The Arctic spring thaw has begun with a bang, with extensive melting of the Greenland ice sheet and sea ice loss that is already several weeks ahead of normal, scientists said.

A stagnant zone of high-pressure air over Greenland last week brought warm air from the south, raising temperatures as much as 40 degrees Fahrenheit above normal. That, coupled with cloudless conditions, led to a pulse of melting across much of the ice sheet surface.

Melting last Wednesday was the most extensive, at 275,000 square miles, or about 45 percent of the surface, said Marco Tedesco, a geophysicist at the Lamont-Doherty Earth Observatory, which is part of Columbia University.

That represents a record early date for such extensive melting, which has been measured by satellites since 1979. In 2012, an exceptionally warm year in the Arctic, there was slightly greater melting a few days later in June .