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When ice sheets melt, some of the water seeps down and refreezes. This creates a distinctive visual layer in an ice core. Because the GISP2 ice core was collected at Summit, we have a record of melt events going back thousands of years.
In fact over the last 4,000 years, the average occurrence in the ice core is about one Summit melt event every 250 years.
However, recent rapid warming in the Arctic is clearly making such rare melt events much more likely.