Introduction

Abstract

Human activity affects the Earth’s climate mainly via two processes: the emission of greenhouse gases and aerosols and the alteration of land cover. Climate research conducted in the past several years indicates that most of the observed increase in global average temperatures over the past few decades is very likely ¹ due to the observed increase in greenhouse gas concentrations from human activities. It is likely that without the cooling effects of atmospheric aerosols, greenhouse gases alone would have caused a greater global temperature rise than has actually been observed. Research also indicates that human influences on the climate are expected to increase in the future, mainly because greenhouse gas emissions will continue to rise. Consequently, global average surface warming is projected for the 21^st century. These projections depend largely on the scenarios used to represent greenhouse gas emissions. In general, however, the projected warming is greatest over the land and most high northern latitudes, with relatively less change over the Southern Ocean and parts of the North Atlantic Ocean. Warming is typically projected to be greater in arid regions than in humid regions. Projected precipitation increases are very likely at high latitudes, while decreases are likely over most subtropical land regions. This projected change in precipitation is a continuation of recent trends.