A key reason for concern about climate change is that it could lead to gradual melting of the polar ice sheets in Greenland and Antarctica, resulting in an inexorable rise in global sea levels. A new set of high-precision measurements (paper in press with the journal ‘Geophysical Research Letters‘) now confirms that the ice sheets are not only melting but that their melt rate has more than doubled over the past seven years. This suggests that sea levels could rise a lot faster than indicated in earlier studies. Increases in sea level by more than 1m during the 21st century now have to be seriously considered.
Numerous studies since the early 2000s indicated that several glaciers that drain the polar ice sheets into the ocean have accelerated as a result of local warming. These observations triggered concerns that the polar ice sheets could loose ice more quickly and sea level could rise more rapidly than climate models had suggested. But it was unclear how representative these rather striking changes were — was it just a temporary ripple running through some glaciers or were the entire ice sheets loosing ice at an increasing rate?
High-precision measurements of the change in the total mass of the polar ice sheets, using a pair of orbiting satellites (the Gravity Recovery and Climate Experiment), have now revealed that both ice sheets are indeed loosing mass, and at an accelerating pace. The satellite data capture the changes in total ice mass and hence give a truly comprehensive picture of changes in the polar ice sheets rather than only spot measurements for particular glaciers.
The assessment by the Intergovernmental Panel on Climate Change (IPCC) published in 2007 found that over the period 1993 to 2003, Greenland and Antarctica had both lost about 75 billion tonnes of ice every year. If this loss of ice continued at that rate for a century, this would result in sea level rise of 4.2cm. The new satellite data now reveal that by 2002/2003, the Greenland ice sheet was already loosing as much as 137 billion tonnes of ice per year, and that this rate more than doubled again to 286 billion tonnes of ice per year during 2007-2009. Meanwhile, Antarctica’s ice loss had increased from 75 to 104 billion tonnes per year during 2002-2006, and more than doubled to 246 billion tonnes per year by 2006-2009. These most recent rates of ice loss would raise global sea levels by 15cm if they continued at current rates over a century.
The trouble is of course that there is little reason to assume that the loss of ice will continue only at current rates while the atmosphere and oceans continue to warm. It would be extremely surprising if the observed melting processes did not increase further, implying a significant risk that sea levels would increase by more — and potentially a lot more — than the 0.6m that the IPCC had projected for the 21st century.
Unfortunately, we still do not sufficiently understand all the processes that take place within and underneath the polar glaciers to understand which aspects of warming are responsible for this acceleration. Candidates for the accelerated ice loss include warming of the atmosphere, increases in local ocean temperatures, and the creation of meltwater on top of the ice sheet that is draining into its base and acts as lubricant for the glacier flow. Until those processes are better understood, extrapolations of the recent trends have to be treated with caution.
However, these recent data are consistent with a raft of other observations and model studies that suggest that we now have to seriously consider that sea levels could rise by more than 1m during the 21st century as a result of climate change. A report earlier this year by Professor Will Steffen at ANU for the Australian Department of Climate Change advised that sea level rise could lie somewhere between 0.5 and perhaps up to 1.5m by 2100. It’ll be a busy time for cartographers as we are in the process of re-designing the world’s coastlines.