The study, which includes 39 records of either freeze dates or breakup dates from 1846 to 1995, represents one of the largest and longest records of observable climate data ever assembled. University limnologist John Magnuson led a team of 13 co-authors who contributed to the report, to be published in the Sept. 8 issue of the journal Science.

Sites ranges from Canada, Europe, Russia and Japan. Of those, 38 indicate a consistent warming pattern. The average rate of change over the 150-year period was 8.7 days later for freeze dates; and 9.8 days earlier for breakup dates. A smaller collection of records going well past 150 years also show a warming trend, at a slower rate.

"We think this is a very robust observation: It is clearly getting warmer in the Northern Hemisphere," says Magnuson. "The importance of these records is that they come from very simple, direct human observations, making them very difficult to refute in any general way."

Magnuson says the observational nature of the study is "both its strength and its weakness," and the results do not offer specific proof that greenhouse gases are driving the warming trend. However, the findings are consistent with computer-generated models that have been developed to estimate climate change from greenhouse gases over a 125-year time period, he says.

The findings also correspond to an air temperature increase of 1.8 degrees Celsius over the past 150 years. A temperature change of 0.2 degrees Celsius typically translates to a one-day change in ice-on and ice-off dates.

Freeze dates were defined in the study as the observed period the lake or river was completely ice-covered; the breakup date was defined as the last ice breakup observed before the summer open-water phase.

Ice records have valuable attributes for climate researchers, Magnuson says. They can be gathered across a wide range of the globe, and in areas traditionally without weather stations. Their primary weakness is that early observers did not document the methods used.

"Of course, 10,000 years ago the Midwest was covered by ice, so we know it's getting warmer," he says. "What's troubling and scary to people is that these rates in recent decades are so much faster."

Climate models have predicted a doubling of total greenhouse gases in the next 30 years or so, a change that could potentially move the climate boundaries for fish and other organisms northward by about 300 miles, approximately the length of the state of Wisconsin, Magnuson says.

The records in this study are part of a decade-long project led by Magnuson and the UW-Madison Center for Limnology to build a database of lake and river ice records from around the world. The project was supported by the National Science Foundation's Long-Term Ecological Research program, which emphasizes tracking and understanding global changes.

"It's kind of a new science, you might call it network science," Magnuson says. "We reached out to colleagues around the world and asked for these records. It turned out some people had very rich stores of data."

The records in this study represent the longest and most intact of 746 records collected through the project. Some individual records are of astonishing lengths, with one dating back to the 9th century, another to the 15th century and two more to the early 1700s.

For example, Lake Suwa in Japan has a record dating back to 1443 that was kept by holy people of the Shinto religion. The religion had shrines on either side of the lake. Ice cover was recorded because of the belief that ice allowed deities on either side of the lake -- one male, one female -- to get together.

Lake Constance, a large lake on the border of Germany and Switzerland, has a peculiar record dating back to the 9th century. Two churches, one in either country, had a tradition of carrying a Madonna figure across the lake to the alternate church each year it froze.

Two other long records come from Canada's Red and McKenzie rivers, which date back to the early 1700s and were kept because ice cover and open water were critical to the fur trade. Records from Grand Traverse Bay and Toronto Harbor, both on the shores of the Great Lakes, reflect their prominence as shipping ports.

Other records included in the study are from lakes Mendota, Monona and Geneva from Wisconsin; lakes Detroit and Minnetonka from Minnesota; lakes Oneida from New York and Moosehead from Maine; Lake Kallavesi from Finland; and the Angara River and Lake Baikal from eastern Russia.

Another finding in the study, based on the 184 ice records from 1950 to 1995, showed the variability in freeze and breakup dates increased in the last three decades. Magnuson says it might be related to intensification of global climate drivers such as the El Nino /La Nina effects in the Pacific Ocean.

Magnuson says the ecological effects of global warming are only beginning to be studied. But studies already exist that have shown the northern ranges of some butterflies and birds have been extending northward.

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