The melting of the Antarctic and Greenland ice sheets may have indirect effects on the global climate system, triggering more variable weather and further melting, reports a paper published this week in Nature. A separate paper in Nature explores a debated process for runaway ice-cliff collapse in the Antarctic.
Global temperatures could potentially rise by three to four degrees Celsius above pre-industrial levels by the end of the century, accelerating ice sheet melting and raising global sea levels. However, the combined effects of Greenland and Antarctic ice-sheet melt on climate have not been widely explored.
Nicholas Golledge and colleagues use satellite measurements of recent ice-mass changes to refine simulations of the melting Antarctic and Greenland ice sheets and the potential impacts. They find that, within a few decades, increasing meltwater will substantially slow the Atlantic Meridional Overturning Circulation (which carries warm water from the tropics into the North Atlantic). Climate variability between years may increase by up to 50 per cent in some regions. Antarctic melt will form a freshwater lens on the ocean surface, allowing rising warmer water to spread out and potentially cause further melting of Antarctic ice underwater.
In a separate paper, Tamsin Edwards and colleagues analyse past sea-level rises (ranging from three million years ago to present day) to determine whether they support the controversial ‘marine ice-cliff instability’ hypothesis. According to this hypothesis, exposed ice cliffs reaching over 100 metres above sea-level become unstable and collapse, causing rapid ice retreat. They find that this mechanism is not required to explain past sea level rises. When the instability hypothesis is discounted from models, the authors calculate that Antarctic contribution to sea-level rise has only a 5 per cent chance of exceeding 39 centimetres by 2100 under very high greenhouse gas concentrations (RCP8.5). Both papers predict the most likely Antarctic contribution to sea-level rise under this scenario to be around 14-15 centimetres.