Lakes, Rivers Analyzed for Warming
Copyright © 2000 The Associated Press
September 8, 2000
By PAUL RECER, AP Science Writer
WASHINGTON (AP) - The Earth may have warmed by an average of more than 3 degrees over the past 150 years, according to an analysis of the freeze and thaw records for lakes and rivers in the Northern Hemisphere.
In a study that adds fresh support to the theory of global warming, researchers say that the annual freeze of 26 bodies of water in North America, Asia and Europe shifted later by about 8.7 days over the last century and a half, while the spring ice breakup came an average of about 9.8 days earlier.
The study appears Friday in the journal Science.
``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.''
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 Fahrenheit over the 150-year period, said Magnuson. An average temperature rise of just a third of a degree is enough to change the icing and de-icing dates by one day, the researchers said.
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.
Data for the ice study were 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. Following the freeze up a year later, the figure is 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.''