At the greatest source of melting ice on the planet, even more ice has melted than we thought. Our estimates were wrong because the crust beneath western Antarctica is rising faster than we thought possible. Besides astounding geologists, this rapid crustal rebound could be a lifesaver for some low-lying cities.
In the last few weeks, so much ground-breaking research has been published about Antarctica, it is easy to get the different studies confused. Most prominently, and disturbingly, the continent's ice loss has accelerated dramatically over the last five years.
The Amundsen Sea Embayment (ASE) is a relatively small sector of West Antarctica, but it accounts for a quarter of all the ice that has melted across the entire planet. The future of low-lying cities depends on what happens here. The impact will be particularly large in a curious place.
"The large amount of water stored in Antarctica has implications for the whole planet, but especially for northern Europe," said Dr Valentina Barletta of the Technical University of Denmark in a statement. "Because of a combination of gravitational effects, surprisingly, the ice lost in Antarctica mostly raises the sea level here, in northern Europe. In contrast, the ice lost in Greenland has no effect here, but it raises the sea level in the southern hemisphere and further destabilizes the WAIS.”
Barletta is first author of a paper in Science reporting that estimates of ice loss in the ASE have been 10 percent too low.
Ice sheets are heavy and can push down the crust beneath them. When the ice melts to the ocean, the weight is removed and the crust rebounds. Usually, however, it does this very slowly; for example, 10 millimeters (0.4 inches) a year in Scandinavia since the ending of the ice age 10,000 years ago.
Consequently, crustal rebound from human-induced global warming has been expected to happen too slowly to affect us much. However, Barletta's co-authors put six GPS stations on exposed rock around the ASE, revealing uplift of 41 millimeters (1.6 inches) a year – four times faster than expected. This indicates the mantle below must be far less viscous than anywhere else we have studied.
This rapid rebound distorted our ice loss estimates. In such a remote region, geologists have relied heavily on satellite measurements of gravity to estimate ice changes, since less ice means slightly less gravitational pull.
After correcting for the additional gravity produced by the rising crust, the team realized this unexpected factor has been partially disguising the amount of ice lost.
All this might seem like another round of grim news we are used to hearing from Antarctica, but this is more complex. The ASE has been melting faster than almost anywhere else because so much of its ice rests on rock below sea level. Water flowing in from the ocean melts glaciers from below and lubricates their flow, greatly hastening more melting.
If the crust rebounds enough, it will block incoming sea water and dramatically slow melting – something previously not anticipated for centuries.
It's too early to tell if these crustal effects will prevent the collapse of the ASE ice sheet, or how applicable this is to other locations, but it's at least possible that future sea level rise will be less catastrophic than we thought.
Coincidentally, just last week a separate team revealed evidence of ocean sediments and fish remains inland of the Weddell Sea in Nature. Their explanation is that at some time in the last 35,000 years the region experienced a similar surprisingly rapid rebound after partial melting of local ice. The Weddell Sea and ASE are on opposite sides of the Antarctic Peninsula.