We might have a little more time than we thought to work out how to stop, or at least cope with, rising sea levels. Antarctic ice will continue to melt faster in a warming world, but the pace may be slowed by some unexpected factors, a paper in Nature Communications argues.
Human-induced global warming is raising sea levels in multiple ways, some of them better understood than others. “The stability of marine sectors of the Antarctic Ice Sheet (AIS) in a warming climate has been identified as the largest source of uncertainty in projections of future sea-level rise,” writes Dr Natalya Gomez of McGill University, Canada, and her co-authors, in the study.
The expansion experienced as water warms, and the retreat of glaciers elsewhere in the world, will eventually be enough on their own to drown many of the world's great cities, but if Antarctica melts at the more rapid end of predictions the pace will be terrifyingly fast.
Gomez argues that current models for the retreat of Antarctica's major ice sheets ignore factors that have been regarded as trivial. However, when these are included, these sheets are delayed enough to buy us some precious time.
One of these factors is bedrock uplift, also known as the elasticity effect. Ice is heavy, and enough of it will compress the land it sits on. At the end of an ice age the loss of ice allows the rock beneath to rise back upwards. The paper notes that this rebound could reduce the speed with which ice melts, particularly in West Antarctica.
This occurs because as the bedrock rises it raises the remaining ice up with it. Most Antarctic melt arises from exposure to the oceans, rather than the air. As the ice rises up less of it is exposed to the waters lapping around Antarctica, and that in turn slows down the rate of melting.
The second effect is even more surprising. The vast Antarctic ice mass exerts its own gravitational pull, which raises sea levels nearby by drawing water towards it. A reduction in the ice sheet will diminish this gravitational effect and lower sea levels along the coast, and similarly reduce the amount of ice exposed directly to the sea.
Small as these effects sound, modeling of scenarios where carbon dioxide levels in the atmosphere double suggests that over the period 200 to 1,000 years in the future, ice loss could be half as fast as without them. The authors do acknowledge that plenty of uncertainty remains, for example the speed of bedrock uplift.
The authors stress their work does not decrease the imperative to cut greenhouse gas emissions, "The fate of the polar ice sheets in a warming world is a major concern for policy makers – and attention is rightly focused on the importance of restraining CO2 emissions and preparing for rising sea levels," said Gomez in a statement, "The greater the emissions, the more the geophysical forces risk being overwhelmed by the strength of warming."
