Global warming - is it real?


Why Files:

Tools of climatology
Global Warming
Polar Science
El Nino
Cooling climate?
Frequent Floods


RIGHT: Practically out our window: here's a view of the ice breaking up on Lake Mendota, in Madison, Wis., one of the most studied lakes in the world.
Courtesy John Magnuson





















In the northern hemisphere, records of at least 150 years show that ice forms later, and disappears earlier, at many locations.
Courtesy John Magnuson

POSTED 11 SEP 2000 New evidence for a century-long warming trend comes from a meticulous combing of historical records from across the northern Hemisphere. The results depend on human observation rather than thermometers, and while they focus on water, they give a good estimation of trends in climate.

Slabs of ice are being pushed toward shore, producing a scaly appearance near the lake shoreFor a decade, University of Wisconsin-Madison lake expert John Magnuson has been asking colleagues for data on when lakes and rivers froze and melted. Today, he published the results of a giant collaborative research effort in Science.

Magnuson assembled 746 records, each representing long-term observations of ice dates from one lake or river. From them, he selected 39 records for the arrival and departure of ice cover which spanned the 150-year period 1846-1995.

Almost unanimous
Fully 38 of those records showed a shortening in ice coverage: On average, at the end of the 150-year period, freezing occurred 8.7 days later, while melting occurred 9.8 days sooner. When translated to air temperature, that indicates a warming of 1.8 degrees Celsius.

While the records were not predominantly from polar regions, other information demonstrates considerable warming near the poles:

Ice has been melting at the North Pole in summer.

Claire Parkinson of Goddard Space Flight Center has calculated that the area of Arctic ice has been shrinking 0.25 percent per year since the 1970s.

Some scientists have estimated that Arctic air temperatures have soared 6 degrees Celsius over the past 30 years.

Scientists have noted major melting in Antarctic ice.

Arrows show the size of major changes in ice dates.

Ice is nice
Ice records, whether from the Northern Hemisphere or the poles, are not perfect, since the definition of what's "ice-free" or completely iced over may vary, even at a particular site. And while the records do not pin down the cause of the warming, they constitute more data for climatologists, most of whom believe that the globe is warming due to an increase in atmospheric greenhouse gases.

No-one questions that greenhouse gases reflect heat back to Earth, but skeptics have argued that global warming theory amounts to scare-mongering that relies more on computer models than on data.

Since the thermometer was not invented until the early 1700s, thermometer records are young. In contrast, ice records have longer legs, since many are driven by factors more compelling than science -- like commerce and religion.

Some of the oldest records, dating to 1443, came from Lake Suwa in Japan. It would seem that holy people of the Shinto religion, who maintained shrines on opposite shores, believed ice allowed their deities -- one male, one female -- to cross the lake and meet.

Missing data!
Sorry. We didn't learn what happened next. But moving from the sacred to the mundane, records dating to the early 1700s from Canada's Red and McKenzie rivers reflect the importance of ice cover and open water to the fur trade.

Like the recent records, the older records showed a warming trend, although a more gradual one.

Overall, the records substantiate the picture of recent climate, says Magnuson. "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."

Magnuson says the observational nature of the study is "both its strength and its weakness." While the results do not prove that greenhouse gases are driving the warming trend, he adds that they are consistent with computer models.





Historical Trends in Lake and River Ice Cover in the Northern Hemisphere, John Magnuson et al, Science, 8 Sept. 2000,

Open Water at Pole Not Surprising, Experts Say, John Noble Wilford, The New York Times, Aug 29, 2000, p. 1, science section.





Credits | Feedback | Search