The highest point in the Greenland ice sheet recorded rainfall last week for the first time in history, another alarming sign of a hot ice sheet melting at an increasing rate, scientists said Friday.

“This is not a healthy sign of an ice sheet,” said Indrani Das, a glaciologist at the Lamont-Doherty Earth Observatory at Columbia University. “The water in the ice is bad … it’s more likely to melt the ice sheet.”

Water is not only warmer than normal snow, but also darker – so it absorbs more sunlight than it reflects.

This molten water flows into the ocean, causing sea level to rise. Scientists estimate that the melting of the Greenland ice sheet – the world’s second largest after Antarctica – has caused a 25% increase in global sea level in recent decades. That share is expected to grow as global temperatures rise.

Scientists at the U.S. National Snow and Ice Data Center said it rained for hours on Aug. 14 at an altitude of 3,216 meters above the ice sheet, where it had been freezing for about nine hours.

Rainfall and high temperatures caused widespread melting across the island, causing the loss of surface ice on August 15, an average of seven times by mid-August.