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Report: Shorter lake and river ice seasons confirm global warming
MADISON, Wisconsin (CNN) -- Records from riverboat captains, Shinto monks and others dating to the 15th century confirm a dramatic warming trend in the Earth's recent history, scientists said Thursday.
Studying climate observations from dozens of sites in the Northern Hemisphere, an international team of researchers concluded that temperatures have risen steadily for at least 150 years.
They compiled data on lake and river ice cover from newspaper articles, business journals and individual diaries, some as far back as 1443.
Piecing together a historic portrait, the researchers said the Northern Hemisphere has experienced increasingly shorter winter seasons since 1840.
"The thing that makes this catchy is that this is a very simple way of looking at what happened over the last 150 years," said John Magnuson, lead author of the report, to be published Friday in the journal Science.
"These are direct observations of people. Some were religious people, some were fur traders," said Magnuson, a freshwater expert at the University of Wisconsin in Madison.
Clerics in Central Europe who walked a Madonna statue over Lake Constance when it first froze each season.
Fur traders and riverboat skippers in Canada who measured river ice levels.
The records, which also come from the United States, Russia and Finland, indicate that lakes and rivers now freeze an average of 8.7 days later and ice cover begins disintegrating 9.8 days earlier than 150 years ago.
The findings are consistent with an increase in air temperatures during the time of 1.8 degrees C (almost 4 degrees F). Climate records confirm a rise of at least 1 degree C (2 degrees F) over the past century.
The trend corresponds with the rise of the Industrial Revolution. Yet significant warming takes place well before its peak, suggesting other causes besides greenhouse gas emissions from human activities.
"These increases are generally consistent with scenarios for greenhouse gas-forced climate warming, but they may be related to other drivers, such as changes in solar activity," wrote Magnuson and his colleagues in Science.
Reuters contributed to this report.
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