Arctic sea ice volume can bounce back during cooler summers

The volume of Arctic sea ice jumped in 2013, suggesting that sea ice may be surprisingly capable of bouncing back temporarily during cooler summers.

Data from the European Space Agency's CryoSat-2 satellite show that Arctic sea ice volume during its annual autumn minimum increased 41 per cent in 2013 compared to the year before, following an unusually cool summer, the researchers reported in the journal Nature Geoscience this week.

• Summary of the paper

"The sharp increase in sea ice volume after just one cool summer suggests that Arctic sea ice may be more resilient than has been previously considered," they wrote in a more detailed analysis of results first reported by the space agency in 2013.

So the events of 2013 will have simply wound the clock back a few years on the long-term pattern of decline.- Andy Shepherd, study co-author

The Arctic has warmed unusually quickly due to human-caused climate change, and that has had a dramatic effect on the proportion of the Arctic covered by sea ice (also known as sea ice extent), which has declined about 40 per cent since the late 1970s. That, in turn, has raised concerns because ice strongly affects the Arctic environment. For example, snow-covered sea ice reflects sunlight, cooling the surrounding area, while dark, melted ice causes increased warming.

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The Cryosat-2 satellite was launched in 2010, and measured a 14 per cent decrease in sea ice volume from 2010 to 2012. The proportion of the Arctic covered by sea ice hit a record low in 2011, according to data from NASA's Aqua satellite. The U.S.-based National Snow and Ice Data Center reported another record low in 2012.

But in 2013, a cooler summer meant there were fewer days when the ice melted. Compared to the average for 2010 to 2012, there was 33 per cent more ice in 2013. The ice was also 21 per cent thicker, on average, than the average for 2010 to 2014 – up to 3.5 metres thick in some places.

"Although models have suggested that the volume of Arctic sea ice is in long-term decline, we know now that it can recover by a significant amount if the melting season is cut short," said Rachel Tilling, lead author and a PhD student at University College London's Centre for Polar Observation and Modelling, in a statement.

But co-author Andy Shepherd, professor of Earth Observation at University College London and the University of Leeds, said Arctic temperatures are expected to keep rising in the future.

"So the events of 2013 will have simply wound the clock back a few years on the long-term pattern of decline."

Paul Kushner, principal investigator of the Canadian Sea Ice and Snow Evolution Network and a physics professor at the University of Toronto, who was not involved in the study, said the amount of the increase in sea ice volume in 2013 isn't surprising.

"It's consistent with a simple relationship the authors find between melting degree days (the number of days above freezing) and sea ice loss each year," he said in an email. "Even with global warming and even stronger Arctic warming, there will be years like 2013 and 2014 where things will cool down a bit."

In 2014, the sea ice volume declined by nine per cent, the study showed.

Without snow cover, ice changes faster

John Yackel, a sea ice scientist and climatologist at the University of Calgary who was not involved in the study, said it also demonstrates how quickly sea ice volume can change when the surface of the ice isn't covered by an insulating blanket of snow, as is happening more often.

"It allows the ice to respond to these changes in the atmosphere really quickly," he said. "It can either grow thick really quick if it's cold or melt really fast if it's warm."

He added that the thicker ice in 2013 was still very thin compared to the thicknesses of five to seven metres thick that were directly measured in parts of the Arctic in the late 1970s, and is likely not as cold as that thicker ice.

That means it's quite susceptible to melting the next summer. This summer there has already been a lot of Arctic sea ice melting, he said, and sea ice extent could be headed for another record low for its minimum extent this fall.

It already set a record low for its maximum spread this past spring. However, in general, the new study found that the Arctic springtime sea ice volume is relatively stable.

Kushner thinks the real significance of the study is it shows how well the Cyrosat-2 satellite is able to measure sea ice thickness and volume – something that has been much harder to measure in the past than the extent of sea ice, which is not necessarily proportional to the volume or thickness.

"We are now for the first time going to be able to put solid numbers behind our understanding of how sea ice thickness and volume will vary in the coming years," Kushner added.