Ice sheets: Warming ice, faster-flowing glaciers
The ice sheets in Greenland and the Antarctic are dwindling as a direct effect of our warming climate, and are losing more and more ice. With regard to Greenland we now know that roughly 40 percent of the losses are due to melting processes and changed precipitation on the surface of the ice sheet; the remaining 60 percent is the result of accelerating glaciers. In other words, the glaciers are now more quickly transporting inland ice out to sea, where it raises the local water level.
What we’re also seeing in Greenland is that a major portion of the meltwater produced by melting snow doesn’t flow away, but instead penetrates and gathers in the topmost, porous layer of firn. Another change: unlike in the past, this water doesn’t always freeze. Instead, near glaciers like e.g. Helheim Glaciers iit can form layers of water within the ice, or aquifers measuring up to 1,000 square kilometres. In other words, the ice sheet now contains more and more meltwater, like a massive sponge, which raises the question of just how much water it now stores.
The answer to that question is a very important one, since heat is released when the water refreezes. Accordingly, the more meltwater that is stored in the ice and e.g. refreezes in winter, the more heat is released. In the long term, this process can substantially warm an ice sheet’s upper layers, making it more sensitive to rising air temperatures. We’ve also learned that there is much more meltwater in the ice and under the ice sheet on Greenland, which can accelerate the flow of ice masses, than we believed just a few years ago. Also, climate-related changes in the ice sheet and its many glaciers can now also be seen in the higher northern regions of the island – and not just in the southeast and western regions.
The loss of ice mass in the Antarctic is chiefly caused by the acceleration of glaciers in West Antarctica. We can’t yet determine whether the ice masses there have become so unstable that the West Antarctic Ice Sheet will inevitably collapse. Moreover, West Antarctica’s contribution to sea level rise is already so high that we have to closely monitor its accelerating glaciers – regardless of whether or not the ‘tipping point’ has been reached. After all, the trend is clear to see: thanks to global warming, the Antarctic ice is melting. Why this is happening, and which processes are involved, are aspects that we’re now coming to understand in more and more detail.
How critical it is to reduce global greenhouse-gas emissions as quickly as possible and meet the goals set in the Paris Agreement, without exceeding the 2-degree target, can be seen in one of our latest model studies. If we can succeed in limiting global warming to 1.5 degrees Celsius through the year 2100, the Greenland Ice Sheet will lose 38 percent less ice than in a world that briefly warms by much more than 1.5 degrees Celsius and subsequently cools again. Accordingly, our actions will have an enormous impact on our planet’s ice sheets.
Prof Angelika Humbert, glaciologist and Head of the Working Group on Ice Sheet Modelling at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research