Global surface temperature is an estimate of global mean air temperature at the Earth's surface, based on thermometer measurements made at landbased weather stations. Because about 70 percent of the Earth's surface is covered by water, landbased data are supplemented by sea-surfacetemperature (SST) measurements. Estimates of air temperature are based on the assumption that there is a simple link between the SST and that of the air above. Usually, SST measurements are based on measurements of the temperature of seawater that is taken aboard ships for use as an engine coolant. In the past, SST measurements were made of water taken from buckets tethered to ropes and thrown overboard. Supplementary SST data are gathered from data buoys, small island stations, and shipboard, nighttime measurements of marine air temperatures.

Three agencies have taken responsibility for the combined global surface temperature record: the Climate Research Unit of the University of East Anglia; the U.S. National Aeronautics and Space Administration's Goddard Institute for Space Studies; and the Global Historical Climate Network of the U.S. National Oceanographic and Atmospheric Administration. To determine global surface temperature changes over time, these agencies locate and analyze anomalous departures from thirty-year temperature averages. These analyses are most commonly based on the area-weighted global average of sea-surface-temperature anomalies and land-surface-air-temperature anomalies. Based on these analyses, the mean global surface temperature of the Earth shows a warming in the range of 0.3œ-0.7œ Celsius over the past century, or a statistical average of about 0.003œ-0.007œ Celsius per year. Global data sets from the various agencies show slightly dissimilar trends, as the data are processed in different ways.

Questions arise as to the representative nature of the data on which global surface temperature calculations have been based. These data come from weather stations unevenly distributed over the Earth's surface, mostly on land, close to towns and cities, and predominantly in the Northern Hemisphere. Land use can have significant effects on local climate. The best-documented examples of such effects are urban heat islands, which are significantly warmer than their rural surroundings. Cities replace natural vegetation with surfaces such as concrete and asphalt that can warm them by several degrees Celsius. Thus, a disproportionate number of weather stations being located in urban environments may skew data regarding global averages. Many weather stations are located at airports, which were originally located in rural areas that have since been developed. Thus, while the data collected at such stations remain reliable measurements of the local, urban environment, they may no longer be equally reliable indicators of global trends.

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