Cloudy Picture of a Changing Globe

Four hundred registrants from fifteen nations are expected to arrive for the International Conference on the Role of the Polar Regions in Global Change, a meeting to be held 11-15 June at the University of Alaska Fairbanks. That impressive turnout doesn't mean all scientists are convinced that global changes is an urgent problem, but the consensus is growing that change of some sort is under way, and that it will show up clearly in high-latitude regions like Alaska.

Change--specifically, a warmer planet--seems inevitable chiefly because human activity is leading to more of the so-called greenhouse gases present in Earth's atmosphere. Though scientists may argue justly about how much warming, it any, has occurred already, there's no arguing with the basic physics involved. More greenhouse gases in the air mean warming; the debate concerns only the accuracy of various computer models that specify whether counterbalancing effects will appear, how much warming will occur, when it will show up, and what its effects will be.

In the April 1990 issue of Environmental Science and Technology, I found a concise and clear list of probable and possible changes to come. The author is Stephen H. Schneider, a climatologist at the National Center for Atmospheric Research in Boulder, Colorado. Schneider is convinced that warming is under way, but even this spokesman for climate change says that the likelihood of specific changes can be given in only the most general terms.

With that understood, he tops the list with a virtually certain cooling--that's right, cooling--of the stratosphere. More carbon dioxide and other gases with similar radiative properties will increase the rate at which heat radiates out of this high layer of the atmosphere. At the same time, other human-added compounds are reducing the concentration of stratospheric ozone. Reduced ozone means reduced absorption of solar ultraviolet radiation in the upper stratosphere, which means less heating will occur there. More heat leaving and less coming in will lead to what Schneider calls "a major lowering of temperatures in the upper stratosphere."

The most probable change at the earth's surface is an increase of global mean temperature. Disagreements appear here because computer models come out with different temperatures depending on what they're told to consider, and no one quite agrees on what to tell the computers. A main source of difficulty is the effect of clouds. (This is one item with which northerners are familiar. Give us clouds in winter, and our weather warms. In summer, clouds mean chilly temperatures.) There's fair accord that Earth should now be receiving between one and two watts of extra radiant energy falling on each square meter of the planet's surface, thanks to inadvertent human meddling with the atmosphere, but there's less agreement about what that means in measurable degrees. The usually quoted figures predict a mean global increase of 1.5 to 4.5 degrees Centigrade by sometime in the next century

Also very probable is an increase in global mean precipitation, especially at high latitudes. Overall, we'll have a wetter world as well as a warmer one, although some areas may actually dry out. Soil moisture in the midlatitude wheat belts may decline, for example, because of earlier snow melt.

Given a "very probable" ranking on Schneider s list are some points of special interest to Alaskans. Reduction of sea ice is one of these; from that, he expects to see a polar winter surface warming. As the extent of arctic sea ice shifts poleward, the computer models predict dramatically warmer winter surface temperatures. The greater fraction of open water and thinner sea ice cover might lead to warming of polar surface air up to three times that of the global average. (Details like that certainly underline why global-change specialists have become keenly interested in monitoring the Arctic.)

Ranking only as "probable" is the change most beloved of headline writers: a rise in global mean sea level. If the climate is warmer, sea level is sure to go up to some extent just from the thermal expansion of the warmed sea water. Much less predictable--so far--is any increase from melting or calving ice sheets and glaciers

In Schneider's view, the only wholly wrong predictions are the extremes of "nothing to worry about" and "it's the end of the world." But he does think we are living in the midst of an uncontrolled experiment involving the whole planet, and that's an uncomfortable thought--even for people who really wouldn't mind consistently warmer winters.