Figuring out what's going on underneath a kilometer (over half a mile) of solid Greenland ice isn't easy for scientists, but the rise and fall of 'water blisters' could offer some vital insights into the deep flow of water and ice, according to a new study.
These blisters form between the ice sheet and the bedrock underneath, created as natural meltwater lakes on the surface drain down through fractures in the ice and fill in cavities. From there the water permeates into the drainage system underneath the glacier.
What researchers have now discovered, through a combination of field measurements, modeling, and lab experiments, is that these blisters can push the ice upwards as they form and cause it to drop back down as they recede.
That means they can be used to estimate transmissivity – the efficiency of the water networks that form between the ice and the bedrock underneath – and to better understand how increased melting caused by climate change could affect the overall stability of the ice sheet.
"We know that as the climate warms in the future, the surface melt zone can expand and migrate to higher elevations than currently observed," says geoscientist Ching-Yao Lai from Princeton University.
"A big question that remains to be answered, however, is how much transmissivity can increase further inland."
Lai and her team looked at five lake drainage events from 2006 to 2012, using methods including GPS tracking to monitor drainage volume, surface displacements, and blister formation happening below the ice.
The length of time the ice took to sink back down as the blisters disappeared varied quite significantly, suggesting key differences in the subglacial drainage system underneath.
After developing analytical models to match the field observations they'd seen, the researchers set up experiments in the lab using a deformable silicone sheet as the ice and a porous material as the bedrock. These tests helped them refine their models further.
"The system is small enough to be held in one hand and the material is transparent, so we were able to directly observe the shape of the blister and the drainage into the porous substrate over time," says mechanical engineer Danielle Chase from Princeton University.
As water flows through the ice and forms blisters, it can act as a lubricant between a glacier and the ground it's sat on. The signs are that this destabilization is likely to get worse as temperatures rise and melting increases.
With the new model now available, scientists should be able to measure this potential destabilization more accurately, through an analysis of transmissivity and how it changes over time.
The next step is to apply these findings to get a better understanding of what's happening under the Greenland ice sheet – especially further inland and higher up, where the current data is patchy and it's much more difficult to take readings.
"A potential impact is that the link between surface melt and subglacial water-network development could be activated not only at lower elevations, as currently observed, but also at higher elevations," says Lai.
"More observations of seasonal changes of subglacial transmissivity in response to surface melting would be needed to really understand what would happen when melt migrates to higher elevation regions."
The research has been published in Nature Communications.