'Ice on, ice off' and climate

Five years ago this fall, a small international group of scientists gathered in a rustic cabin at a research station in northern Wisconsin to ask a question: Could records of freezing and thawing on lakes, ponds, and rivers be used to track changes in climate?

In the current issue of the journal Science, the 14-member team answer the question with a resounding yes. They report that freeze-thaw records from 38 sites around the Northern Hemisphere show an increase in average air temperatures of 1.8 degrees Celsius during the past 150 years.

From 1846 to 1995, the lakes and rivers studied froze an average 8.7 days later at the end of the period, while thaw dates came nearly 9.8 days earlier than at the beginning. The researchers say that a change of 0.2 degrees Celsius results in a one-day change in freeze and thaw dates. Historical records came from a variety of sources, including monasteries, newspapers, and shipping records.

The results themselves show no surprises, acknowledges John Magnuson, a University of Wisconsin limnologist who led the effort. The temperature increase recorded in the "ice-on, ice-off" data generally agrees with other measures and is consistent with computer models of human-induced greenhouse warming.

Yet the approach's simplicity and its reliance on direct observation led to what Dr. Magnuson calls "very robust" observations. The technique could be used, he says, to track temperature changes in regions where standard meteorological stations are sparse. The method also could be used to add historical depth to records in regions where formal temperature statistics don't reach back very far. In one case, that of Lake Constance along the border between Germany and Switzerland, "ice-on, ice-off" data stretch back to AD 900, because the information was vital to performing an annual religious ceremony that involved carrying a statue of Mary across the lake.

Magnuson notes that, faced with budget cuts in Canada, scientists there are experimenting with a program to train volunteers in rural areas to report "ice-on, ice-off" data in place of high-tech climate sensors.

(c) Copyright 2000. The Christian Science Publishing Society

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