Tracking Greenland’s melting ice with seismic waves

Greenland’s ice cap contributes nearly 20% to Earth’s mean rate of sea-level rise of 3 mm/y. To monitor the thinning ice sheet, scientists typically rely on periodic surveys taken by airborne and satellite radar, laser altimetry, and NASA’s Gravity Recovery and Climate Experiment (GRACE) mission, whose twin spacecraft orbit Earth and measure perturbations in its gravity field. MIT’s Germán Prieto, his postdoc Aurélien Mordret, and their colleagues have now adapted a seismic-wave method to do the same job. Originally developed to monitor active volcanoes and fault zones, the method exploits ambient noise signals, such as the crashing of ocean waves on the shoreline, recorded at an array of seismometers inland. (See the article by Roel Snieder and Kees Wapenaar, Physics Today, September 2010, page 44.) Seismic waves propagate at speeds that depend on the porosity of the crust and upper mantle: The more porous the rock, the slower...