Get your ice skates sharpened. At least 150 years of steady global warming in the Northern Hemisphere has dramatically shortened the outdoor rink season, a new study notes. The report, published in the September 8th issue of Science, tracks patterns of freezing and thawing ice between 1846 and 1995. As such, it is one of the largest and longest records of climate data ever assembled.

Limnologist John Magnuson of the University of Wisconsin-Madison and a team of 13 scientists compiled data on 39 lakes and rivers across the U.S., Canada, Europe, Russia and Japan by drawing on newspaper archives, transportation ledgers and religious observances. At all but one site, they found evidence of rising temperatures over 150 years, with freezing dates occurring on average 8.7 days later and breaking up dates, 9.8 days earlier. "The importance of these records," Magnuson says, "is that they come from very simple direct human observations, making them difficult to refute."

Magnuson's team at the UW-Madison Center for Limnology plans to continue this work over the course of a decade, building a database of lake and river ice information. Some of their information already dates back much further than 150 years: they know, for instance, when Lake Constance, between Germany and Switzerland, froze back to the 9th century. Whenever it did, a cleric would carry a Madonna figure over the ice from one church to another on the other side.