Greenland’s Ice Sheet Becoming Top-Heavy

20 Oct 2005 - Articles, Ice & Snow, Arctic

Glacier Greenland

Scientists from Norway, Russia and the United States have released findings from their study published in Science Express of satellite data over the period 1992-2003 - revealing a new picture of what is happening to Greenland's massive ice-sheet.

Ola Johannessen, Founding Director of the Nansen Environmental and Remote Sensing Center in Bergen, Norway, said that "careful analysis of data collected by European Space Agency satellites over this period showed that the winter height of the glacier increased on average".

So is the ice sheet melting, as you'd expect from global warming, or growing?

The answer, it seems, is that it's doing both - but in different parts of the ice sheet. The study by the group of scientists shows that the interior (above an elevation of 1.5 kilometres) is significantly increasing in size while the rest, particularly around glacial outlets, is decreasing to a much lesser degree. Overall there has been an increase of over 5 centimetres on average per year in the studied period. Previous studies detected the decline in the ice shelves and glacial outlets but indicated that the high interior sheet was in balance, rather than growing.

So what is happening in Greenland?

The pronounced overall warming trend in the Arctic over the last few decades, as documented in the 2004 Arctic Climate Impact Assessment (ACIA) report, has already led to significant retreats in Arctic sea-ice over the last decade. Intuitively you'd expect there to be a similar impact on the Greenland ice-sheet - which is forecast to begin a process of completely disappearing once mean temperatures in the Arctic increase by 3 degrees centigrade (fortunately such a process, leading to global sea-level rise of around 7 metres, would take thousands of years!). But the situation is more complex.

The increase in the elevation of the ice sheet's interior is driven by greater snowfall accumulation. Some 75% of the increased snowfall can be accounted for by changes in what is known as the North Atlantic Oscillation (NAO) index which reflects very large scale weather patterns across the North Atlantic Ocean. When this is positive (ie high sea-level pressure readings in the Azores outweigh low readings in Iceland) Greenland gets cold dry winters. When it is negative Greenland gets warmer wetter winters - and lots of snow. In the period studied the NAO was negative on average and with a couple of strongly negative years.

The NAO varies considerably naturally although models of global warming suggest that it will increasingly tend to be positive (as it was for most of the second half of the 20th century) if greenhouse gases continued to accumulate in the atmosphere. The global warming model also indicates that warming temperatures should lead to higher rain and snowfall levels in general for Greenland - and this is perhaps what accounts for the 25% of increased elevation that cannot be explained by the observed changes in the NAO.

So the impact of global warming in the Arctic is, in the short-term, likely to have added to a naturally occurring variation which yielded increased snowfall in Greenland between 1992 and 2003. Longer term the picture is likely to be different, with the overall outcome likely to be a slow melting of the Greenland ice sheet. But Professor Johanessen and the other scientists stress in their article that there is a need to continue to monitor the situation as eleven years of data, while providing for the first time a continuous record, remain insufficient to draw firm conclusions about long-term trends.

By: Gauthier Chapelle