BOSTON — IN Washington, D.C. and in Japan, cherry trees bloomed 10 days earlier this year than in any previous year on record. It now appears that this was just one sign of an exceptionally warm spring across the Northern Hemisphere. Climatologists warn that such a single unusual event is not necessarily a sign of a global climatic warming trend due to a pollution-enhanced greenhouse effect. However, James Angell, who has ``been rather a skeptic'' about global warming, says ``this shakes me up a little bit.''
The National Oceanic and Atmospheric Administration climate analyst says the 1990 spring (March, April, May) is by far the warmest in his data set, which goes back to 1958.
For the north polar zone, the springtime average temperature was 3.9 degrees C above the long-term average temperature for that zone. The largest previous springtime warm departure in Dr. Angell's data set was only 2 degrees. The north temperate zone's spring average temperature was 1.8 degrees above normal, while the tropics were about normal. This gave an overall Northern Hemisphere anomaly of 1.4 degrees above normal - twice the largest positive springtime anomaly in the data set.
James Hansen, director of NASA's Goddard Institute for Space Sciences in New York, who works with a larger data network and date set, says his group also finds the spring very warm. Unlike Angell, Dr. Hansen has long believed that global warming is beginning to express itself. However, he notes that this is a matter of personal intuition. Like Angell, he warns that single anomalous events don't prove global warming. ``Scientists would like to see a clear trend,'' he says. Average temperatures vary
Hansen explains that natural short-term variations in seasonal average temperatures are very large as compared to the rate of long-term trends. February, for example, was near normal for the Northern Hemisphere. But March was 1.5 degrees above the average temperature for 1950 to 1980 for the Northern Hemisphere while, for the planet as a whole, the March average was 0.9 degrees above normal - the largest warm global temperature anomaly for March on record, Hansen says.
If there is an underlying warming trend, this unusual spring doesn't prove it. Yet, Hansen notes, if such unusual events become more common they should tend to erode skepticism.
So far, the data are ambiguous. The 1980s included several of the warmest years in this century. Yet the largest overall change so far found in long-term records is only about half a degree of global warming over the past century. That's too small to define a trend.
Indeed, analyses of some specific data sets show no warming trend at all. For example, Roy Spencer of the NASA Marshall Space Space Flight Center and John Christy of the University of Alabama studied global-satellite temperature data from 1979 to 1988. They found no net warming or cooling trend, although the decade included some exceptionally warm years. Greenhouse effect detection
The National Aeronautics and Space Administration has launched an ``Enhanced Greenhouse Effect Detection Project'' to try to find stronger clues to any warming trend. This includes 12 separate studies to look at all possible greenhouse-related data or phenomena such as tree rings and sea level records. It's one of 10 studies under the auspices of the international Space Agency Forum on the International Space Year (1992). That year is to be the culmination of such comprehensive space-based environmental studies.
Hansen says he expects an increasing number of possible greenhouse-related events, such as this spring, during this decade. He explains that the buildup of heat-trapping gases such as carbon dioxide has forced the global climate system out of equilibrium in a direction that favors surface warming. This, he says, is likely to bias the atmosphere so that, even with considerable variability, warming events should occur with increasing frequency.