Estimate of polar ice loss is the most accurate yet

Estimate of polar ice loss is the most accurate yet

30 November 2012, by Tamera Jones

In the pioneering study, published in Science, researchers show that between 1992 and 2011, melting ice from Greenland and Antarctica has contributed just over a centimetre to global sea-level rise.

This amounts to a fifth of all sea-level rise over the 20-year study period. The remaining four fifths comes from water from melting mountain glaciers around the world, and from the way water takes up more volume as the temperature rises, known as thermal expansion.

This is the first time data from so many satellites has been combined in this way, with the resulting figure ending 20 years of uncertainty around ice loss from the poles.

It's imperative that scientists find out how much ice has been lost in recent years from glaciers and ice sheets in the polar regions so that they can accurately predict how global sea levels will change. Rising sea-levels are one of climate change's biggest threats, because so much of the world's population lives in major cities either on or close to the coast. Cities like London, Tokyo and New York are all potentially at risk.

Before this study, estimates of polar ice loss were typically based on data from a few satellites. Indeed, the Intergovernmental Panel on Climate Change (IPCC) used data from just a handful in its 2007 report, leading to a broad spread of estimates, ranging from a drop in sea level of 0.2 millimetres per year to a rise of 1.9 millimetres a year. It wasn't even clear if Antarctic ice was growing or shrinking.

Now, the new combined assessment from an international team of nearly 50 satellite experts, led by Andrew Shepherd, professor of Earth observation at the University of Leeds, is two to three times more accurate than previous calculations.

Shepherd and his colleagues also found differences in the pace of change at each pole: around two thirds of the melted ice came from Greenland, while the rest was from Antarctica. Both ice sheets are losing more than three times as much ice as they were in the 1990s, but the speed of ice loss from Greenland has surged five-fold since the middle of that decade.

The study confirms that both Greenland and Antarctica are losing ice faster than snow is building up on their surfaces. This difference between build-up of snow-fall and loss of ice through melting is known as mass balance.

'Our study confirms that many of the different techniques that scientists use to estimate ice loss from the poles do agree with each other. But the best estimates come from long-term monitoring. Shorter studies should be taken with a pinch of salt, because there can be so much natural fluctuation in ice gain or loss,' explains Shepherd.

The team also identified regions where data isn't complete enough to make a reasonable estimate of ice loss or gain.

'The Antarctic Peninsula and the East Antarctic ice sheet would benefit from longer-term monitoring,' says Shepherd.

Though the rise in global sea-levels seems fairly modest, it's the next hundred years or so that scientists are most concerned about. Even the small rises seen today could trigger substantial ice loss through accelerated melting of ice shelves and outlet glaciers.

'Without the coming together, effort and cooperation of the international scientific community, we wouldn't be in a position to tell people with confidence how the Earth's ice sheets have changed, and to end the uncertainty that has existed for many years,' adds Shepherd.

The Ice Sheet Mass Balance Inter-comparison Exercise (IMBIE) is a collaboration between 47 researchers from 26 laboratories, and was supported by the European Space Agency (ESA) and the National Aeronautics and Space Administration (NASA).