Winter ice cover on the Great Lakes has dropped dramatically over the past four decades, according to a new report. Peak ice has dropped by 71 percent on average, with Lake Michigan ice decreasing by even more.

Researchers at the National Oceanic and Atmospheric Administration compared satellite photos going back to 1973. Jia Wang, an ice climatologist with the National Oceanic and Atmospheric Administration, said the changes are stark. In a year like 1979, ice covered about 94 percent of the lakes in the dead of winter.

“This winter the maximum ice cover is about 5 percent,” Wang said. “It’s the lowest ever since the satellite era.”

The drop in ice cover is driven by rising temperatures due to climate change, There are also other factors at play this year in particular, such as La Nina weather patterns.

Wang says losing winter ice can cause a number of problems for the Great Lakes ecosystem. It can speed up wintertime evaporation from the lakes, which could reduce water levels. The trend could also fuel more and earlier algae blooms, which damage water quality and habitat. And it leaves shoreline more exposed to waves, accelerating erosion.

Wang’s findings are published in the Journal of Climate.