Chapter 1 discusses the nature of the climate system and the climate variability and change it may undergo, both naturally and as a consequence of human activity. The projections of future climate change discussed in this chapter are obtained using climate models in which changes in atmospheric composition are specified. The models “translate” these changes in composition into changes in climate based on the physical processes governing the climate system as represented in the models. The simulated climate change depends, therefore, on projected changes in emissions, the changes in atmospheric greenhouse gas and particulate (aerosol) concentrations that result, and the manner in which the models respond to these changes. The response of the climate system to a given change in forcing is broadly characterised by its “climate sensitivity”. Since the climate system requires many years to come into equilibrium with a change in forcing, there remains a “commitment” to further climate change even if the forcing itself ceases to change.

Observations of the climate system and the output of models are a combination of a forced climate change “signal” and internally generated natural variability which, because it is random and unpredictable on long climate time-scales, is characterised as climate “noise”. The availability of multiple simulations from a given model with the same forcing, and of simulations from many models with similar forcing, allows ensemble methods to be used to better characterise projected climate change and the agreement or disagreement (a measure of reliability) of model results.

The heat balance
Broad aspects of global mean temperature change may be illustrated using a simple representation of the heat budget of the climate system expressed as:

dH/dt = F - T.

Here F is the radiative forcing change as discussed in Chapter 6; aT represents the net effect of processes acting to counteract changes in mean surface temperature, and dH/dt is the rate of heat storage in the system. All terms are differences from unperturbed equilibrium climate values. A positive forcing will act to increase the surface temperature and the magnitude of the resulting increase will depend on the strength of the feedbacks measured by T. If is large, the temperature change needed to balance a given change in forcing is small and vice versa. The result will also depend on the rate of heat storage which is dominated by the ocean so that dH/dt = dH_o/dt = F_o where H_o is the ocean heat content and F_o is the flux of heat into the ocean. With this approximation the heat budget becomes F = T + F_o, indicating that both the feedback term and the flux into the ocean act to balance the radiative forcing for non-equilibrium conditions.

Radiative forcing in climate models
A radiative forcing change, symbolised by F above, can result from changes in greenhouse gas concentrations and aerosol loading in the atmosphere. The calculation of F is discussed in Chapter 6 where a new estimate of CO₂ radiative forcing is given which is smaller than the value in the SAR. According to Section 6.3.1, the lower value is due mainly to the fact that stratospheric temperature adjustment was not included in the (previous) estimates given for the forcing change. It is important to note that this new radiative forcing estimate does not affect the climate change and equilibrium climate sensitivity calculations made with general circulation models. The effect of a change in greenhouse gas concentration and/or aerosol loading in a general circulation model (GCM) is calculated internally and interactively based on, and in turn affecting, the three dimensional state of the atmosphere. In particular, the stratospheric temperature responds to changes in radiative fluxes due to changes in CO₂ concentration and the GCM calculation includes this effect.

Equivalent CO₂
The radiative effects of the major greenhouse gases which are well-mixed throughout the atmosphere are often represented in GCMs by an “equivalent” CO₂ concentration, namely the CO₂ concentration that gives a radiative forcing equal to the sum of the forcings for the individual greenhouse gases. When used in simulations of forced climate change, the increase in “equivalent CO₂” will be larger than that of CO₂ by itself, since it also accounts for the radiative effects of other gases.

1%/yr increasing CO₂
A common standardised forcing scenario specifies atmospheric CO₂ to increase at a rate of 1%/year compound until the concentration doubles (or quadruples) and is then held constant. The CO₂ content of the atmosphere has not, and likely will not, increase at this rate (let alone suddenly remain constant at twice or four times an initial value). If regarded as a proxy for all greenhouse gases, however, an “equivalent CO₂” increase of 1%/yr does give a forcing within the range of the SRES scenarios.

This forcing prescription is used to illustrate and to quantify aspects of AOGCM behaviour and provides the basis for the analysis and intercomparison of modelled responses to a specified forcing change (e.g., in the SAR and the CMIP2 intercomparison). The resulting information is also used to calibrate simpler models which may then be employed to investigate a broad range of forcing scenarios as is done in Section 9.3.3. Figure 9.1 illustrates the global mean temperature evolution for this standardised forcing in a simple illustrative example with no exchange with the deep ocean (the green curves) and for a full coupled AOGCM (the red curves). The diagram also illustrates the transient climate response, climate sensitivity and warming commitment.

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