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New science upsets calculations on sea level rise, climate change

Ice sheet melt massively overestimated, satellites show

Opinion A new analysis of data from dedicated satellites shows that one of the main factors predicted to drive rising sea levels in future has been seriously overestimated, with major implications for climate talks currently underway in Doha.

The new methods involve filtering out noise from the data produced by the Gravity Recovery And Climate Experiment (GRACE) spacecraft, sent into orbit with the aim of finding out just how much ice is melting from the world's ice sheets and glaciers. Such water then runs off into the sea, providing one of the main potential drivers of sea level rise - which is itself perhaps the main reason to worry about climate change.

"GRACE data contain a lot of signals and a lot of noise. Our technique learns enough about the noise to effectively recover the signal, and at much finer spatial scales than was possible before," explains professor Frederik Simons of Princeton uni. "We can 'see through' the noise and recover the 'true' geophysical information contained in these data. We can now revisit GRACE data related to areas such as river basins and irrigation and soil moisture, not just ice sheets."

Simons and his colleague Christopher Harig tried their new methods out on GRACE data covering the Greenland ice sheet, which is of particular interest as the rest of the Arctic ice cap floats on the sea and so cannot contribute directly to sea level rise by melting. Meanwhile the Antarctic ice cap is actually getting bigger, so Greenland is probably the major worry.

According to a Princeton statement highlighting the new research:

While overall ice loss on Greenland consistently increased between 2003 and 2010, Harig and Simons found that it was in fact very patchy from region to region.

In addition, the enhanced detail of where and how much ice melted allowed the researchers to estimate that the annual acceleration in ice loss is much lower than previous research has suggested, roughly increasing by 8 billion tons every year. Previous estimates were as high as 30 billion tons more per year.

The rate of loss of ice from Greenland is estimated at 199.72 plus-or-minus 6.28 gigatonnes per year. So the possible acceleration of losses is only barely larger than the margin of error in the readings: it's very difficult to tell the supposed loss curve from a straight line.

In other words the possible acceleration in ice losses is barely perceptible: it may not really be happening at all. Similar results were seen not long ago in GRACE data for central Asian mountain glaciers, another suggested source for sea-level rises.

If the Greenland ice losses aren't accelerating, there's no real reason to worry about them. According to the Princeton statement:

At current melt rates, the Greenland ice sheet would take about 13,000 years to melt completely, which would result in a global sea-level rise of more than 21 feet (6.5 meters).

Put another way, in that scenario we would be looking at 5cm of sea level rise from Greenland by the year 2130: a paltry amount. Authoritative recent research drawing together all possible causes of sea level rise bears this out, suggesting maximum possible rise in the worst case by 2100 will be 30cm. More probably it will be less, and there will hardly be any difference between the 20th and 21st centuries in sea level terms.

The new GRACE research was published in the Proceedings of the National Academy of Sciences. Details of the computer code can be found here.

Doha delegates take note. ®

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