Climate Change 2001:
Working Group I: The Scientific Basis
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Executive Summary

Introduction
This chapter assesses regional climate information from Atmosphere-Ocean General Circulation Models (AOGCMs) and techniques used to enhance regional detail. These techniques have been substantially improved since the IPCC WGI Second Assessment Report (IPCC, 1996) (hereafter SAR) and have become more widely applied. They fall into three categories: high and variable resolution Atmosphere General Circulation Models (AGCMs); regional (or nested limited area) climate models (RCMs); and empirical/statistical and statistical/dynamical methods. The techniques exhibit different strengths and weaknesses and their use depends on the needs of specific applications.

Simulations of present day climate
Coarse resolution AOGCMs simulate atmospheric general circulation features well in general. At the regional scale the models display area-average biases that are highly variable from region-to-region and among models, with sub-continental area-averaged seasonal temperature biases typically within 4ºC and precipitation biases mostly between -40 and +80% of observations. In most cases, these represent an improvement compared to the AOGCM results evaluated in the SAR.

The development of high resolution/variable resolution AGCMs since the SAR shows that the models' dynamics and large-scale flow improve as resolution increases. In some cases, however, systematic errors are worsened compared with coarser resolution models although only very few results have been documented.

RCMs consistently improve the spatial detail of simulated climate compared to General Circulation Models (GCMs). RCMs driven by observed boundary conditions show area-averaged temperature biases (regional scales of 105 to 106 km2) generally within 2ºC and precipitation biases within 50% of observations. Statistical downscaling demonstrates similar performance, although greatly depending on the methodological implementation and application.

Simulation of climate change for the late decades of the 21st century

Climate means
The following conclusions are based on seasonal mean patterns at sub-continental scales emerging from current AOGCM simulations. Based on considerations of consistency of changes from two IS92a-type emission scenarios and preliminary results from two SRES emission scenarios, within the range of these four scenarios:

Results from regional studies indicate that at finer scales the changes can be substantially different in magnitude or sign from the large area average results. A relatively large spread exists between models, although attribution is unclear.

Climate variability and extremes
The following conclusions are based on patterns emerging from a limited number of studies with current AOGCMs, older GCMs and regionalisation studies.

Tropical cyclones
Despite no clear trends in the observations, a series of theoretical and model-based studies, including the use of a high resolution hurricane prediction model, suggest:

Recommendations
The material assessed identifies key priorities for future work:

GCMs:

RCMs:

Empirical/statistical and statistical/dynamical methods:

Tropical cyclones:

Cross-cutting:



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