Here is a non-conclusion: after nine years of close observation, researchers still cannot be sure whether the planet is losing its ice caps at an accelerating rate.
That is because the run of data from one satellite is still not long enough to answer the big question: are Greenland and Antarctica melting because of global warming, or just blowing hot before blowing cold again in some long-term natural cycle?
The question is a serious one. If the loss of ice that seems to be happening now is really going to accelerate, then by 2100, mean sea level will rise 43 centimetres higher than the original notional prediction, and hundreds of millions of people who live on estuaries, deltas, coral atolls and great city river basins face serious losses.
Bert Wouters, a glaciologist at the University of Bristol in the UK and the University of Colorado at Boulder, Colorado, in the US, and colleagues report in Nature Geoscience that their most up-to-date and consistent measuring system, a satellite called Grace, needs to run for a lot longer before there can be a clear answer.
Grace stands for Gravity Recovery and Climate Experiment and it measures changes in mass in the landscape over which it flies, and the biggest variations in mass come from the changes in ice cover.
The main targets of the study are the ice sheets of Greenland and Antarctica because these amount to more than 99% of the planet’s snow and ice, and were these to melt completely, sea levels would rise by 63 metres, with calamitous consequences.
Shared caution
To be sure of detecting an accelerating mass loss of give or take 10 billion tonnes a year per year, the experiment needs at least 10 years for Antarctica and perhaps 20 years for Greenland.
But the results so far are ominous. “It has become apparent that ice sheets are losing substantial amounts of ice – about 300 billion tonnes each year – and the rate at which these losses occur 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,” said Dr Wouters.
But he is talking, of course, of a consistent finding from one experiment: other research has shown that the melting so far is real enough. The question is: could this just be the consequence of some natural rhythm so far unidentified?
Dr Wouters’ caution is echoed through the glaciological community. “Although ice is lost beyond any doubt, the period is not long enough to state that ice loss is accelerating,” said Wolfgang Rack of the University of Canterbury in New Zealand.
“This is because of the natural variability of the credit process, snowfall, and the debit process, melting, and iceberg calving, which both control the ice sheet balance.”
