11.1 Introduction

Increasingly reliable regional climate change projections are now available for many regions of the world due to advances in modelling and understanding of the physical processes of the climate system. Atmosphere-Ocean General Circulation Models (AOGCMs) remain the foundation for projections while downscaling techniques now provide valuable additional detail. Atmosphere-Ocean General Circulation Models cannot provide information at scales finer than their computational grid (typically of the order of 200 km) and processes at the unresolved scales are important. Providing information at finer scales can be achieved through using high resolution in dynamical models or empirical statistical downscaling. Development of downscaling methodologies remains an important focus. Downscaled climate change projections tailored to specific needs are only now starting to become available.

11.1.1 Summary of the Third Assessment Report

The assessment of regional climate projections in the Third Assessment Report (TAR; Chapter 10 of IPCC, 2001) was largely restricted to General Circulation Model (GCM)-derived temperature with limited precipitation statements. The major assessment of temperature change was that it is very likely all land areas will warm more than the global average (with the exception of Southeast Asia and South America in June, July and August; JJA), with amplification at high latitudes. The changes in precipitation assessed to be likely were: an increase over northern mid-latitude regions in winter and over high-latitude regions in both winter and summer; in December, January and February (DJF), an increase in tropical Africa, little change in Southeast Asia, and a decrease in Central America; an increase or little change in JJA over South Asia and a decrease over Australia and the Mediterranean region. These projections were almost entirely based on analysis of nine coarse-resolution AOGCMs that had performed transient experiments for the 20th century with the specifications for the A2 and B2 emission scenarios. Chapter 10 of the TAR noted that studies with regional models indicate that changes at finer scales may be substantially different in magnitude from these large sub-continental findings.

Information available for assessment regarding climate variability and extremes at the regional scale was too sparse for it to be meaningfully drawn together in a systematic manner. However, some statements of a more generic nature were made. It was assessed that the variability of daily to interannual temperatures is likely to decrease in winter and increase in summer for mid-latitude Northern Hemisphere (NH) land areas, daily high temperature extremes are likely to increase and future increases in mean precipitation are very likely to lead to an increase in variability. In some specifically analysed regions, it was assessed that extreme precipitation may increase and there were indications that droughts or dry spells may increase in occurrence in Europe, North America and Australia.