150 Year Record of Freezes Shows Warming Trend

By Cat Lazaroff

MADISON, Wisconsin, September 8, 2000 (ENS) - A 150 year record of freeze and ice breakup dates for lakes and rivers in locales as far apart as Wisconsin and Japan chronicles a steady warming trend.

The report offers more evidence that the Earth is experiencing a period of global climate change, said the 13 coauthors of the study, which appears today in the journal "Science."


Ice breakup on Lake Mendota in Madison, Wisconsin during the spring of 2000. Lake Mendota is one of the lakes with 150 year ice formation and breakup records included in the journal "Science" study. (Photo by John Magnuson, three photos courtesy University of Wisconsin-Madison)
From sources as diverse as newspaper archives, transportation ledgers and religious observances, the scientists gathered lake and river ice records spanning the Northern Hemisphere.

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.

The team's method of gathering information stands in contrast to the many global warming studies that draw their conclusions from computer modelling, yet the study's findings are consistent with those of computer generated models.

The group studied sites ranging from Canada, Europe, Russia, Japan and the United States. 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 ice breakup dates.

A smaller collection of records going back in time past 150 years also show a warming trend, though at a slower rate.

University of Wisconsin-Madison John Magnuson led the team that compiled the report. Magnuson is a limnologist - a scientist who specializes in the study of bodies of fresh water such as lakes and rivers.


John Magnuson, professor of limnology, sitting along the Lake Mendota shoreline (Photo by Jeff Miller)
"We think this is a very robust observation. It is clearly getting warmer in the Northern Hemisphere," said 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 said 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.

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 said.

Greenhouse gases which trap the heat of the sun near the surface of the Earth are generated by the burning of coal, oil and gas. These fuels powered the factories of the Industrial Revolution that spread across Europe and North America from the early 1800s.

The temperature increase could be related to other "drivers" of climate change, such as fluctuations in solar activity, as well.

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 freeze and ice breakup 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 before the summer open water phase.


Candled ice during breakup of Lake Kallevesi in Finland during the Spring of 1997 (Photo by Esko Kuusisto)

Ice records are valuable to climate researchers, Magnuson said. They can be gathered across a wide range of the globe, and in areas traditionally without weather stations. Their primary weakness is that early observers did not document the methods used.

"Of course, 10,000 years ago the Midwest was covered by ice, so we know it's getting warmer," said Magnuson. "What's troubling and scary to people is that these rates in recent decades are so much faster."

Climate models have predicted a doubling of total greenhouse gases in the next 30 years or so, a change that could potentially move the climate boundaries for fish and other organisms northward by about 300 miles, about the length of the state of Wisconsin, Magnuson said.

"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 records in this study are part of a decade long project led by Magnuson and the University of Wisconsin-Madison Center for Limnology to build a database of lake and river ice records from around the world. The project was supported by the National Science Foundation's Long-Term Ecological Research program, which emphasizes tracking and understanding global changes.

"It's kind of a new science, you might call it network science," Magnuson said. "We reached out to colleagues around the world and asked for these records. It turned out some people had very rich stores of data."

The records in this study represent the longest and most intact of 746 records collected through the project. Some individual records are of astonishing lengths, with one dating back to the 9th century, another to the 15th century and two more to the early 1700s.

For example, Lake Suwa in Japan has a record dating back to 1443 that was kept by holy people of the Shinto religion. The religion had shrines on either side of the lake. Ice cover was recorded because of the belief that ice allowed deities on either side of the lake - one male, one female - to get together.


The researchers found ice records from Lake Constance dating back to the 9th century (Photo courtesy International Lake Environment Committee)

Lake Constance, a large lake on the border of Germany and Switzerland, has a record dating back to the 9th century. Two churches, one in either country, had a tradition of carrying a Madonna figure across the lake to the alternate church each year it froze.

Two other long records come from Canada's Red and McKenzie rivers, which date back to the early 1700s and were kept because ice cover and open water were critical to the fur trade. Records from Grand Traverse Bay and Toronto Harbor, both on the shores of the Great Lakes, reflect their prominence as shipping ports.

Other records included in the study are from lakes Mendota, Monona and Geneva from Wisconsin; lakes Detroit and Minnetonka from Minnesota; lakes Oneida from New York and Moosehead from Maine; Lake Kallavesi from Finland; and the Angara River and Lake Baikal from eastern Russia.

Another finding in the study, based on the 184 ice records from 1950 to 1995, showed that the variability in freeze and breakup dates have increased in the last three decades. Magnuson says it might be related to intensification of global climate drivers such as the El Nino /La Nina effects in the Pacific Ocean.

"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 Niņo. 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.


The investigators also found lengthy records from Lake Baikal in eastern Russia, the deepest, oldest and largest freshwater lake on earth (Photo by Mark Cherrington, courtesy Earthwatch Institute)
Magnuson says the ecological effects of global warming are only beginning to be studied. But studies already exist that have shown the northern ranges of some butterflies and birds have been extending northward.

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 hope that scientists and public policy officials will incorporate their findings in future analyses of global warming.