From sources as diverse as newspaper archives, transportation ledgers and religious observances, scientists have amassed lake and river ice records spanning the Northern Hemisphere that show a steady 150-year warming trend. 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.
Sites ranges from Canada, Europe, Russia and Japan.
A smaller collection of records going well past 150 years also show a warming trend, at a slower rate. 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. The findings also correspond to an air temperature increase of 1.8 degrees Celsius over the past 150 years. Ice records have valuable attributes for climate researchers, Magnuson says. They can be gathered across a wide range of the globe, and in areas traditionally without weather stations.