Accelerating ice loss from the fastest Greenland and Antarctic glaciers

Ice discharge from the fastest glaciers draining the Greenland and Antarctic ice sheets – Jakobshavn Isbrae (JI) and Pine Island Glacier (PIG)– continues to increase, and is now more than double that needed to balance snowfall in their catchment basins. Velocity increase probably resulted from decreased buttressing from thinning (and, for JI, breakup) of their floating ice tongues, and from reduced basal drag as grounding lines on both glaciers retreat. JI flows directly into the ocean as it becomes afloat, and here creep rates are proportional to the cube of bed depth. Rapid thinning of the PIG ice shelf increases the likelihood of its breakup, and subsequent rapid increase in discharge velocity. Results from a simple model indicate that JI velocities should almost double to >20 km a−1 by 2015, with velocities on PIG increasing to >10 km a−1 after breakup of its ice shelf. These high velocities would probably be sustained over many decades as the glaciers retreat within their long, very deep troughs. Resulting sea‐level rise would average about 1.5 mm a−1. Key Points Progressive increase in ice discharge from fastest Greenland/Antarctic glaciers Key imortance of floating ice shelves to future behavior of many similar glacier Likelihood of continued, very large increases in ice discharge