Global temperatures are rising. Observations collected over the last century suggest that the average land surface temperature has risen 0.8-1.0°F (0.45-0.6°C) in the last century. The surface of the ocean has also been warming at a similar rate. Studies that combine land and sea measurements have generally estimated that global temperatures have warmed 0.5-1.0°F (0.3-0.6°C) in the last century. About two-thirds of this warming took place between 1900 and 1940. Global temperatures declined slightly from the 1940s through the 1970s; but have risen more rapidly during the last 25 years than in the period before 1940.
Surface temperatures are not rising uniformly. Night-time low temperatures are rising on average about twice as rapidly as daytime highs. The winters in areas between 50 and 70° North Latitude (the latitude of Canada and Alaska) are warming relatively fast, while summer temperatures show little trend. Urban areas are warming somewhat more rapidly than rural areas, because of both the changes in land cover and the consumption of energy that take place in densely developed areas (a feature known as the "urban heat island" effect).
Note: cooling in Southeast U.S. may be due to sulfate aerosol influence.
In the United States, temperatures in the last 50 years have cooled in the East while warming in the West. Over the last 100 years, the pattern is similar, except that New England is warmer than 100 years ago because it warmed more in the first half of the 20th century by more than it cooled in the second half. This pattern of warming and cooling may be part of a worldwide pattern: while most of the earth has warmed, the regions that are downwind from major sources of sulfur dioxide emissions have generally cooled (see the discussion on sulfates in the Atmospheric Change section). This pattern is evident when one compares the two world maps below. The first map of the world shows the areas that have warmed and cooled from 1951-99. The second map of the world shows the amount of incoming solar radiation blocked by the cloud of atmospheric sulfates downwind from industrial emissions of sulfur dioxide.
Red circles reflect warming — blue circles reflect cooling.
Solar Energy Blocked By Atmospheric Sulfates
(Radiative Forcing in Watts per Square Meter)
Although scientists have incontrovertible evidence that the surfaces of the land and oceans have been warming, some scientists are not yet convinced that the atmosphere is also warming. Satellite data on temperatures in the lower 4.8 miles of the atmosphere, spanning a period from 1979 to the present, show little if any warming trend compared with the surface-based record during the same period. However, the 1979-2000 satellite data series may be too short to show a trend in atmospheric temperature. There also are physical reasons (such as the different responses of the atmosphere and surface to stratospheric ozone depletion and El Niņo events) to expect that changes in atmospheric temperatures may not exactly match temperature changes on the surface during this period.
Balloon-borne instruments, which researchers have used to measure temperatures in the lower 4.8 miles of the atmosphere since 1958, show an overall warming trend from 1958-2000 similar to that of the surface record. But when just the period 1979-2000 is considered, the balloon data resemble the satellite data (see the charts below). This finding suggests that atmospheric and surface temperature trends may diverge in the short term.
Measurement errors associated with the satellite-based technology, and short-term variations in temperature due to ozone depletion and El Niņos (see glossary), may be responsible for the lack of a warming trend in the relatively short satellite record. Nevertheless, to many scientists, the absence of a warming trend in the satellite data provides an important caution that there is still much to learn about the global climate.