Climate change is not only making Arctic sea ice disappear — it's also changing the type of ice that forms. Researchers are now trying to determine how an increase in 'pancake ice' is affecting the far north, including whether it's accelerating local warming.

In the past, Arctic waters have been dominated by thick slabs of sea ice that last from one year to the next. But sea-ice cover is diminishing and thick ice that lasts for several years is disappearing fast, with researchers seeing a greater proportion of thin, newly formed ice.

New ice can form in several different ways. When water is surrounded by ice packs, as has been common in the Arctic, areas of open water are small and there is little chance for wind to work up vigorous waves. In such calm conditions, ice forms in unbroken sheets called 'nilas.'

But now the Arctic has larger areas of open water, and more waves. "As soon as you introduce swell, you get an entirely different form of ice," says Jeremy Wilkinson of the Scottish Association for Marine Science in Oban, UK. Under these conditions, globs of ice crystals tossed about in the water combine to form first a soupy mixture called 'grease ice', and then 'pancakes' of thin ice a metre or two in diameter.

This can have all sorts of knock-on effects. Because the pancakes are round, for example, they have areas of open water between them when joined up, making the surface darker overall. This could have a warming effect as a result of less of the Sun's radiation being reflected. Water also slops up from these holes over the ice so that falling snow melts rather than settling, keeping the surface darker. "This whole cycle is not in models of the Arctic or the Antarctic. It's one of these conundrums that people haven't looked into," says Wilkinson.

Pancakes in the lab

Wilkinson and his colleagues this month completed a series of controlled experiments to measure the differences between nilas and pancake ice in the lab, including differences in ice thickness, water temperature, salinity and albedo — the fraction of incident sunlight that's reflected. The team used a wave tank 30 metres long and 1.5 metres deep at the Arctic Environmental Test Basin in Hamburg, Germany, to test ice formation in calm conditions compared with choppy water and storms¹. The team could not replicate the metre-high waves that might be seen in the ocean, so to mimic stormy conditions they increased the frequency of waves tens of centimetres high.

Their tests confirmed that, as expected, pancake ice dominated when waves were around. Their as-yet unpublished quantitative data, which will take some months yet to analyse, will be incorporated into new climate models being developed by team member Dirk Notz and others at the Max Planck Institute for Meteorology in Hamburg to see how the changing ice type might affect future Arctic temperatures.

There are effects other than a change in albedo. Ice accumulates on the bottom of a single sheet more slowly than it does around crystals bobbing up and down in the water, so pancake ice 0-15 cm thick can form in the same time as 1 cm thickness of nilas ice. As ice formation extracts fresh water from the ocean, faster ice formation should mean saltier seas, which could in turn have an impact on ocean circulation, ice growth and air temperature.

"Young ice isn't that well studied because there didn't used to be that much of it," says David Barber of the University of Manitoba in Winnipeg, Canada, who led an over-wintering project investigating sea ice conditions in the Arctic that ended last year. He says there is clearly more open water during the winter freeze-up now, causing cyclonic storms that bring wind and snow, and more pancake ice. "We had to take our ship out of the open water because the storms were too rough for us," says Barber. "We couldn't land our helicopters because there was too much snow." He notes that although the extent of the sea ice was greater in 2008 than during the record low of 2007, this was because of additional young ice; the multi-year ice had in fact declined from 2007 levels.

Such rare observations of the winter ice pack along with lab and modelling work should help to determine the likely future of the Arctic. "We're trying to work out what the net result of all these processes are," says Barber.

• References

  1. Wilkinson J. et al. Eos Trans. AGU 10, 81-82 (2009).

Link to article: http://www.nature.com/news/2009/090323/full/news.2009.183.html