"Two things are happening," said study co-author William D'Andrea, a paleoclimatologist at Columbia University's Lamont-Doherty Earth Observatory. "One is you have a very gradual decrease in the amount of sunlight hitting high latitudes in the summer. If that were the only thing happening, we would expect these glaciers to very slowly be creeping forward, forward, forward. But then we come along and start burning fossil fuels and adding carbon dioxide to the atmosphere, and glaciers that would still be growing start to melt back because summer temperatures are warmer."Glaciers are dynamic and heavy. As a glacier moves, it grinds the bedrock beneath, creating silt that the glacier's meltwater washes into the lake below. The larger the glacier, the more bedrock it grinds away. Scientists can take sediment cores from the bottom of glacier-fed lakes to see how much silt and organic material settled over time, along with other indicators of a changing climate. They can then use radiocarbon dating to determine when more or less silt was deposited.