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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, T2x, that results when
the climate system, or a climate model, attains a new equilibrium with the forcing
change F2x resulting from a doubling of the atmospheric CO2
concentration. For this new equilibrium dH/dt = 0 in the simple heat budget
equation and F2x = 
T2x
indicating a balance between energy input and output. The equilibrium climate
sensitivity
T2x = F2x/
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
- dHo/dt) = T / (F - Fo)
and the effective climate sensitivity is calculated as
Te = F2x/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.
