Out of Balance
Since the 1990s, the Greenland Ice Sheet has been seriously out of balance. Just as in the past, every year the snowfall there forms about 730 billion tonnes of new ice. But at the same time, Greenland is losing nearly 995 billion tonnes of ice each year, as reported in an article in the journal Nature, prepared by 89 researchers, who combined data from eleven different satellites. Accordingly, since 1992 Greenland has lost approximately 3,800 billion tonnes, and the meltwater produced has raised the global sea level by more than a centimetre.
This also means that Greenland now loses seven times more ice per year as it did in the early 1990s. This loss rate is roughly equivalent to that calculated by the ‘business as usual’ IPCC climate scenario, representing only minimal efforts to combat climate change worldwide. “I wasn’t exactly surprised by the outcome,” says Ingo Sasgen from the Alfred Wegener Institute (AWI), Helmholtz Centre for Polar and Marine Research in Bremerhaven. He and his fellow AWI researcher Veit Helm and Ludwig Schröder supplied important calculations for the Nature study, which was led by Andrew Sheperdfrom the University of Leeds, UK, and Erik Ivins from NASA’s Jet Propulsion Laboratory in Pasadena, California. “After all, emission rates of the greenhouse gas carbon dioxide have been approximately as high as in the IPCC’s worst-case scenario over the past years,” the AWI expert adds.
Moreover, the annual rise in air temperatures produced by these greenhouse gases is much more pronounced in the ?High North? than in the rest of the world. This additional warmth is impacts on air currents, and with them, impacts Arctic weather. “As a result, much more surface ice melted than in the past,” Sasgen explains.
Consequently, the team of 89 researchers had good reason to take a closer look at changes of the ice sheet that took place between 1992 and 2018. To do so, they employed data from eleven satellites operated by the European Space Agency (ESA)NASA and other agencies, which use for example radar waves or gravity measurements to determine the ice’s volume and mass, as well as the speed at which the glaciers flow. In this regard, AWI researchers used crucial measurements of satellite CryoSat-2 dedicated to ice observations and and the twin satellites GRACE measuring the gravitity field, which were co-designed by the German Research Centre for Geosciences (GFZ) in Potsdam.
According to the satellite data, Greenland has increasingly lost ice since the 1990s. The trend peaked between 2010 and 2012, with record-breaking average losses of more than 335 billion tonnes in each year, which corresponds to an global mean sea-level rise of almost one millimetre. “Afterwards, the summers of 2013, 2017 and 2018 were comparatively cool, and the ice loss decreased,” says Sasgen. Nevertheless, in the past three decades, every year the Greenland Ice Sheet lost approximately one third more ice than it replaced.
From 1960 to 1990, Greenland lost an average of 421 billion tonnes of ice per year, by discharge of solid ice into the ocean. Melting contributed another 260 billion tonnes on the surface of the ice cap. In recent years, this surface melting has nearly doubled, climbing to 487 billion tonnes. In comparison, the ice loss at the glaciers’ mouths has ‘only’ risen by twenty percent, to about 508 billion tonnes per year.
In essence: climate change is already substantially thinning the Greenland Ice Sheet, and produced a global mean sea-level rise of around 10.6 millimetres between 1992 and 2018. And these changes will have very real consequences: “As a rule of thumb, for every centimetre rise in global sea level another six million people are exposed to coastal flooding around the planet,” says Andrew Shepherd from the University of Leeds, putting the effects into perspective.
Original publikation: Andrew Shepherd et al.: Mass balance of the Greenland Ice Sheet from 1992 to 2018, Nature (2019), DOI: 10.1038/s41586-019-1855-2