|Working Group I: The Scientific Basis|
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9.2 Climate and Climate Change
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.9.2.1 Climate Forcing and Climate Response
The heat balance
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 = dHo/dt = Fo where Ho is the ocean heat content and Fo is the flux of heat into the ocean. With this approximation the heat budget becomes F = T + Fo, 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
1%/yr increasing CO2
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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