11 September 2000
Warming Trend Seen In Late Freeze, Early Thaw
by Kate Melville
A 150-year record of freeze and ice breakup dates for lakes and rivers in such far-flung locales as Wisconsin and Japan chronicles a recent climate warming trend in the Northern Hemisphere, say researchers in the 8 September issue of the international journal Science.
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 freeze and thaw data, combined with yearly records that typically stretch back further in time than air temperature data, make patterns of lake and river ice cover a unique proxy for climate change, say the researchers.
Magnuson and colleagues combed through a variety of historical records of lake and river freeze and thaw to compile the database for their analysis. These records exist for a number of reasons, including religious, cultural, and commercial purposes, said the Science authors.
At Lake Constance in Central Europe, a centuries-old record of ice cover has been linked to the transport of a Madonna figure between two churches--one in Germany and one in Switzerland--located on opposite sides of the frozen lake. Similar records exist at Lake Suwa in Japan, marking the appearance of the spring thaw that created ramparts of ice connecting male and female Shinto temples across the water.
At other sites, recorded freeze and thaw dates were important to agriculture, and to trade on the transportation corridors created by the rivers and lakes. In some cases, Magnuson said, these dates may have been noted out of pure curiosity.
"It's similar to seeing the first robin, a sign of the season changing. A river breaking up in the spring can be quite a spectacular site," said Magnuson.
Along with the later freeze and earlier thaw pattern observed in the Science study, previous research has confirmed an increase in the year-to-year variability of Northern Hemisphere freeze and ice breakup dates during the last half of the 20th century. An increase in extreme climate events like the recent powerful El Niño-La Niña cycles, Magnuson suggested, may be behind this rise in variability.
"One of the things that we are beginning to do with these types of data is to separate out the effects of different climate signals, like El Niño. We are optimistic that these long and geographically widespread ice cover records can play some role in sorting out the effects of greenhouse warming as well," said Magnuson.
Magnuson and colleagues hope to expand on this part of their analysis in future studies, taking a closer look at interannual variability, and expanding and updating the database to look at other climate oscillations. In particular, they would like to fill in a crucial gap in their study--the last five years.
"We would like to know what has happened with these ice records in the recent years, especially in light of other climate data that show a warming trend over this time period," Magnuson said.
The researchers have deposited the compiled data from their study in a publicly accessible database, with the hopes that scientists and public policy officials will incorporate the findings in future analyses.