 0 in the simple heat budget equation and F_2x = T_2x indicating a balance between energy input and output. The equilibrium climate sensitivity

T_2x = F_2x/

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
Although the definition of equilibrium climate sensitivity is straightforward, it applies to the special case of equilibrium climate change for doubled CO₂ and requires very long simulations to evaluate with a coupled model. The “effective climate sensitivity” is a related measure that circumvents this requirement. The inverse of the feedback term a is evaluated from model output for evolving non-equilibrium conditions as

1/_e = T / (F - dH_o/dt) = T / (F - F_o)

and the effective climate sensitivity is calculated as

T_e = F_2x/_e

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.

Warming commitment
An increase in forcing implies a “commitment” to future warming even if the forcing stops increasing and is held at a constant value. At any time, the “additional warming commitment” is the further increase in temperature, over and above the increase that has already been experienced, that will occur before the system reaches a new equilibrium with radiative forcing stabilised at the current value. This behaviour is illustrated in Figure 9.1 for the idealised case of instantaneous stabilisation at 2x and 4xCO₂ . Analogous behaviour would be seen for more realistic stabilisation scenarios.

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