Arguing over when the Arctic ocean will be ice-free might be missing the point
This year the UK media has given the record Arctic sea ice loss a great deal of coverage, including plenty of prominent reports that the Arctic Ocean will be almost entirely free of sea ice within the next four years, a prediction which comes from the most pessimistic end of expert scientific opinion.
But other polar scientists tell us that arguing over precisely when the Arctic will be sea ice-free in summer misses the point – we don’t need to lose all of the Arctic sea ice to feel the most significant impacts on climate and transport.
Arctic sea ice has been declining over the last four or five decades, with the seasonal low summer ice coverage shrinking particularly quickly. But something new is happening – on top of the long-term decline, scientists now believe that conditions in the Arctic have changed markedly in recent years.
Since 2007, Arctic weather patterns have changed, and sea ice extent has seen some dramatic falls – the six lowest ice extents in the satellite record occurred in the last six years. Climate models that capture sea ice volume up to around 2006 are now failing to model it well – sea ice has become thinner than models estimate it should be.
What does this year’s record low mean?
Scientists widely accept that Arctic sea ice decline over the last three decades is in large part due to manmade climate change. The most recent estimate suggests that between 70 and 95 per cent of Arctic sea ice loss in the past three decades is due to human-induced warming.
Over the last six years sea ice looks like it’s been declining faster than this long term trend, and from parts of the media you might get the impression that scientists agree the more dramatic sea ice decline of the last six years is all down to manmade climate change.
But Arctic experts report that they cannot rule out the possibility that natural variability is playing a role in the recent shift in Arctic weather patterns and rapid decline in sea ice. So the dramatic sea ice decline of the last six years isn’t necessarily due just to manmade climate change.
One reason Arctic sea ice loss is of such widespread interest, is that it provides a particularly visible sign of environmental change. But experts expect to see significant knock-on effects from losing the much of the sea ice at the top of the planet. For example, sea ice loss will lead to vast stretches of open ocean during the summer months, amplifying warming in the region as the open water absorbs heat which would previously have been reflected by ice.
Ice loss is also enabling access to previously iced-up shipping routes and mineral exploration. This summer, for example, a three man sailing expedition navigated the Northwest Passage – a traditionally frozen stretch of water. Future sea ice loss might even affect weather in the northern hemisphere.
2016 – a lower bound?
Climate models cannot pinpoint a precise year or even decade when the Arctic Ocean will be completely free of ice in summer. Most recent projections put it within the next few decades.
Professor Peter Wadhams, ocean physicist at the University of Cambridge, disagrees with these climate model projections, telling a number of news outlets and a parliamentary committee that the Arctic Ocean will be almost entirely free of sea ice by the summer of 2016, and that this is down to manmade climate change. In an email to the Guardian, he writes:
“I have been predicting [the collapse of sea ice in summer months] for many years. The main cause is simply global warming: as the climate has warmed there has been less ice growth during the winter and more ice melt during the summer. […] This collapse, I predicted would occur in 2015-16 at which time the summer Arctic (August to September) would become ice-free. The final collapse towards that state is now happening and will probably be complete by those dates”.
Wadhams’s predictions have not been published in a peer reviewed journal as far as we can tell. We think he has come to this conclusion by extrapolating forward in time from the University of Washington’s PIOMAS (Pan-Arctic Ice-Ocean Modeling and Assimilation System) estimates of sea ice volume based on observations, as shown in the diagram below:
Image - Extrapolation from PIOMAS (note)
How we think Wadhams extrapolated when the Arctic might be sea ice-free from sea ice volume data. Source: BBC News
It’s worth noting this is one end of the spectrum of expert opinion. Other scientists differ on the likely date for a ‘sea ice-free’ Arctic. It also raises questions about whether projecting into the future from PIOMAS is likely to produce useful results.
This includes the PIOMAS team themselves, who question whether it makes sense to predict future sea ice decline simply by extending the recent trend from the last six years into the future. This is because it assumes all of the recent ice loss is down to global warming and that the decline will continue in precisely the same way into the future. Such a method ignores any changes to sea ice caused by natural climate variability.
PIOMAS scientist Dr Axel Schweiger tells us:
“I don’t think this year’s record in ice extent changes anything fundamentally. Given the continuing trend in reduced ice thickness, new records are bound to happen […] The fact that observed changes seem to outpace predictions made by climate models as a group isn’t sufficient reason for me to substitute climate model results with extrapolations of observations. Even as intuitively compelling it may be. I don’t extrapolate the growth rate of my teenage children.”
Dr Ted Scambos, senior research scientist at the US National Snow and Ice Data Center, a research body which maintains the most widely used record of ice extent, tells us:
“I think [Arctic sea ice extent] is unlikely to go below [one million square kilometres] by 2016 (four summers from now) because the final [two million square kilometres] should be thicker ice. Even a single winter produces some thick ice by buckling [building up] against the northern Canadian and Greenland coasts.”
But physical oceanographer Professor Wieslaw Maslowski, of the Naval Postgraduate School in California argues in a new research paper that these extrapolations can play a useful role in trying to understand the changes in the region. He writes that we can consider 2016 to be the earliest date by which the Arctic Ocean might be practically sea ice-free.
In any case, model simulations of when the Arctic might be sea ice-free in summer are likely to improve. Dr Helene Hewitt, manager of the polar climate group at the Met Office, tells us:
“We continually evaluate the models against the ongoing record of observations. As we develop a greater understanding of Arctic processes, climate models will be improved which may lead to future refinements of our projections of an ice-free Arctic.”
Missing the point?
But Ted Scambos makes an important point when discussing such predictions, telling us that in his view the focus on a sea ice-free Arctic misses the point. He says that in reality “ice-free is a hard thing to achieve”. He adds that by the time there’s around one million square kilometres of sea ice left in the Arctic Ocean, “essentially all the changes to access to the Arctic (e.g. shipping) and all the climate effects (warming of the ocean and surrounding land) will be in force.”
Helene Hewitt tells us that one million square kilometres of Arctic sea ice in summer “can be taken as an indicative date of when the Arctic is likely to appear largely as open water as well as when there will be relatively easy access in the Arctic for a few weeks in the summer.”
There is no doubt that Arctic sea ice is in decline, and has been over course of the satellite record, but right now it’s not clear how much of the recent more dramatic decline is down to manmade climate change.
It might all be down to manmade warming. Or we could be seeing shorter-term natural variability superimposed on the long-term declining trend. As yet, scientists cannot say for certain which of these is the case.
Either way, the overall effects are the same. Whether it takes another four year or forty years, enough of the Arctic sea ice will be gone in the summer months for us to feel the effects.