Climate change is causing sea levels to rise as the ice on land melts and the oceans expand. How quickly and how quickly sea level rises in the near future will depend, in part, on the frequency of glacier shedding events. These occur when large chunks of ice break off from glaciers that end in the ocean (known as tidal water glaciers) and fall into coastal fjords as icebergs. The faster these glaciers flow from the ground to the ocean, the more ice enters the ocean, increasing the rate of sea level rise.
During the warmer summer months, the surface of Greenland’s glaciers can melt and form large lakes that can then drain to the base of the glacier. Studies of the Greenland inner ice sheet have shown that this reduces friction between the ice and the ground, causing the ice to slide faster for a few days. So far, however, it has not been clear whether such drainage events affect the flow rate of tidal glaciers and thus the speed of calving events.
To investigate this, a research team from the Department of Earth Sciences of the University of Oxford, the Oxford University Mathematical Institute and Columbia University used GPS (Global Positioning System) observations of the flow velocity of the Helheim Glacier, the largest contribution of a single glacier to sea level rise in Greenland. GPS captured a near-perfect natural experiment: high-resolution temporal observations of glacier flow response to lake drainage.
The results found that the Helheim Glacier behaved very differently from the inner ice sheet, which shows rapid downward movement during lake drainage events. In contrast, the Helheim Glacier exhibited a relatively small “pulse” of motion in which the glacier accelerated for a short time and then moved more slowly, resulting in a net increase in motion.
Using a numerical model of the subglacial drainage system, the researchers found that this observation was likely caused by the Helheim Glacier which had an efficient system of channels and cavities along its bed. This allows draining waters from the glacier bed to be quickly evacuated without causing an increase in total net movement.
Although this appears to be positive news in terms of implications for sea level rise, the researchers suspected that a different effect could occur for glaciers without an efficient drainage system where surface melting is currently low but will increase in the future to due to climate change (as in Antarctica). .
They ran a mathematical model based on the conditions of the coldest Antarctic tidal glaciers. The results indicated that lake drainages under these conditions would produce a marked increase in glacier movement. This was largely due to the less efficient winter subglacial drainage system which was unable to evacuate floodwaters quickly. To date, however, there are none on site observations of glacier responses to Antarctic tide water to lake drainage.
The study questions some common approaches to infer glacial drainage systems based on glacier speeds recorded using satellite observations (which are currently used in sea level rise models).
Lead author, Associate Professor Laura Stevens (Department of Earth Sciences, University of Oxford) said: ‘What we have observed here in Helheim is that there can be a large supply of meltwater into the drainage system during a lake drainage event, but this melt input does not translate into an appreciable change in glacier velocity when averaged over the week of the drainage event. ‘
With the highest temporal resolution of satellite-derived glacier speeds currently available of around a week, lake drainage events such as that captured in Helheim’s GPS data usually go unnoticed.
‘These tidal glaciers are complicated,’ added Professor Stevens. “We have a lot more to learn about how meltwater drainage works and modulates tide-glacier water velocities before we can confidently model their future response to atmospheric and ocean warming.”
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