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Is global warming upon us? Many climate experts say it is only a matter of time before heat trapped in the atmosphere by man-made chemicals raises world temperatures, melting polar ice caps, raising sea levels and generally wreaking havoc. Most also go on to caution that it is too soon to declare that the warming has begun. But two studies released last week evoked concern that a shift is already under way. Teams of scientists in the U.S. and Britain found that 1990 was the warmest year in more than a century of record keeping. The average worldwide temperature, said the Americans, was 15.5 degrees C (59.8 degrees F), while the British, using a different set of readings, pegged it at 15.4 degrees C (59.7 degrees F).

One hot year does not make a trend, since temperatures naturally vary from year to year. But the Americans, at NASA's Goddard Institute for Space Studies, pointed out that the new data mean that the seven warmest years since 1880 have all occurred in the past 11 years. On average, the 1980s were about .6 degrees C (1 degrees F) warmer than the 1880s, while 1990 was .7 degrees C (1.25 degrees F) warmer. That may not sound like much, but a mere 5 degrees C (9 degrees F) rise was enough to bring the earth out of the last ice age.

If global-warming theorists are correct, temperatures could rise by another 1 degrees to 4 degrees C (2 degrees to 8 degrees F) over the next half- century. Unfortunately, no one can say whether even a decade-long heat wave confirms this view or is merely a glitch. Worse, says Stephen Schneider of the National Center for Atmospheric Research: "By the time the evidence is irrefutable, it could be too late to do anything."