Scientists concerned about climate change are looking for Earth's climatic ''switch.''
Recent research shows that climate can shift between warm and cold modes rapidly and repeatedly. These sudden changes occur within a human lifetime, sometimes within as short a period as a single year.
''It almost acts as if there's a switch in the system somewhere,'' says climatologist Richard Alley of Pennsylvania State University at University Park. He explains that ice cores from Greenland, which reflect the global climate, show the following sequence:
''You go along, and it's cold, and it's dusty, and it isn't snowing much. Then it goes warm-cold, warm-cold, warm. And then it sits warm for 1,000 years. And then it goes warm-cold, warm-cold ... then it sits cold for 1,000 years.''
Present assumptions rosy
As presented by Dr. Alley and other scientists during the recent annual meeting of the American Association for the Advancement of Science, these findings highlight the poorly understood complexities of the climate system that make climate forecasting difficult. They also challenge the complaisant notion that our present climate is relatively benign.
Alley bases his findings on examinations of ice ''records'' in Greenland. He is part of an American consortium that, together with a European consortium, drilled two cores where the ice is about 3 kilometers thick. They can identify annual layers in these cores that mark off the years, like tree rings. The chemical composition of the ice depends partly on temperature. Trapped bubbles contain samples of ancient air. Dust residues reflect wind patterns.
In short, Alley says, the ice cores ''serve as ... a weather station'' whose records go back over many tens of thousands of years. Furthermore, he adds, there is worldwide evidence that they reflect global climate changes.
These cores reveal a few large, rapid climate fluctuations over much of the past 100,000 years. Alley explained that by ''large,'' he means swings of 7 or 10 degrees C (15 degrees F.), a factor of 2 or 3 in snowfall, and a factor of 10 to 100 in atmospheric dustiness. By rapid, he means changes of that magnitude that take place within a few decades or less -- or in ''as little as a year sometimes.''
It's true that there have been few large, rapid fluctuations during the past 10,000 years. However, Alley warns there are signs that ''we are not necessarily immune from such changes.''
This implies that until the climate system is better understood, it will be hard to predict any changes attributable to man-made global warming.
It's a system in which air, sea, land, and ice interact in complex ways. ''We are trying to make predictions based on a set of physics'' of how that system works, explains oceanographer Nick Pisias of Oregon State University at Corvallis. But, he adds, ''are those physics ... described properly'' in the forecasters' computer-based climate models?
To find out, says Yale University's Jeffrey Park, scientists are ''going to historical climate data and trying to find variability ... that makes itself known.''
Searching the depths
Other scientists, such as geophysicist Gerard Bond of Columbia University's Lamont-Doherty Earth Observatory near New York, look to deep sea sediment cores. Dr. Bond told how two cores taken between 60 degrees and 50 degrees latitude in the subpolar North Atlantic ''record'' climate back into the last two or three glacial cycles. They show the same fluctuations that Alley reported.
Yet other scientists, such as Oregon State's Pisias are studying a variety of geological records that span the last 6 million years. The point, Pisias explains, is to try ''to understand the sensitivity of the climate system to external forces'' known to be operating over that period of time. These forces include changes in Earth's orbit as well as dust from major volcanic eruptions.
He adds that it's interesting ''that, during the past million years, the climate has been extremely sensitive'' -- more so than in the more distant past. These are the kind of mysteries climatologists must unravel to put climate forecasting on a solid basis.