Story highlights About 12% of the massive ice sheet saw some melting, according to Danish researchers

Previous melts began in May, scientists say

(CNN) Greenland's massive ice sheet has started its annual summer melt earlier than ever before, according to stunned scientists who said they had to recheck their calculations before releasing the results.

Nearly 12% of Greenland's nearly 656,000 square miles of ice saw some melting this week, the earliest date on record for the start of the summer melt season, according to scientists with the Danish Meteorological Institute.

The previous earliest dates were all weeks later, in May, according to Denmark's polar research site, Polar Portal.

"We had to check that our models were still working properly," DMI climate scientist Peter Langen said.

To be clear -- the agency isn't saying 12% of the world's second-largest ice sheet melted away this week. Rather, researchers are saying that percentage of the ice melted just a bit, a millimeter or so, in most of the area.

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