During the summer melt season, melt water accumulates in undulations on the surface of the Greenland ice sheet. Eventually, these melt lakes drain through crevasses or moulins (tunnels under the ice sheet surface), delivering water to the bottom of the ice sheet. This melt water lubricates the interface between the ice and the bedrock, causing the ice to flow faster toward the sea during summer. As summer melt increases and more melt water is available, the greater its effect on summer ice sheet flow rates.
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