Warm water flowing into the Arctic Ocean may be speeding the melting of sea ice, according to a new analysis of NASA satellite images. In 2012, for example, a year in which Arctic sea ice reached a record low for the past 30 years, Canada’s Mackenzie River delta was fringed with a swath of ice during late spring (light blue stripe at lower center of top panel, image taken 14 June) and open waters offshore remained near freezing (dark blue). But 3 weeks later (image at bottom), river runoff had broken through the ice barrier, warming the surface waters of a Poland-sized area of open water by an average of 6.5°C, the researchers report in Geophysical Research Letters. A combination of factors has rendered Arctic sea ice vulnerable in recent years, the team contends: The overall volume of river runoff dumped into the Arctic Ocean has grown (by more than 7% between 1965 and 2000), that water has been increasingly warm (picking up heat from the sun-warmed land before it flows into the sea), and sea ice cover has become ever thinner and more fragmented (thus making it easier to melt). Presuming that the runoff from the 72 major rivers flowing into the Arctic Ocean has an average temperature of 5°C, the amount of energy transported to the sea each and every year is equivalent to the amount of electric power used by the entire state of California in 50 years at today’s usage rates, the researchers estimate.