Ice records reveal warming trend

Lake Kallavesi in Finland as the ice breaks up

By environment correspondent Alex Kirby

Researchers have found records showing a clear warming trend in the northern hemisphere stretching back at least 150 years.

They say the documentary evidence they have unearthed offers robust support for believing that the climate is changing.

Their evidence bears out computer models of climate change over roughly the same period.

And they also found even earlier records which suggest that the warming trend set in much earlier than the mid-nineteenth century.

The leader of the team, whose work is reported in the journal Science, is Professor John Magnuson, a limnologist, of the University of Wisconsin-Madison.

The researchers examined records from a variety of sources, including newspaper archives, transport ledgers and religious groups. The records came from Japan, Russia and other parts of Europe, and Canada.

Consistent pattern

The study details 39 records of either freeze dates or break-up dates, from 1846 to 1995.

Ice break-up on Lake Mendota in Wisconsin

Freeze dates are defined as the observed period when a lake or river was completely ice-covered, and break-up dates as the last ice break-up seen before the summer open-water phase.

Of the 39 records, 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 break-ups.

The researchers believe the study covers one of the largest and longest records of observable climate data ever assembled.

Professor Magnuson said: "We think this is a very robust observation. It is clearly getting warmer in the northern hemisphere.

"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."

He acknowledged that the results did not prove that greenhouse gases were responsible. But he said they were consistent with the models developed to estimate climate change from the gases over a 125-year period.

The findings also correspond to an air temperature increase of 1.8 degrees Celsius over the past 150 years. A change of 0.2 degrees C typically translates to a one-day change in ice-on and ice-off dates.

Methods unknown

Professor Magnuson said ice records were valuable for researchers in several ways - they could be gathered across a wide stretch of the world, and in areas traditionally without weather stations.

Their main weakness was that early observers did not document the methods they were using.

Professor John Magnuson

The records he and his team examined are part of a project to build a database of lake and river ice records from around the world.

They represent the longest and most intact of 746 records collected, some of them far older.

Lake Suwa in Japan has a record dating back to 1443, kept by members of the Shinto religion. They kept records because they believed that the ice allowed deities on either side of the lake to meet.

Greater variability

Records on Lake Constance, bordering Germany and Switzerland, date from the ninth century. Two churches, one in each country, had a tradition of carrying a statue of the Madonna across the lake to the other every year it froze.

The study also found, on the basis of the 184 records from 1950 to 1995, that the variability in freeze and break-up dates had increased in the last three decades.

Professor Magnuson said this might be related to the intensification of global climate drivers such as El Nino and La Nina in the Pacific.

Photos courtesy of the University of Wisconsin-Madison