Overview

The IPCC projections of future global warming are dependent on the efficacy of computermodels and their ability to realistically represent the processes of the climate system. IPCCclaims that the ability of computer models to provide useful projections of future climate hasimproved because of the demonstrated performance on a range of space and time scales. If suchclaims have foundation then the computer models must be sensitive to internal variability ontimescales that are observed and responsive to external forcing consistent with natural forcingof the climate system. The IPCC draws attention to positive developments in computer modelling,including:

• over long simulations the models are stable, they adequately represent a variety ofstatistical characteristics of the climate, and they exhibit only limited internalvariability;

• simulations for the 20th century with imposed forcing that mimics natural forcing (solarirradiance variations and vulcanism) and anthropogenic forcing (aerosols and increasing greenhousegas concentrations) project a pattern of global mean surface air temperature that has many of thecharacteristics of the observed temperature pattern for the period.

It is true that computer models of the climate system are undergoing a rapid rate ofdevelopment. The increasing speed of computers is being translated directly to improved resolutionof the atmospheric and ocean circulations, and to improved representation of local processes,including those associated with clouds, sea ice and exchanges between the underlying surface andthe atmosphere. Also, many uncertainties relating to atmosphere and ocean processes are beingresolved through research and these advances are being incorporated into computer models.

Notwithstanding the improvements over recent decades, computer models of the...