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More grounding for global warming.

More grounding for global warming

Have the increasing levels of carbon dioxide and other "greenhouse' gases in the atmosphere begun to warm the planet yet?

The answer to this question is urgently needed by scientists who are trying to verify climate models predicting that temperatures will rise significantly in the next century due to greenhouse gases (SN:9/14/85, p.170). These gases are thought to trap heat escaping from the earth and send it back to warm the planet. One reason scientists have been uncertain about the answer is that past estimates of global temperatures have been based almost exclusively on readings taken on land, which covers at most 30 percent of the globe.

Now, in the July 31 NATURE, climatologists P.D. Jones, Thomas M. L. Wigley and P.B. Wright present the first comprehensive estimates of global mean surface temperatures that are based on calibrated ocean data as well as on land measurements. "This is a big step forward,' says James Angell at the National Oceanic and Atmospheric Administration's Air Resources Laboratory in Silver Spring, Md. Adds Murray Mitchell, also at the lab, "The importance of this study is that it confirms that we have indeed seen a general warming of the global climate over the last 100 years.'

Records of air and water temperatures have been kept by ships at sea for more than a century. The problem with these marine data, however, is that different measurement techniques of varying degrees of accuracy have been used, and the methods were not always reported.

One group of researchers had attempted to correct these discrepancies by trying to remedy each source of inconsistency individually. But because so much uncertainty is involved with this approach, Jones and his colleagues at the University of East Anglia in Norwich, England, instead corrected the marine data by calibrating them to measurements from nearby spots on land.

Their new temperature estimates, based on data taken from 1861 through 1984, show a slow warming trend over that period of about 0.7|C--consistent with the temperature changes predicted by greenhouse models. The researchers also note that the three warmest years in their temperature estimates were 1980, 1981 and 1983 and that five of the nine warmest years over the entire record occurred after 1978.

In general, these estimates confirm those based solely on land measurements. The only noteworthy difference is that land data showed a slight decrease in temperature between the late 1930s and 1970s, while the addition of marine data for this period causes the temperature to level off. The lack of growth during this period has puzzled scientists for several years. Jones's group writes that these steady conditions mean either that something is compensating for the greenhouse warming or that the climate system is less sensitive to greenhouse gases than was thought. Mitchell and others, for example, think that volcanic eruptions, which occurred far less frequently in this period than in the previous decades, counteracted greenhouse warming by cooling the atmosphere.

However, Jim Hansen at NASA Goddard Institute for Space Studies in New York City thinks that neither the argument for compensating factors nor that for lower sensitivity is required. He says greenhouse warming could be obscured by natural climate fluctuations of several tenths of a degree every decade or so.

Mitchell observes that for the majority of researchers, the new temperature estimates are "another piece of the puzzle that needs to fall into place before we ask the politicians to take us more seriously than they have. As for the minority who believe that carbon dioxide changes could affect the temperature only a little bit, if at all, I think when they see these data they're going to find that they have less ammunition than they thought they did to support their conservative view.'
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Title Annotation:greenhouse effect
Author:Weisburd, Stefi
Publication:Science News
Date:Aug 9, 1986
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