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A Guide to Information on Greenhouse Gases and Ozone Depletion
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Item #d97mar73

Results from three independent analyses of worldwide surface temperatures show that, although 1996 was a fraction of a degree Celsius cooler than 1995, the year still ranks among the ten warmest on record. The surface measurements also indicate that this is the hottest decade on record, and the 1980s are second.

Satellite measurements of temperatures in the lower layers of the atmosphere also show 1996 to have been slightly cooler than 1995. But in contrast to the recent warming trend displayed by the surface temperature record, the 18-year satellite record exhibits a slight global cooling trend. The two data sets are not necessarily inconsistent because they measure somewhat different properties of the atmosphere. However, this difference complicates interpretation of the observational record, and fuels debate over climate change. (See Science News, p. 38, Jan. 18, 1997; The New York Times, pp. C1, C6, Jan. 14; Global Environ. Change Rep., pp. 1-3, Jan. 17.)

At least two regional phenomena probably contributed to the lowered global temperature for 1996 compared with 1995. La Niña, the cool phase of the El Niño-Southern Oscillation of the Pacific Ocean, prevailed early in 1996, helping to cool broad portions of the Northern Hemisphere. Even more dramatic was the abrupt reversal in late 1995 of the North Atlantic Oscillation (NAO). The NAO is a long-recognized tendency of the atmosphere to exhibit either one of two patterns of circulation over the North Atlantic Ocean for a decade or so, before shifting to the other pattern. Until 1995, the NAO had been in the phase that is thought to have kept Europe and North America relatively warm for the last two or three decades. The recent reversal probably accounts for the two latest severe winters in Europe, and could contribute to lower Northern Hemisphere temperatures in the near future.

Recently, oceanographers have proposed that the NAO and its reversals may be controlled by pulses of warm and cool water traveling along a vast ocean current that starts with the Gulf Stream, swings past Ireland, then heads west to the Labrador Sea. (See feature articles in Science, pp. 754-755, Feb. 7, and The New York Times, pp. C1, C6, Mar. 18.) A full understanding of the NAO could lead to regional climate forecasts for Europe and North America. But its existence also complicates the search for a human "fingerprint" of climate change, as the Science feature discusses. Natural agents of climate change such as the NAO and El Niño could be masquerading as a strengthening greenhouse. On the other hand, elevated greenhouse gases could be altering these otherwise natural oscillations. Only time, and continued observations, will tell.

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