16th February 2013
Arctic sea ice volume has collapsed
New data from CryoSat-2 confirms that Arctic sea ice volume shrank by 36% between 2003 and 2012.
In August 2012, the surface area (or extent) of Arctic sea ice cover was the lowest ever recorded, breaking the previous record in 2007. Now the underlying volume, seen above – a far more accurate indicator of the overall changes taking place – has been shown to have collapsed at an even faster rate, as the ice becomes thinner and more fragile.
Since 1979, a system developed at the University of Washington (UW) has provided a monthly estimate of what's happening to Arctic sea ice volume. The Pan-Arctic Ice Ocean Modeling and Assimilation System (PIOMAS) combines weather records, sea-surface temperature and satellite pictures of ice coverage to compute ice volume. It then verifies the results with actual thickness measurements from individual moorings or submarines that cruise below the ice. However, because the ice is so variable, these are unable to provide a complete picture. The PIOMAS model has been the only way to reconstruct a time series that spans multiple decades.
From 2003-2008, NASA's ICESat (Ice, Cloud, and land Elevation Satellite) provided ice measurements across the Arctic to within 15" (37 cm). It was followed in April 2010 by the more powerful and sophisticated CryoSat-2, launched by the European Space Agency (ESA). With its high precision radar altimeter, this has been tracking changes across the entire Arctic Ocean Basin with a resolution of just 0.5" (1.3 cm).
Using CryoSat-2's new data, combined with measurements from the older ICESat spacecraft, researchers have observed a 36% decline in autumn sea ice volume between 2003 and 2012. This confirms the continuing trend in volume decline simulated by the PIOMAS model.
The results are published in Geophysical Research Letters. Co-author of the study, Axel Schweiger, a polar scientist in the UW Applied Physics Laboratory: "These data essentially confirm that in the last few years, for which we haven't really had data, the observations are very close to what we see in the model. So that increases our confidence for the overall time series from 1979 to the present."
Dr Katharine Giles, from the Centre for Polar Observation and Modelling (CPOM) at University College London: "Thin ice grows more quickly than thick ice in the winter. Ice acts as an insulator – the thinner the ice, the more heat can be lost to the atmosphere and the faster the water beneath the ice can freeze. But even with an increased ice growth during the winter, we can see from the Cryosat data that it's still not fully compensating for the deep summer melt."
Credit: Bonnie Light, University of Washington
If recent volume trends continue, the Arctic will soon be ice-free during short periods each year, with dire implications for climate change. Perhaps as early as 2015, we may see the first photo from space that shows nothing but clear water at the North Pole. Make no mistake about it: this will be a profound and serious change; the first of many tipping points during the 21st century. Since open water is darker than white ice, the Arctic will enter a phase shift – from reflecting light and heat back into space – to a new state, an ocean area sucking up sunlight and producing heat.
What happens to the jet stream and weather patterns when the high Arctic becomes a giant heat sponge? Water will warm very quickly in summer, raising temperature across the bordering tundra areas. Feedback loops will accelerate. Permafrost in Siberia, Canada and Alaska will begin to melt, releasing fossil carbon and methane. A recent study by the U.S. Geological Survey found that 850 billion tons of carbon are stored in the frozen Arctic ground: equal to the level already in the atmosphere today. We ignore scientists' warnings at our peril.
CryoSat-2 is funded until 2017, but may operate until the end of the decade, for overlapping coverage with ICESat-2, successor to NASA's earlier satellite. This potential overlap greatly improves the prospects for better knowledge of Arctic sea ice volume. Ron Kwok, at NASA's Jet Propulsion Laboratory: "The hope is that we'll be able to create a multi-decadal record using ICESat, CryoSat-2 and ICESat-2."