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Global Temperatures
Trends in global mean surface temperatures are best revealed by plotting
temperature "anomalies," which are changes from the long-term (120 year) average
over the period for which we have instrument records.
Global land, ocean, and land-ocean combined surface data (Figure 1) show that the last 4-5 years have been near the record high year of 1998 (an El Niño year). The high temperatures in recent years are observed both over oceans and over land.
Global mean surface temperatures have increased over the past 120 years at a reate near 0.06 C/decade (0.11 F/decade) but with periods of larger and smaller trend. During the past 25 to 3- years this trend has approximately 0.18 C/decade (0.32 F/decade). The most recent period of increasing surface temperatures has a warmning rate comparable to the rates of warming projected by the global climate models using observed changed in greenhouse gases, statospheric ozone concentration, volcanoes, and fluctuations of the output of the sun. This recent trend is expected to continue throughout the next century with continued increases of anthropogenic greenhouse gases.
Temperature measurements above the Eath's surface have been made over the past 50 to 60 years using balloon-borne instruments (radiosondes) and for the past 28 years using satellites. These measurements show that the middle tropospheric temperatures (14 to 22 km above the surface) have been below average since the dissipation of the warming effects from the 1991 Mt. Pinatubo eruption. Depletion of the ozone in the lower statosphere, combine with the effects of rising greenhouse gas concentrations, would lead to lower temperatures int eh stratosphere of magnitude consistent with these observations.
