Monitoring the health of ice sheets is important for working out just how much sea levels are going to rise in the years ahead. Unfortunately, it is also very difficult. Traditionally, researchers have had to fly over ice sheets and reflect a laser off of them to calculate their thickness. This is only possible a few times a year and does not grant a very detailed image of how the sheets are changing. Satellites have shown some promise here too but there are not many that are equipped to do the job and, at their best, they can can only pass over the same ice sheet every few months. More frequent monitoring would be helpful and now a team led by Aurélien Mordret at the Massachusetts Institute of Technology is suggesting that it has found a way to accomplish this by using the vibrations produced by ocean waves.
A glacier that was collapsing during my trek in Alaska in 2010.
While people can only feel the crashing of ocean waves when they are standing along the coast, many sensors are capable of detecting these vibrations thousands of miles inland. More importantly, the seismic waves generated by the ocean propagate through the crust at speeds that depend on how much pressure the crust is under. Glaciers and ice sheets put a lot of pressure on the crust beneath them and this alters the speed at which the sound of crashing waves travels.
Dr Mordret questioned whether he might be able detect the changing size of ice layers by studying the vibrations of ocean waves traveling through the crust on a daily basis and decided to test this out in Greenland. He and his colleagues looked through seismic data collected from January 2012 to January 2014 by a small seismic sensor network situated on the western side of Greenland’s ice sheets. While the sensors were put into place to study earthquakes, they proved sensitive enough to detect ocean wave vibrations.
The researchers speculated that the speed of ocean wave movement through the crust would reflect the volume of the ice sitting above and, when they compared the data that they collected to data on ice sheet size collected by NASA's GRACE satellites, they found that their measurements mirrored the observations made by the satellites almost perfectly.
Given that we currently have no way of looking at melting in high resolution over short time periods, like the shift from summer to winter, this technique is one that should be seriously considered in regions that need close monitoring.
This research published in Science Advances in May, 2016. A link to my coverage of these findings in The Economist can be found here.