A new study has found that a network of lakes underneath a major Antarctic glacier underwent large drainage events in 2013 and in 2017. After completely emptying in 2013, the lakes regained volume over four years, only to empty again — this time faster and with more water discharged — in 2017. This was the first time repeat drainage and recharge of subglacial lakes under Thwaites Glacier has been recorded. Recent improvements in satellite instrumentation permit these precise measurements of the movement of water two kilometers below the permanent ice surface.
The presence of meltwater at the bed of the glacier lubricates the base and reduces friction between the rock and ice, preconditioning the glacier above to slide over the slippery surface more easily and flow faster toward the ocean. The velocity of the glacier directly influences its rate of mass loss and accordingly, sea level rise contribution. This study highlights yet another mechanism weakening Antarctica’s ice. Another concern is that the study’s results suggest that previous modelled estimates of melting at the bed have been substantially underestimated.
Thwaites Glacier is world-renowned for its importance in regulating Antarctic ice loss. It is known to be one of the fastest-changing glaciers in the region, transporting vast amounts of ice from the interior of the enormous West Antarctic Ice Sheet out to the sea. Scientists have been monitoring Thwaites Glacier for over two decades and have recognized it as one of the first glaciers to respond significantly to climate change. This critical glacier also accounts for a major proportion of Antarctica’s contribution to global sea level rise.
The results from the study found that four lakes that drained in 2013 were also involved in the 2017 event. George Malczyk, a glaciologist at the University of Edinburgh and the study’s lead author, told GlacierHub that “what is interesting about this second drainage event is how different it is from the first, with a much faster transfer of water and increased discharge. Our observations highlight that there were potentially significant modifications to the subglacial system between these two events.” This is intriguing because large changes occurred over a short time scale — once again highlighting the rapid ability of the Antarctic ice sheet to respond to change.
“What takes place under the ice sheet is critical to how it responds to changes in the atmosphere and ocean around Antarctica, and yet it is hidden from view by kilometres of ice, which makes it very difficult to observe,” explains Noel Gourmelen, also from the University of Edinburgh, study co-author and experienced researcher in Antarctic glaciology. He went on to add that the movements of water give key information about the kind of environment and hydrology network that is present beneath the glacier — information which is needed when trying to project the role of Thwaites Glacier in Antarctic contribution to sea level rise.
To map the subglacial lakes and movement of water, Malczyk’s team used radar altimetry, a method that sends radio wave pulses from a satellite and measures the time it takes for the signal to bounce back from the Earth’s surface. The satellite, CryoSat-2, was launched by the European Space Agency in 2010. “It collects data at the same point in space every month and allows us to build a time series of elevation change,” explained Malczyk. When water is routed out from a lake, the ice above moves downward, creating a local depression at the surface of the glacier which can remarkably be detected by the satellites.
Noting that the 2017 drainage event was faster and more substantial than in 2013, Malczyk revealed his team had two hypotheses why this may have been. If the channels that formed to carry water between the lakes in 2013 did not fully close, the series of conduits left behind would pre-condition the system for efficient and rapid transfer of water in 2017. The other hypothesis suggests that the lake drainage in 2013 may have eroded the channels, which also could allow for more rapid flow. Either way, the complex network of streams and lakes between the bedrock and the glacier are critical in the future of Antarctic ice.
Future events might not display the same efficiency in transfer speed of water between the lakes. As the lakes begin to refill again, the subglacial channels connecting the lakes may not remain fully open as they were immediately after the discharge of water in 2017. However, Malczyk points out that if the drainage did erode the channels, then there will be more long term modification of the system, meaning future events might display similar or perhaps faster rates of transfer.
In this remote environment, any observations are very rare. The use of high-resolution satellite data is important, but it still leaves some questions unanswered. Malczyk explains that scientists are aware that water discharged at the grounding line (the point where the glacier reaches the end of land and begins to float on the sea surface) generates enhanced melting by bringing warm ocean water in contact with the ice. The extent to which this is important for ice sheet evolution and the routing of water out of the ice sheet remains unknown.
Melting at the bed is not just initiated by frictional heat as the glacier slides over the rocky bed, but also through geothermal heat from inside the Earth. Thwaites Glacier is located near the West Antarctic Rift System — a tectonic area of thinning crust, responsible for volcanic activity on the continent. These large-scale geologic movements within the Earth create enough heat to melt the ice above bedrock. A problem, however, is that estimating the heat flux from tectonic movement here is very difficult, so its contribution to melt is rather unknown.
This new study raises concerns that repeat draining of subglacial lakes underneath one of Antarctica’s most important glaciers could accelerate its flow and, as a result, speed up its contribution to sea level rise. Monitoring this vast, remote expanse of ice over long periods of time is crucial in understanding how this highly sensitive environment may respond to climate change through the twenty-first century and beyond.