Paleoclimate proxies reveal Greenland instability

The Greenland ice sheet contains nearly 3 million km³ of ice—enough that, if melted, global sea levels would rise by more than 7 m. Over recent decades, the ice sheet has been losing mass at an accelerating rate, leading to concern that it may be headed for a total collapse. New data from ocean sediment cores now suggest that the Greenland ice sheet may be even less stable than previously thought (Proc. Natl. Acad. Sci. U.S.A. 2019, DOI: 10.1073/pnas.1911902116). Scientists used microscopic fossilized plankton called foraminifera preserved in seafloor sediments to construct a record of ocean temperatures near Greenland. By determining the elemental ratios of magnesium and calcium in the foraminifera, the researchers could infer the ocean’s temperature over the past four warm interglacial periods. They found that although all four periods were warmer than the present, warmer temperatures didn’t necessarily mean more melting. Instead, milder warm conditions sustained over a longer period led to the most severe deglaciation. This suggests that the critical temperature for Greenland’s stability may be on the lower end of what had been predicted, says University of Bergen paleoclimatologist Nil Irvali, who led the study. Irvali says that the amount of warming forecast for the remainder of the century suggests that a collapse of the Greenland ice sheet “may already be unavoidable.”