What caused last summer’s Greenland surface melt?

Data image showing extreme Greenland surface melt in summer 2012

These satellite-derived maps show the extent of surface melt over Greenland’s ice sheet during the summer of 2012. On July 8, about 40 percent of the ice sheet had melted at or near the surface. By July 12, nearly 97 percent of the ice sheet surface had melted. High pressure ridges that persisted over Greenland created a lingering heat dome that caused the extensive melt. Credit: Nicolo E. DiGirolamo, SSAI/NASA GSFC, and Jesse Allen, NASA Earth Observatory

During the summer of 2012, scientists noticed something unusual in Greenland. On July 8, about 40 percent of the Greenland Ice Sheet surface showed signs of melting. But a mere four days later, 97 percent of the surface was melting, an extent that was unprecedented in the satellite record.

Just how unprecedented was this event? Does it mean that the Greenland Ice Sheet is possibly on the way out, like summer sea ice in the Arctic? Continue reading

The Arctic Oscillation, winter storms, and sea ice

The Arctic Oscillation (AO) is a large-scale climate pattern that influences weather throughout the Northern Hemisphere. It alternates between a positive phase (left) and a negative phase (right). In its positive phase the AO tends to bring warmer weather to the middle latitudes, while in its negative phase, winter storms are more common in the Eastern United States and Europe. Credit: J. Wallace, University of Washington.

Last year, many scientists blamed the winter storms that blasted the Northeastern United States and Europe on the negative mode of a weather pattern called the Arctic Oscillation. This winter, the Arctic Oscillation started out in the opposite mode, which scientists connect to the warmer-than-average temperatures and unusually low snowfall over much of the U.S. The swings of the Arctic Oscillation also help control how sea ice moves in the Arctic Ocean, which is of great interest to climate scientists. Readers often write in to ask us about this powerful but mysterious climate phenomenon, and how it affects weather where they live. What is the Arctic Oscillation, and how does it affect Arctic sea ice and the rest of the Northern Hemisphere? Continue reading

An Arctic hurricane?

This satellite image from November 8 shows the hurricane-like storm that hit Western Alaska earlier this month. Credit: NASA Earth Observatory image created by Jesse Allen, using data obtained from the Land Atmosphere Near real-time Capability for EOS (LANCE).

On November 8 and 9, a strong storm hit the Western Alaska coast, bringing blizzard conditions, storm surge of up to 10 feet and wind gusts as fast as 93 miles per hour. Along the Western Alaskan coastline, towns and villages prepared for the worst. “Up here, cities are much more sparse, but a storm like this still impacts the people that live there,” said Kathleen Cole, an ice forecaster at the National Weather Service. Damage reports after the storm indicated extensive flooding, wind damage to buildings, as well as power outages, which led to many evacuations to higher ground and to shelters with generator power. Some reports referred to the storm as a “blizzicane,” or an Arctic hurricane. What was unusual about this storm—and was there any connection to changes in the Arctic climate? Continue reading