Recently released maps have revealed that the Greenland ice sheet is growing thicker and moving slower in the interior, while melting and moving faster at its edges.
This phenomenon, which has been widely observed by scientists, is thought to be part of a longer-term deceleration that has been taking place during the last 9,000 years. By mapping the history of ice speed across Greenland, geophysicists are beginning to understand why this continues even as the climate warms.
‘Like many others, I had in mind the ongoing dramatic retreat and speedup along the edges of the ice sheet,’ says Dr Joseph MacGregor, researcher at the University of Texas and lead author of the study, ‘so I’d assumed that the interior was faster too. But it wasn’t. We have found that most of the Greenland Ice Sheet interior moves slower now than it did, on average, during the Holocene.’
It is difficult to imagine processes on the millennial timescales of glaciers, however, the maps can help compare the movements of today’s ice to those of the ‘last several thousand years,’ according to MacGregor.
So why the slowdown? MacGregor and his team puts it down to three causes: the increase in snowfall since the start of the Holocene; the stiffening of glaciers over time; and the collapse of the Innuitian ice sheet – an ‘ice bridge’ that once connected Greenland to Ellesmere Island near the northwest coast.
‘The snow that was deposited during the previous glacial period is three times “softer” than ice deposited more recently during the Holocene,’ says William Colgan, researcher at York University in Canada and a co-author of the study. This means the new, harder snow moves more slowly than the softer snow. ‘Plus, if an ice sheet slows down in a particular region, then it will also thicken,’ says MacGregor, ‘because the snow that is falling can’t be evacuated out by ice flow as quickly.’
However, this does not account for the slowing up of glaciers to the northwest of Greenland, where there is less snowfall. Here the scientists believe that the slowdown is a hangover from the disappearance of the Innuitian ice bridge around 10,000 years ago, which once blocked the open water from Greenland to Ellesmere island, locking the glaciers firmer in place. The ice bridge collapse caused the glacier speeds to quicken. Relative to those initial speeds, the northwest has been experiencing a deceleration during the Holocene, an effect which we are still seeing today.
The researchers say it does not change the fact that the ice sheet is losing mass overall. ‘The ice growth does not offset the overall loss by much,’ says MacGregor, however the slow thickening – at typically less than one cm per year – is widespread across the interior of the ice sheet, so it’s important to understand the origins of the phenomenon.’
‘Reconciling these observations is critical to predicting the future of the Greenland ice sheet amid ongoing climate change,’ the study concludes.