Climate change is causing the sea level to rise as land ice melts and oceans expand. How much and how quickly sea levels will rise in the near future will depend in part on how often glaciers calve. Tidewater glaciers break off from glaciers that finish in the ocean (called tidewater glaciers) and fall into coastal fjords as icebergs. More ice enters the ocean, increasing the rate of sea level rise. The faster these glaciers move over the earth towards the ocean,
During the warmer months of summer, the surface of Greenland’s glaciers can melt and generate huge lakes, which may drain to the glacier’s base. According to studies of the interior Greenland ice sheet, this reduces friction between the ice and the earth, causing the ice to flow more quickly for a few days. To date, however, it has not been found out if these drainage events affect how fast tidewater glaciers move and, in turn, how often they break off.
To investigate this, a team of scientists from the Earth Sciences department of Oxford University, the Oxford Mathematical Institute, and Columbia University utilized Global Positioning System (GPS) observations of the flow speed of Helheim Glacier, the largest single glacier contributor to sea level rise in Greenland. The GPS caught a near-perfect natural experiment: observations of the glacier’s flow reaction to lake outflow at high temporal resolution.
According to the findings, Helheim Glacier behaved substantially differently than the inland ice sheet, which moves rapidly downhill during lake drainage events. Helheim Glacier, on the other hand, showed a small “pulse” of movement in which the glacier moved faster for a short time and then slowed down, so there was no net acceleration.
Using a computational model of the subglacial drainage system, the researchers determined that this observation was likely the result of Helheim glacier’s bed containing an effective network of channels and cavities. This makes it possible for the water to drain quickly from the glacier bed without making the glacier move more overall.
Although this looks to be good news for sea level rise, the researchers predicted that glaciers without an adequate drainage system where surface melt is currently low but will increase in the future owing to climate change may experience a different effect (such as in Antarctica).
They developed a mathematical model based on the conditions of colder, tidewater glaciers in the Antarctic. The results suggested that under these conditions, lake drainage would result in a net increase in glacier movement. This was mostly owing to the inability of the wintertime subglacial drainage system to quickly discharge flood waters. There are not yet any in-situ measurements of how Antarctic tidewater glaciers change when lakes drain.
The work casts doubt on a number of common ways of figuring out how glaciers drain from measurements of glacier speed taken from satellite images (which are currently used in models of sea level rise).
Associate Professor Laura Stevens of Oxford University’s Department of Earth Sciences said, “What we’ve seen here at Helheim is that a large amount of meltwater entering the drainage system during a lake drainage event does not cause a significant change in glacier speed when averaged over the week of the drainage event.”
Since the best accuracy of satellite-measured glacier speeds is about a week, lake draining events like the one recorded by the Helheim GPS data are usually not reported.
As Associate Professor Stevens noted, “These tidewater glaciers are problematic.” Before we can properly forecast their future reaction to atmospheric and oceanic warming, we have a great deal to learn about how meltwater drainage operates and affects tidewater-glacier speeds.
