WASHINGTON (AP) — Crucial Arctic sea ice this summer shrank to its second lowest level on record, continuing an alarming trend, scientists said Tuesday.

The ice covered 1.74 million square miles on Friday, marking a low point for this summer, according to NASA and the National Snow and Ice Data Center in Boulder, Colo. Last summer, the sea ice covered only 1.59 million square miles, the lowest since record-keeping began in 1979.

Arctic sea ice, which floats on the ocean, expands in winter and retreats in summer. In recent years it hasn't been as thick in winter.

Sea ice is crucial to worldwide weather patterns, both serving as a kind of refrigerator and reflecting the sun's heat. Given recent trends, triggered by man-made global warming, scientists warn that within five to 10 years the Arctic could be free of sea ice in the summer.