Ice Loss Shifts Arctic Cycles Record Shrinkage Confounds Models and Portends Atmospheric and Ecological Change

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Larger areas of open ocean and warming surface waters will alter evaporation, cloudiness, precipitation and storm patterns across the Arctic and beyond.

CLIMATE Ice loss shifts Arctic cycles Record shrinkage confounds models and portends atmospheric and ecological change.

BY QUIRIN SCHIERMEIER (NSIDC) in Boulder, Colorado, announced the NSIDC, says that much of the Arctic pack record decline on 26 August, saying that the is now thin first-year ice — frozen only last efore indifferent satellite eyes, the top ice extent had dropped to 4.10 million square winter — which requires much less energy of the world is undergoing a transfor- kilometres (see ‘Going, going...’). The figure is to break apart and disperse than multi-year mation. The Arctic ice pack, a primary 70,000 square kilometres less than the previ- ice. “We have entered a new regime,” he says. Bindicator of climate change, has shrunk in ous record low, set in 2007, and it came at least “The sea ice is in such poor health in spring recent weeks to an extent that no computer two weeks before the annual low is typically that large parts of it can’t survive the summer model and few scientists had thought possible. reached. According to the NSIDC, by 9 Sep- melt season, even without boosts from extreme After five years that all saw less ice than tember that figure had dropped by another weather.” previously documented in the 34-year satel- 14%, to around 3.52 million square kilometres. Computer models that simulate how the ice lite record, this year’s record loss has scien- The massive melt has occurred in relatively will respond to a warming climate project that tists questioning their models. They are also normal weather con- the Arctic will be seasonally ‘ice free’ (defini- striving to understand the complex cascade of ditions, with only one NATURE.COM tions of this vary) some time between 2040 effects — from shifting weather patterns to dis- strong summer storm For more on the and the end of the century. But the observed placed marine species — that the accelerating to hasten the break-up receding Arctic downward trend in sea-ice cover suggests that retreat could trigger. of the pack ice. Mark ice, see: summer sea ice could disappear completely The US National Snow and Ice Data Center Serreze, director of the go.nature.com/s5w2pr as early as 2030, something that none of

the models used for the next report by the GOING, GOING … Maximum, much more of the Arctic was ice- Sea-ice coverage over the Arctic Ocean as 5 Intergovernmental Panel on Climate Change measured on 26 August — a record-breaking low. free than it is today — with no mass-extinc- 1 comes close to forecasting . R tion events known to have occurred. U “There’s a tremendous spread S Farther afield, this year’s record sea- between observations and model S ice melt might foreshadow a harsh Chukchi I Arctic projections,” says Serreze. “It might Sea A winter in parts of Europe and be that natural variability is larger Ocean North America. Recent research, than assumed, or perhaps models A although preliminary, suggests a Kara don’t get the change in ice thick- S connection between late-sum-

Sea CENTER AND ICE DATA SNOW NATIONAL ness right.” Uncertainty also U mer Arctic sea-ice extent and Beaufort remains over the strength of Sea the location of areas of high various natural ‘feedbacks’. For and low atmospheric pressure North Pole Barents

C example, an exposed ocean is Sea A over the northern Atlantic. A I S darker than an ice-covered sur- N S The highs and lows can remain U A Y R face and so absorbs more solar A relatively fixed for weeks, D

W heat, causing yet more warming A R FINLAND shaping storm tracks and sea-

O and melting. N sonal weather patterns such as A lack of fine detail about cir- SWEDEN extended cold surges6. Norwegian culation patterns in the Arctic G R E E N L A N D Sea Ralf Jaiser, a climate scientist at Ocean could also be throwing off the ( D E N M A R K ) the Alfred Wegener Institute for Polar models. For example, a survey carried and Marine Research in Potsdam, Ger- out in 2008 revealed 20 formerly unob- Median 18 September measurement 1979–2000 many, found a significant correlation in served eddies, each some 15 to 20 kilome- Previous record low, 18 September 2007 1989–2011 meteorological data between tres in diameter, in waters north of Canada. late-summer Arctic sea-ice extent and atmos- “Whether these are new features, and what pheric-pressure anomalies that favour extreme role they might play for ocean-mixing pro- weather such as prolonged cold snaps in win- cesses, we don’t know yet,” says Yves Gratton, dominant Arctic zooplankton — the copepods ter7. He reasons that in autumn, the open an oceanographer and Arctic researcher at Calanus hyperboreus and C. glacialis — are Arctic Ocean sheds heat to the high-latitude the National Institute of Scientific Research in being replaced by Atlantic C. finmarchicus. atmosphere. The warming tends to reduce the Montreal, Canada. Meanwhile, Arctic cod (Arctogadus glacialis) is large-scale atmospheric-pressure gradient and Ice loss could also accelerate if the ice pack’s increasingly being out-competed by its larger weakens the dominant westerly winds in the underlying waters warm up. Unlike in most Atlantic cousin Gadus morhua3. Northern Hemisphere. Those winds normally of the world’s oceans, the coldest water in the Other species are showing unexpected resil- sweep warm, moist Atlantic air to western Arctic, at −1 °C to −2 °C, is at the surface; below ience. In January, a research cruise into the Europe; their weakening leaves the region a depth of 200–300 metres, saltier and warmer pack ice north of Svalbard, Norway, found evi- more prone to persistent cold. water of about 1 °C flows in from the Atlantic. dence that Apherusa glacialis — amphipods, or “The impacts will become more apparent The cold surface layer — called the halocline small crustaceans, that feed on algae growing in autumn, once the freeze-up is under way — isolates the sea ice from the warmer water beneath the ice and are the main food source and we see how circulation patterns have influ- below. for many Arctic sea birds — are less dependent enced the geographical distribution of sea ice,” But the halocline is vulnerable to warming on extensive sea ice than previously thought. says Judith Curry, a climate researcher at the from above, says Henning Bauch, a marine Just as some coastal organisms drift with the Georgia Institute of Technology in Atlanta. geologist at the GEOMAR research centre in tide to retain their positions in river estuaries, But, she adds, “We can probably expect some- Kiel, Germany. A thinning halocline — some- these amphipods can sink to depths at which where in the mid/high latitudes of the North- thing that has not yet been observed — would pole-ward ocean currents sweep them farther ern Hemisphere to have a snowy and cold not only jeopardize the sea ice but could also north, where they can find and colonize sur- w i nt e r.” ■ melt the carbon-rich permafrost beneath shal- viving sea ice4. 2 1. Stroeve, J. C. et al. Geophys. Res. Lett. 39, L16502 low coastal waters , releasing greenhouse gases “This looks like an adaptive trait that allows (2012). into the atmosphere. key Arctic-ice fauna to persist in changing 2. Vonk, J. E. et al. Nature 489, 137–140 (2012). Meanwhile, Arctic biology is already chang- sea-ice environments,” says Berge, who led 3. Schiermeier, Q. Nature 446, 133–135 (2007). 4. Berge, J. et al. Biol. Lett. http://dx.doi.org/10.1098/ ing, as the retreat and thinning of sea ice allows the study. He points out that the Arctic Ocean rsbl.2012.0517 (2012). more sunlight to penetrate the upper ocean has been ice-free during the summer several 5. Funder, S. et al. Science 333, 747–750 (2011). and deprives certain species of habitat, says times during the past 2.5 million years, and 6. Liu, J., Curry, J. A., Wang, H., Song, M. & Horton, R. M. Proc. Natl Acad. Sci. USA 109, 4074–4079 (2012). Jørgen Berge, a marine biologist at the Uni- that as recently as 8,000–6,500 years ago, dur- 7. Jaiser, R., Dethloff, K., Handorf, D., Rinke, A. & versity of Tromsø in Norway. For example, the ing a period known as the Holocene Thermal Cohen, J. Tellus A 64, 11595 (2012).