MADISON - From sources as diverse as newspaper archives, transportation ledgers and religious observances, scientists have amassed lake and river ice records spanning the Northern Hemisphere that show a steady 150-year warming trend.
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 of Wisconsin-Madison 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
Contact: John Magnuson
University of Wisconsin-Madison