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Polar ice loss speed-up may not last – experts respond

Posted in Expert Reactions on July 15th, 2013.

Satellite observations have measured accelerating polar ice mass loss over the past decade as part of the Gravity Recovery and Climate Experiment (GRACE). The unprecedented speed of recent ice loss has prompted some scientists to warn of the potential for sea level rise as much as 35 cm higher than the projections contained in the most recent IPCC report.

Getz_Ice_ShelfHowever, whether this increasingly rapid ice loss will continue into the future or settle into a more steady rate of melt over coming years remains an area of debate among climate scientists.

Now a new study examining the satellite record concludes that natural variability can’t yet be ruled out, and that recent trends could be down to short-term variations, a form of ‘ice sheet weather’. The research was published in Nature Geoscience this week.

“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,” lead author Dr Bert Wouters said.

The study suggests that although another year’s worth of satellite data may be enough to detect a speed-up in mass loss of the Antarctic ice sheet with a reasonable level of confidence, another ten years of satellite observations will be needed to do so for Greenland.

The Science Media Centre received the following comments from experts in climate and ice science on the new study:

Dr Wolfgang Rack, Senior Lecturer at Gateway Antarctica, University of Canterbury, comments:

“Both the Antarctic and Greenland ice sheet are shrinking and contributing to global sea level rise. For ten years now, it has been possible to weigh the ice sheets from space using a ‘satellite scale’. It can detect the change of the ice sheet’s gravity, in other words its changing ice mass. If we would like to know by how much the sea level will rise in the next 100 years, we need not only to know if the ice loss will continue at the average observed rate, but if it is going to speed up, and by how much.

“Ten years is a very short period of time to measure the health of an ice sheet. Although ice is lost beyond any doubt, the period is not long enough to state that ice loss is accelerating. 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 mass balance. Satellite measurements need to continue for another decade to answer with certainty if ice sheet melt is speeding up, which could add another half a meter sea level rise by 2100 to the present more conservative estimates.”  

Professor Tim Naish, Director of the Antarctic Research Centre, Victoria University of Wellington comments:

“The science community has always been aware that it may not be appropriate to extrapolate the acceleration of ice mass loss observed in the satellite-based observations out to 2100 given the short length of the observations (which extend back to the late 1980s). This gets to the nub of the problem…are we observing short-term variability (ice weather) or a longer-term response of the ice sheets to greenhouse gas increases?

“The latest Nature geosciences paper sounds a further warning. This is why the community and the IPCC do not base their assessment of future ice sheet contribution to sea-level rise on the satellite data alone. For example actual field observations from the Antarctic continent and a rapidly evolving understanding of the  processes that melt polar ice sheets show that for the margin of the West Antarctic Ice Sheet in the Amundsen Sea region the ice is thinning and retreating  in response to coastal ocean warming. Models suggest  this  could trigger and abrupt and rapid response involving even more rapid loss of the interior ice sheet…consistent with the type of acceleration being measured by the satellites”Indeed if warming of the Southern Ocean, which is attributed to global warming, is driving the accelerated ice loss observed in satellite measurements, then we may not be looking at short term variability at all. Moreover in the immortal words of oceanographer “Wally” Broecker,  “heating of the ocean is an enormous thing”,  and it will take decades to centuries for the Southern Ocean to cool even if we managed to stabilise greenhouse gases in the atmosphere.

“The Nature geosciences paper is cautioning us not too read too much into just one line of evidence concerning future ice mass loss from the polar regions.

“Paleo-sea level observations show rapid ice sheet loss in the past driving sea-level rise upwards of 2m per century, but there is an open discussion amongst scientists as to whether this is representative of our future climate, given that we no longer have large ice sheets on the Northern hemisphere as we did in the past.

“So while the jury remains out as to whether recent accelerations observed by the satellites are capturing a first order response of the large polar ice sheets to global warming of the West Antarctic Ice Sheet, other lines of evidence suggest they just might be.”

Professor Gary Wilson, the Director of NZARI and Professor in Departments of Marine Science & Geology at the University of Otago:

“This paper confirms that mass is being lost from the Greenland Ice Sheet and hence contributing to sea level rise. The authors also show loss of mass from Antarctica’s ice sheets. What is in question is whether or not we can detect an acceleration in the rate of ice loss. The conclusion is quite simply that the degree of year to year and seasonal variability makes that very difficult to detect on the short timescale of data thus far available from the gravity recovery and climate experiment – so far only 9 years of observations. Looking at the data presented in the paper, I would expect that another 4-5 years of observations will confirm whether or not there is a short term acceleration in ice loss.

“Why does this concern us? Between the Greenland and West Antarctic Ice Sheets, there is potential to raise global sea level by more than 10 m, furthermore these more vulnerable ice sheets are likely to be the first indications of an acceleration in global impact of warming climates. While it may take some time for such an eventuality, detecting acceleration in the rate of course means the timeframe that we have to plan for such an eventuality is shorter than we first thought.”

From the UK SMC:

Prof Andrew Shepherd, Professor of Earth Observation at the University of Leeds, said:

“Their comparison of satellite and climate data helps us to better interpret changes of Earth’s polar ice sheets. It seems that studies based on less than a decade of satellite measurements are too short to establish, with confidence, whether the ice sheet losses are accelerating over time, and so we should be cautious about extrapolating short term trends into the future.

“Fortunately, we can appeal to data from other, longer satellite missions to get a long term perspective, and our own analysis of their data confirms that the rate of ice sheet losses has indeed accelerated over the past 20 years.”

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