Satellite observations of the ice sheets of Greenland and the Antarctic haven't been around long enough, and prior methods were too inaccurate, to be able to say whether the loss of ice today will persist in the future.
Predictions of the contribution of both ice shields to the sea level by the year 2100 may be off by more than 35 centimeters - but whether they will be too high or too low is unclear. Too high is obviously no problem. Too low could be a real worry.
The researchers writing in
analyzed nine years' worth of data from the gravity field satellite GRACE. The GRACE measurements showed that both ice sheets are losing significant amounts of ice - about 300 billion tons per year. At the same time, the rate at with which these losses occur is increasing: The contribution of both ice shields to sea level rise in recent years has almost doubled when compared to the first years of the GRACE mission.
The causes of this accelerated reduction in ice mass are still a challenge for scientists: In addition to anthropogenic warming, the ice sheets are influenced by a variety of natural processes, such as variations in snowfall and slow changes in ocean currents.
In climatological terms, nine years are a very short period of observation. "It would be more prudent to speak of weather rather than climate," says co-author Bert Wouters from the University of Bristol.
"This 'ice sheet weather' can cloak long-term acceleration, or suggest an increase in the depletion of ice mass that could actually be compensated over a longer period," adds co-author Ingo Sasgen from GFZ German Research Centre for Geosciences. "The results highlight the need for a continuous monitoring of the ice sheets with satellites."
In particular, deriving long-term trends of processes that interact with the climate from short observation data sets only have limited meaning. In order to improve the identification and prediction of the contribution of melting ice sheets to future sea level rise, the observation will be continued with the GRACE-Follow On mission from 2017 onwards.
Citation: B. Wouters, J. L. Bamber, M. R. van den Broeke, J. T. M. Lenaerts and I. Sasgen, 'Limits in detecting acceleration of ice sheet mass loss due to climate variability', Nature Geoscience (2013) doi:10.1038/ngeo1874