ABSTRACT
A number of lines of evidence indicate that the Earth's climate has warmed during the 20th century. Foremost among these are compilations of the variation in global mean sea surface temperature and in surface air temperature over land and sea. Supplementing these indicators of surface temperature change is a global network of balloon based observations of atmospheric temperature since 1958. There are also several indirect or proxy indications of temperature change, including satellite observations of microwave emissions from the atmosphere and records of the width and density of tree rings. The combination of surface-, balloon-, and satellite-based indicators provides a more complete picture than can be obtained from any indicator alone, while proxy records from tree rings and other indicators allow the temperature record to be extended back several centuries at selected locations. Apart from temperature, changes in the extent of alpine glaciers, sea ice, seasonal snow cover, and the length of the growing season have been documented that are consistent with the evidence that the climate is warming. Less certain changes appear to have occurred in precipitation, cloudiness, and inter-annual temperature and rainfall variability. Finally, information from tide gauges scattered along the world's coastlines indicates that sea level has been rising more rapidly during the past few decades than during the preceding several centuries, and this is also consistent with a recent warming of the climate.
