Sea-level rise predictions could be flawed

Bristol researchers say the satellite record for the Greenland and Antarctic ice sheets needs to improve if we are to examine ice loss correctly.

It is currently too short to tell if the recently reported speed-up of ice loss will be sustained in the future or if it results from natural processes.

The research, led by Dr Bert Wouters, highlights the need for continuous satellite monitoring of the ice sheets so scientists can better identify and predict melting rates and the impact this will have on sea-levels. Since 2002, the satellites of the Gravity Recovery and Climate Experiment (GRACE) detect tiny variations in Earth’s gravitational field to monitor the state of the ice sheets at monthly intervals.

Wouters, who is currently a visiting researcher at the University of Colorado said: “In the course of the mission, it has become apparent that ice sheets are losing substantial amounts of ice – about 300 billion tonnes each year – and that the rate at which these losses occurs is increasing. Compared to the first few years of the GRACE mission, the ice sheets’ contribution to sea level rise has almost doubled in recent years.”

However, scientists have failed to reach a consensus about the cause of this recent increase in ice sheet mass loss observed by the satellites. Anthropogenic warming is a suggestion, but sheets can also be affected by many natural processes such as multi-year fluctuations in the atmosphere and changes in ocean currents.

“So, if observations span only a few years, such ‘ice sheet weather’ may show up as an apparent speed-up of ice loss which would cancel out once more observations become available,” said Wouters.

Wouters and his team compared nine years of satellite data from the GRACE mission with reconstructions of about 50 years of mass changes to the ice sheets. They discovered that the ability to accurately detect an accelerating trend in mass loss depends on the length of the record itself.

The ice loss detected by GRACE is larger than what we would expect to see just from natural fluctuations, but the speed-up of ice loss over the last years is not. The Bristol study therefore urges caution in extrapolating current measurements to predict future sea-level rise.

The findings are published in Nature Geoscience.