WASHINGTON (AP) -- In a study that adds weight to global warming theories, an analysis of freeze and thaw records for lakes and rivers in the northern hemisphere shows the Earth's temperature has risen steeply over the last 150 years.
Researchers report Friday in the journal Science that the annual freeze of 26 bodies of water in North America, Asia and Europe shifted about 8.7 days later over the last century and a half, while the ice breakup dates have occurred about 9.8 days earlier.
''The strength of this paper is the robust nature of direct human observations,'' said John J. Magnuson, a researcher at the University of Wisconsin, Madison, and first author of the study. The results, he said, ''are not calculations,'' which are subject to bias and instrument error, but ''direct human observations of a 150-year trend of ice freeze and thaw'' that are difficult to refute.
''It is clearly getting warmer in the northern hemisphere,'' he said. ''This is very strong evidence of a general warming from 1845 to 1995 in areas where there is ice cover.''
Other researchers said the ice findings are consistent with recent instrumented temperature readings and tend to support the idea that the Earth is getting warmer. Some scientists have said readings taken by temperature gauges and by satellites are subject to interpretation errors.
''This provides independent evidence that the warming we have seen over the 20th century is real,'' said David R. Easterling, chief scientist at the National Climatic Data Center, an archive center of the National Oceanic and Atmospheric Administration.
Temperature trends are a part of the ongoing research to determine if the burning of fossil fuels, such as coal and oil, is causing global warming. The theory is that carbon dioxide and other gases added to the atmosphere by industry and transportation are trapping heat from the sun and causing the Earth to warm up.
Some scientists have contended the warming is not real or that it is part of a natural cycle unaffected by human actions.
Magnuson said his study does not address the cause of the warming trend, but clearly shows it is occurring.
The change in the ice-on and ice-off days found in the study corresponds to an air temperature warming of about 3.24 degrees F over the 150-year period, he said. An average temperature rise of just a third of a degree is enough to change the icing and deicing dates by one day, the researchers said.
The data was compiled from records kept at lakes or rivers in Canada, Finland, Switzerland, Siberia, Japan and at 14 sites in five northern U.S. states.
Some of the records date back far longer than the 150 years studied.
The oldest records are those kept in two churches on the shore of Lake Constance on the Swiss-German border. Since about the 9th century, there has been a tradition of carrying a Madonna figure from one church to the other on the day that the lake froze enough to walk across. A year later, following freeze up, the figure was returned to the alternate church.
In Japan, ice records have been kept since 1443 at a shrine on Lake Suwa. Shinto religious leaders believe male and female deities separated by the lake are united when it freezes over.
Lake and river ice records long have been important in Canada and the northern United States because the bodies of water often were used to transport people and commercial goods. The main U.S. lakes included in the study were Mendota, Monona and Geneva in Wisconsin; Detroit and Minnetonka in Minnesota; Oneida in New York, and Moosehead in Maine.
Scott Collins of the National Science Foundation, which funded the study, said it ''is important because it humanizes the impacts of global environmental change by using a simple measurement that is relevant and meaningful to the public.''
On the Net:
Science magazine release: http://www.eurekalert.org/news.pub.html
University of Wisconsin pictures: http://www.news.wisc.edu/newsphotos/lakeice.htm
University of Wisconsin release: http://www.news.wisc.edu
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