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New study says the Greenland ice sheet is becoming more sensitive to atmospheric warming

• 02 Aug 2012, 19:00
• Verity Payne

For the first time scientists have produced a detailed 30 year record for ice loss in northwest Greenland. The new research provides evidence that the massive ice sheet might be more robust on short timescales than some climate models and observational studies have indicated. But it also suggests that the ice sheet is becoming more sensitive to warming of the atmosphere.

Until now evidence for whether Greenland is losing or gaining ice has largely come from satellite data, which only dates back to 2002 - a pretty short amount of time for trying to work out trends caused by climate change.

By combining the recent satellite data with old aerial photos, researchers managed to extend ice sheet observations of northwest Greenland back to 1985 - giving them three decades of information about how the ice sheet has thinned.

Visualization of the Kong Oscar Glacier, Northwest Greenland generated from the aerial photographs used in the study. The image covers 17 kilometers by 24 kilometers and shows iceberg calving in 1985. The position of the glacier front from 2005 and 2010 deducted from Landsat satellite imagery are also marked. Credit: Anders A. Bjørk, Natural History Museum of Denmark.

This also let them identify two different types of melting - melting on the ice sheet surface, and 'dynamic ice loss' involving ice sheet movements - glaciers flowing towards the ocean where icebergs calve, causing the ice sheet to become thinner.

The researchers find that in northwest Greenland there have been two periods of rapid ice loss in the last three decades. The first, between 1985 and 1993, was mainly dynamic ice loss, probably triggered by ocean warming. The later ice loss event, between 2005 and 2010, involved both dynamic and surface ice loss, suggesting "that the Greenland ice sheet is becoming increasingly sensitive to atmospheric warming."

The authors conclude that their study shows glaciers in northwest Greenland can be 'self-stabilising' - even after a period of rapid ice melt, the ice sheet can stabilise again. Lead author of the study,  Kurt H. Kjær of the Natural History Museum of Denmark, says:

"Our results show that the thinning of the ice sheet at the end of the 80's and beginning of the 90's eased over a 4-8 year period, after which a period of stability occurred until 2003. Our conclusion is therefore, that if we judged against longer periods of time, the current thinning of the ice sheet is likely to ease within an 8-year period."

We spoke with glaciologist Ruth Mottram, of the Danish Meteorological Institute, who agrees that the results show "the glacier system can be self-stabilising at least on short timescale", adding "we see that in other places in Greenland and on other glaciers".

Implications for future ice melt

The authors of this study suggest that their research might cast doubt on projections of future sea level rise which are based on short satellite records of Greenland ice melt. Kjær explains:

"Variations in the amount of thinning that we are able to document since the 80's make it difficult to predict how much the world's oceans will rise over a longer period of time - a century for instance - as a result of Greenland glacial melt-water runoff.
However, it is certain that many of the present calculations and computer models of ice sheet conditions that built upon a short range of years since 2000 must be reassessed. It is too early to proclaim the 'ice sheet's future doom'..."

But Mottram told us that caution is needed in assuming that future ice loss will inevitably be followed by stabilisation, based on the earlier ice loss event. She pointed out that the analysis shows "[surface melt] played a much bigger role in the mass loss of the second event from 2005 - 2010."

Mottram also highlighted the fact that the air and sea temperatures in the 1985 - 1993 event were very different from those of the 2002 - 2010 event - the latter event involved both warmer air and sea surface temperatures. It is possible that surface melting might play more of a role as warming progresses.

Mottram agrees with the authors that coming up with realistic sea level rise projections is difficult, saying:

"[O]ne of the main sources of uncertainty is in fact the forcing, that is how much CO2 will be emitted into the future, then there are many uncertainties related to the precise nature of feedbacks between ocean, atmosphere and ice that still need to be clarified. However, the models we use to help understand these processes have been developed over a long period of time and continue to be developed so a study of this nature offers an excellent opportunity to validate and improve these models."