The study, conducted by an international research team and led by John J. Magnuson of the University of Wisconsin in Madison, analyzed historical records of annual first freeze and first thaw events observed for 39 time series from 26 lake and river sites in the United States, Canada, Finland, Switzerland, Russia, and Japan.
The researchers found consistent evidence of later freeze and earlier breakup of ice on these waterways over the 150-year span (1846-1995) covered by these historical records. During this interval, the date of first freeze was postponed by an average of 9.8 days, while thaw occurred an average of 8.7 days earlier.
These changes correspond to a 1.8 degrees Centigrade increase in air temperature over 150 years, the researchers calculated.
A few of the study's longer-term records, from Finland, Japan, and Siberia, suggest that the trend toward a shrinking period of ice cover may have been under way as early as the 16th century, although the trend seems to have accelerated in the last 150 years.
This temperature increase is consistent with a scenario of global warming caused by greenhouse gases, according to the authors, but could be related to other "drivers" of climate change, such as fluctuations in solar activity, as well.
"This is exciting as a climate indicator because it's a simple, direct measure of climate change that humans can relate to," said Magnuson.
"Climate change can be relatively abstract, but when these changes are easily observed in places as familiar as a nearby lake or river, they become more relevant."
The broad geographical range of sites with easily measured
Contact: Ginger Pinholster
American Association for the Advancement of Science