WASHINGTON (AP) — Arctic sea ice melted this summer to the second lowest level since record-keeping began more than 50 years ago, scientists reported Thursday, mostly blaming global warming.

“This is not a random event,” said oceanographer James Overland of the U.S. National Oceanic and Atmospheric Administration. “It’s a long-term change in Arctic climate.”

The new measurements were taken by the National Snow and Ice Data Center in Boulder, Colo. It reported that the amount of ice covering the Arctic hit its lowest point late last week, covering just 1.67 million square miles. Only in 2007 was there less summer sea ice, which has been dramatically declining since scientists began using satellites to monitor melt in 1979. Other records go back to 1953.

Each summer, sea ice melts and then refreezes starting in the fall. The summer minimum is a key measurement for scientists monitoring man-made global warming. This year’s level is 36 percent below the average minimum of 2.59 million square miles.

The University of Bremen in Germany, which uses a different satellite sensor and has been monitoring levels since 2003, reported last week that this year’s sea ice actually fell below the record set in 2007.