For years, climate scientists have been predicting a grim forecast for the world’s ice sheets. Now that dire forecast is coming true. The ice sheets in Greenland and Antarctica are melting at a rate consistent with scientists’ worst-case scenarios, raising the global sea level by 0.7 inches over the past 20 years. If this pace continues sea levels could rise another 6.7 inches.

Tom Slater, a climate researcher at the Centre for Polar Observation and Modelling at the University of Leeds, is the lead author of a new study in Nature Climate Change. According to Slater, “Although we anticipated the ice sheets would lose increasing amounts of ice in response to the warming of the oceans and atmosphere, the rate at which they are melting has accelerated faster than we could have imagined. The melting is overtaking the climate models we use to guide us, and we are in danger of being unprepared for the risks posed by sea level rise.”

The melting of ice sheets and glaciers has been the primary cause of rising oceans over the past five years.

Anna Hogg, the study’s co-author, said, “If ice sheet losses continue to track our worst-case climate warming scenarios we should expect an additional 17 cm [6.7 inches] of sea level rise from the ice sheets alone. That’s enough to double the frequency of storm-surge flooding in many of the world’s largest coastal cities.”