WHEN it comes to the question of whether carbon dioxide pollution is causing global warming, the public is understandably confused. The NASA Goddard Institute for Space Studies has just labeled 1990 the hottest year on record - yet another example of a global warming trend. Not so, say scientists at the NASA Marshall Space Flight Center and the University of Alabama. They rank the hottest years of the 1980s, in descending order, as 1987, 1988, 1983, and then 1990. They see no consistent warming trend.
What are lay people to think if even experts within the National Aeronautics and Space Administration can't agree on what the weather records show?
The quarreling experts aren't just being ornery. Climate trends are elusive. Every such ``discovery'' is likely to be challenged. In this case, the debaters aren't even using the same data.
Goddard analysts studied surface-temperature data from 1,000 sites around the world. The Alabama-Marshall scientists used global satellite measurements. They criticize the Goddard data set as being too limited. It doesn't cover the ocean at all.
Goddard researchers note that satellite data reflect a substantial thickness of atmosphere, not just the surface. Trying to decide who has a better handle on climate in this case is a difficult technical issue.
But there is a fundamental point on which climate experts do agree. CO2 pollution has already enhanced the atmosphere's ``greenhouse'' heat-trapping effect, whether or not this has begun to raise the planet's average surface temperature.
Satellite monitoring shows that Earth's surface, on average, emits 390 watts per square meter of infrared (heat) radiation. The atmosphere's natural load of heat-trapping gases - notably CO2 and water vapor - capture 153 watts of that escaping heat. Without this natural greenhouse effect, Earth would be a lifeless icebox. CO2 pollution has added about 2.2 watts per square meter to the natural greenhouse trapping.
Climate experts aren't arguing about whether human activity is enhancing the greenhouse effect. They're trying to decide how the climate system will adjust to the extra few watts per square meter it now has to work with. There could be changes in average cloudiness, ocean currents, and other effects, as well as a general global warming.
Scientists will continue to debate warming trends and their consequences. Don't be fooled by the seeming confusion. There isn't the slightest doubt that our pollution has perturbed the climate system. That's reason enough to consider curbing the pollution itself.
A good start can be made at an international conference in Washington next month on a treaty to control emissions of heat-trapping gases. It's to be hoped that the US will overcome its previous reluctance to set emission targets for these gases, including carbon dioxide, and will join other nations to make a down payment on our planet's climatic future.