The research team led by Osman, a geoscientist at the University of Arizona, came to Greenland to extract a 140-meter-long ice core. This core stretches nearly to bedrock and, in the gases and chemicals housed within, holds evidence of climate change over the past 2,000 years. Their analysis of the core shows that in this place during the Medieval Warm Period, a roughly 400-year phase of higher global temperatures around 1,000 years ago, the ice was growing thicker and advancing—the opposite of what it’s doing today.

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