Good vibrations
POUR some water into a partly full bathtub and the level in the tub will rise. Calculating how much it will rise is straightforward, as long as the surface area of what was already there and the amount being added are known. The same should apply to working out how much the sea level will rise as the worlds ice sheets melt in response to rising global temperatures. But in practice it is not that easy. Though geographers know the surface area of the oceans, measuring how the masses of the worlds ice sheets are changing has proved hard. Aurelien Mordret of the Massachusetts Institute of Technology, however, thinks he has found a way to make it simpler. He proposes to do it using the vibrations created by ocean waves. At the moment, geographers monitor the mass of ice sheets in two ways. One employs aircraft to fly over such sheets and reflect laser beams from their surfaces, to record their topography. The other uses satellites to track gravitational fluctuations caused by variations in the amount of ice present. Both techniques work, but both have limitations. Laser overflights create high-resolution images but are expensive, so can be done only a few times a year. Satellites pass overhead more often, but generate fuzzier pictures. Dr Mordret, however, knew from work conducted by other research groups that vibrations created by waves crashing onto the shore are transmitted inland through the Earths crust, sometimes travelling thousands of kilometres from the coast. These vibrations can be picked up by seismometers of the sort used to monitor earthquakes. He also knew that the speed at which the vibrations propagate varies with the amount of pressure being exerted on the crust by mountains or glaciers sitting above. That gave him his idea. Propagation speeds vary because many types of rock have small voids within their structure. These lower the velocity of passing vibrations. The greater the proportion of a rocks volume that is void, the more slowly vibrations will travel through it. If a piece of rock is dry, compressing it will shrink the voids and speed the vibrations up. Water, though, is famously incompressiblemore so than most rock-forming minerals. If the voids are filled with water, they will thus resist compression. So, putting pressure on wet rock increases the relative volume of the voids, which slows down any passing vibrations. Since an icy overburden will both fill the voids with water and compress the rock, Dr Mordret reasoned that changes in the speed of transmission in rocks under ice sheets will reflect the thickness of the overlying sheet. To test this idea, he and a team of colleagues set up an experiment which made use of existing earthquake sensors at seven monitoring stations in western Greenland, an island almost completely covered by ice sheets. Using these devices, they were able to detect the crashing of ocean waves at all seven sites, and monitor the velocity at which the resultant vibrations were travelling throughout 2012, when an enormous amount of snow and ice melted during the warm months, and 2013, when there was not much melt and a lot of snow from the winter remained throughout the year. The researchers found, as they had predicted, that the velocity of the vibrations was slowest several months after winter, when the crust was at its most compressed, after which it rose. But it rose differently in the two years in question. Between June and September of 2012 the velocity of the vibrations increased by 0.1%. By contrast, during the same months of 2013 the velocity increased by only 0.05%. To double check that their findings made sense, the team compared them with data collected by monthly satellite scans. The satellites generated much the same picture, albeit at lower resolution. Based on these results Dr Mordret and his colleagues argue in this weeks Science Advances that if more sensors are put into place, then Greenlands ice sheets (and, presumably, those of other places) can be monitored on a daily basis. This will yield a far higher-resolution image of how they are changing and help researchers better predict how much the level of the oceanic bathtub will rise.