This report, the fourth biennial review of THE GLOBAL CLIMATE SYSTEM, is based on the current scientific understanding of the climate system and provide.s a basis for the monitoring of global change. This monitoring supports efforts such as the newly formed Climate Change Detection Project of the WCDMP by facilitating the identification of persistent anomalies on both global and regional scales, which is a necessary first step toward separating the "signal" of climate change from the "noise" of natural variability. Due to deficiencies in the global observing system, the diagnostic analyses of cause-effect relationships are preliminary for some regions and some climatic events or processes.