Dr. Meier said that during the winter much of the ice near the edges of the ice sheet covering the Arctic is thin and seasonal, whereas some of the older and thicker ice melts during the summer. The differences between these seasonal processes mean winter ice cover does not help predict how much ice there will be by the end of summer. “When you lose summer ice you aren't really just losing it for that year, you're also losing some ice from many years ago,” he said. “That makes it harder for things to go back towards normal.”