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Global Climate Change Digest

A Guide to Information on Greenhouse Gases and Ozone Depletion
Published July 1988 through June 1999

FROM VOLUME 11, NUMBER 12, DECEMBER 1998

NEWS...
Global Temperatures


Item #d98dec34

Annual temperature anomalies since the beginning of the industrial age. The average global temperatures for every year from 1880 to 1998 were added and divided by the number of years to get the mean global temperature for the period. The departure of each year’s average global temperature from that mean (in degrees Celsius) was then graphed as the annual mean temperature anomaly (dashed line). The five-year moving average of those values was then calculated and superimposed on the graph (solid line) to show a smoothed trend of the data. (Illustration courtesy of James E. Hansen, NASA GISS.)

At the end of November (the end of the meteorological year), the World Meteorological Organization released its annual statement on global climate, which said that the Earth’s mean surface temperature in 1998 would be 0.58° C above the average for the benchmark period 1961–1990 and the highest since such records were first kept in 1860. The WMO said that its calculations, which are based on readings from stations around the world, showed that

  • 1998 would be the 20th consecutive year in which global surface temperatures were above normal;
  • the ten warmest years on record have all occurred since 1983;
  • by October, new monthly temperature records had been set in each of the previous 18 months;
  • global temperatures were almost 0.7° C above those at the beginning of the century; and
  • from January through November, regional temperature patterns were all above average except for the northern sections of Eurasia.

At the same time, researchers James Hansen, Reto Ruedy, Jay Glascoe, and Makiko Sato of the NASA Goddard Institute for Space Studies reported on their website (http://www.giss.nasa.gov/research/observe/surftemp/) that global surface temperatures in the 1998 meteorological year set a new record for the period that instrumental measurements have been made. Data collected from several thousand meteorological ground stations around the world and satellite measurements of sea-surface temperatures indicated that the 5-year mean temperature since 1975 has increased about 1/2° C or almost 1° F. Temperature changes are not necessarily the same at ground level as they are higher in the atmosphere, but the warming in 1998 is so large that the NASA researchers believe that record warm temperatures should be found by all observing systems measuring from the surface through the troposphere. The 1998 warmth was associated partly with a strong El Niño that warmed the air over the eastern tropical Pacific Ocean in the first half of the year and affected weather around the world. The El Niño by itself does not account for either the observed long-term global warming trend or the extreme warmth of 1998. The largest temperature anomalies in 1998 were in northern Canada, but almost the entire world was warmer than normal. Temperatures in the United States, which covers only about 2% of the world area, were also warm, but did not match the record warmth of 1934 during the Dust Bowl era.

On page A3 of its Dec. 8 edition, the Washington Post reported that Jonathan Overpeck, head of the paleoclimatology program of the National Oceanic and Atmospheric Administration, presented his findings on the Earth’s temperature at the annual meeting of the American Geophysical Union in San Francisco in mid-December. He said that the warming observed during this century is without precedent during the past 1,200 years and cannot be fully explained by natural processes. At the same meeting, James Hansen, director of NASA’s Goddard Institute for Space Studies, stated that scientists know too little about climate to make accurate predictions. Overpeck noted that this century’s warmer temperatures are appearing more anomalous as scientists improve their understanding of past climate changes. He cited new dendrochronological and glacial evidence that calls into question whether the Medieval Warm Period ever happened elsewhere than in northern Europe and North America. That warming, he said, was likely caused by changes in ocean currents. On the other hand, the current warming trend is truly global, with the Earth’s average temperature rising about 1° F since the 1880s.

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