Several coupled models are able to reproduce the major trend in 20th century surface air temperature, when driven by historical radiative forcing scenarios corresponding to the 20th century. However, in these studies idealised scenarios of only sulphate radiative forcing have been used. One study using the ECHAM4/OPYC model that includes both the indirect and direct effects of sulphate aerosols, as well as changes in tropospheric ozone, suggests that the observed surface and tropospheric air temperature discrepancies since 1979 are reduced when stratospheric ozone depletion and stratospheric aerosols associated with the Pinatubo eruption are included. Systematic evaluation of 20th century AOGCM simulations for other trends found in observational fields, such as the reduction in diurnal temperature range over the 20th century and the associated increase in cloud coverage, have yet to be conducted.

The inclusion of changes in solar irradiance and volcanic aerosols has improved the simulated variability found in several AOGCMs. In addition, some evaluation studies aimed at the reproduction of 20th century climate have suggested that changes in solar irradiance may be important to include in order to reproduce the warming in the early part of the century. Another study has suggested that this early warming can be solely explained as a consequence of natural internal climate variability.

Taken together, we consider that there is an urgent need for a systematic 20th century climate intercomparison project with a standard set of forcings, including volcanic aerosols, changes in solar irradiance and land use, as well as a more realistic treatment of both the direct and indirect effects of a range of aerosols.

Land-cover change can occur through human intervention (land clearance), via direct effects of changes in CO₂ on vegetation physiology and structure, and via climate changes (Chapter 7). Evidence from observational studies (see Chapter 7, Section 7.4.2) and modelling studies (e.g., Betts et al., 1997, 2000; Chase et al., 2000; Zhao et al., 2001) demonstrate that changes in land cover can have a significant impact on the regional scale climate but suggestions that land clearance has an impact on the global scale climate is currently speculative. Evidence from palaeoclimate (Section 8.5) and modelling work (Section 8.5 and Chapter 7, Section 7.4) indicates that these changes in vegetation may lead to very significant local and regional scale climate changes which, in some cases, may be equivalent to those due to increasing CO₂ (Pitman and Zhao, 2000).

On time-scales of decades the impact of land cover change could significantly influence the rate of atmospheric CO₂ increase (Chapter 3), the nature and extent of the physical climate system response, and ultimately, the response of the biosphere to global change (Chapter 8). Models currently under development that can represent changes in land cover resulting from changes in climate and CO₂ should enable the simulations of these processes in the future. If these models can be coupled with scenarios representing human-induced changes in land cover over the next 50 to 100 years, the important effects of land- cover change can be included in climate models. While the inclusion of these models of the biosphere is not expected to change the global scale response to increasing CO₂, they may significantly effect the simulations of local and regional scale change.

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