WASHINGTON, D.C. -- Just how warm is it, anyway? According to a new study, the average annual surface temperature of the whole world is 14.0 degrees Celsius (57.2 degrees Fahrenheit). It is a little warmer (14.6 degrees C; 58.3 degrees F) in the northern hemisphere, where there is more land, than in the southern hemisphere, where there is more water (13.4 degrees C; 56.1 degrees F). The global annual cycle reflects the land-dominated northern seasons, with a July average maximum of 15.9 degrees C (60.6 degrees F) and a January average minimum of 12.2 degrees C (54.0 degrees F).
These figures appear in the May issue of Reviews of Geophysics, published by the American Geophysical Union. They are based on extensive research by British and American scientists, headed by Prof. Phil Jones of the University of East Anglia in Norwich, England. Reviews of Geophysics is a quarterly journal presenting peer reviewed papers that provide an overview of geophysical science and the directions of current research.
The researchers studied actual temperature readings for the period starting in the mid-nineteenth century and proxy, or indirect, data for earlier years. Their principal conclusions are:
* Annual global surface temperatures warmed by 0.57 degrees C (1.03 degrees F) from 1861 to 1997. From 1901 to 1997, the gain was 0.62 degrees C (1.12 degrees F). Over both periods, the gain was greater in the southern than in the northern hemisphere.
* The warmest years on record all occurred in the 1990s: in descending order, they were 1998, 1997, 1995, and 1990. Their average temperatures ranged from 0.57 degrees C (1.03 degrees F) in 1998 to 0.35 degrees C (0.63 degrees F) in 1990 above the 1961- 1990 average.
* Most warming in the 20th century has occurred in two distinct periods: 1925-1944 and 1978-1997. In both periods, warming was greatest over the northern continents and during the December-February and March-May seasons.
* Arctic temperatures have warmed slightly on an annual basis, with statistically significant increases from 1961 to 1990 during the months of May and June.
* Much of the recent increase in average temperature has occurred at night. From 1950 to 1993, the minimum nighttime temperature warmed by 0.18 degrees C (0.32 degrees F) per decade, while the maximum daytime temperature increased 0.08 degrees C (0.14 degrees F) per decade.
* The coldest year of the millennium was 1601, at the start of the coldest century, the 17th. Based on proxy data, that century was on average only 0.5-0.8 degrees C (0.9-1.4 degrees F) cooler than the 1961-1990 average. The warmest century of the millennium was the 20th.
The study was funded in part by the U.S. Department of Energy; the U.K. Natural Environmental Research Council; the U.K. Department of Environment, Transport, and the Regions; and other agencies.
Materials provided by American Geophysical Union. Note: Content may be edited for style and length.
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