Puzzles in the Greenhouse

As interior Alaska basked in a splendid September, people praised the skies: "Thank heaven for the greenhouse effect---this weather is wonderful!" Sometimes these rejoicings came from the same people who swore last winter was the worst they'd ever seen.

Meanwhile, residents of Anchorage were deciding whether to settle for gumboots or put on swim fins just to get to the car. They could note that if Alaska was supposed to get drier as the greenhouse effect takes hold, somebody must have left the greenhouse door open. Some of these people had earlier complained about having to water gardens suffering in a hot dry spell.

So Alaskans may well ask: Just what is going on here? Is there a greenhouse effect? Is it doing anything? To which scientists could answer, in order: Nobody yet knows; of course; and Probably, but it's too soon to tell.

That, at least, is how I translate a recent review article in the journal Nature. Author John E. Walsh, who is in the department of atmospheric sciences at the University of Illinois in Urbana, considered evidence from the Arctic for the coming of the greenhouse.

Walsh assumed all his readers would know that the greenhouse effect is real. Certain gases, such as methane and carbon dioxide, let the sun's heat enter the atmosphere and warm the planetary surface. They do not let Earth's heat radiate so easily back out to space. Thus they act like the glass in a greenhouse.

But, as home greenhouse builders know, a greenhouse constructed with double-pane glass holds heat far better than one covered with thin plastic. Similarly, higher percentages of green-house gases in the atmosphere make a better heat trap. More of those gases are present now than at any time since the Industrial Revolution, but the climatologists' computer models depend upon concentrations about twice present levels. If Earth really were a greenhouse, so far we've made the plastic thicker.

Then too, how well a greenhouse holds heat depends in part on how many jugs of water and pots of soil it contains. When the jugs are oceans and the pots are continents...the difficulty in finding answers to the first and third questions above becomes more obvious. A whole planet is a most complicated greenhouse.

Nevertheless, scientists have been able to develop working global climate models. These generally agree that the warmer climate from the greenhouse effect will first appear in the Arctic.

Walsh considers what has appeared so far, and what it might mean. Some studies do document a warmer Arctic. Sea ice northeast of Greenland was thinner in 1987 than in 1976, and the Arctic Ocean's ice pack retreated northward between 1978 and 1987, decreasing the ice-covered area by a little over two percent.

The subarctic has been warmer, too. Some lakes in central Canada warmed up by several degrees and their ice-free season grew longer by several weeks between the late 1960s and late 1980s. Alaska's snow cover melted off about two weeks earlier in the 1980s than in the 1940s and 1950s. The snow-covered area of all the Northern Hemisphere's continents was smaller in 1990 than at any time since 1972, when homogeneous snow data first became available.

Add to those details the half-degree increase in global mean surface temperature over the past century, and it would seem greenhouse heating has arrived. But there are other details. Global temperatures, for example, increased from about 1900 through the 1940s, then declined through the mid-1960s, and have gone up again since then.

Furthermore, while Alaska and northern Asia are apparently warmer, other parts of the north are cooler. The Labrador Sea, southern Greenland, and northernmost Europe are chillier now than in the 1960s. Are these arguments against the greenhouse or are they anomalies caused by things (like E1 Ninos) not in the climate models? Nobody knows---yet.

Walsh concludes, "the Arctic will bear close watching over the next decade or two." That we can do, and from ringside seats.