| Summary: | Sea levels are currently rising, which may lead to an increased likelihood of flooding in many parts of the world, as well as a plethora of other problems. One of the biggest contributors to the rise in sea levels is the melting of the Greenland ice sheet. Large portions of the ice sheet are covered by old snow, called firn. Here, we investigate the role that firn plays on the melting of the ice sheet. Firn acts as a sponge that can absorb meltwater into its pores, which prevents it from running off into the ocean. It is thus important to understand how firn interacts with meltwater, and what may lead to an increase or decrease in firn cover. We find that the position of the polar jet stream, a band of strong westerly winds encircling the Northern hemisphere, plays a large role in determining if a region gains or loses firn. Changes in the jet stream caused firn loss in the beginning of this century, but lead to more firn after 2012. When meltwater is absorbed into the firn, it can either refreeze into layers of ice, which are called ice slabs, or large amounts of water can be stored inside the firn as a liquid. Locations of firn that hold water year-round are called firn aquifers. Ice slabs and firn aquifers impact the runoff of meltwater very differently. Our research finds that the amount of snowfall and melt determine whether ice slabs or aquifers form, and that they are likely much more widespread than previously assumed. In a warmer climate, these phenomena may appear more often and accelerate ice-sheet mass loss.
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