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A clear signal

  • Written by  Mark Rowe
  • Published in Polar
A clear signal Photoshot Holdings/Alamy
01 Mar
2014
With the summer ice melting and temperatures rising at unprecedented rates, scientists are all but certain that changes taking place in the Arctic can be attributed to anthropogenic climate change

In  September 2012, the world’s attention turned to the Arctic as warmer than average summer temperatures melted the sea ice around the North Pole. By the time temperatures began to cool down again, the average extent of the Arctic sea ice had shrunk to just 3.61 million square kilometres. This was 690,000 square kilometres less than the previous record low in 2007. Between March 2012, when the ice reached its maximum extent for that year, and 16 September, the Arctic Ocean lost a total of 11.83 million square kilometres of ice (which isn’t far off the total land area of Antarctica).

But this record loss is merely part of a long-term trend that scientists are all but certain is directly attributable to anthropogenic climate change, which has seen temperatures rise across the Arctic region at a staggering rate. The most pronounced warming has taken place in the East Siberian region, where surface air temperature increased by 5°C between 2000 and 2005.

 

RECORD DECLINE

Passive microwave satellites have been circling the North Pole several times a day since 1978, able to look through cloud and the polar night to see where the sea ice is – and where it isn’t. Over that time, September Arctic sea ice extent has declined by 13.7 per cent per decade, according to the University of Colorado’s National Snow and Ice Data Center (NSIDC). ‘It has dropped from around eight million square kilometres over that time to around four million square kilometres,’ says Dr Jeremy Wilkinson, lead investigator at the British Antarctic Survey (BAS). ‘We can be 99.9 per cent sure that that is due to a warming planet.’

But the retreat of the sea ice – which is mainly occurring around the large continental shelves of Canada, Alaska and Siberia – is just one part of the story. For decades, US and UK nuclear submarines have travelled under the Arctic ice, where they deploy an upward-looking sonar system (partly for safety, allowing them to find thinner ice through which to surface) to measure its thickness.

Analysis of these data has shown how the ice has thinned from an average of 3.5 metres in the 1970s to about 2.5 metres today. Wilkinson can point to another profound development. ‘About five years ago, the ice shifted from predominantly multi-year ice [more than one year old] to predominantly first-year ice. These changes influence the thickness and the strength of ice, and as a result, the ice sheet can become much more dynamic.’ On top of that, Wilkinson points out, an anomalously warm summer
may not significantly affect the thicker multi-year ice, but can have a detrimental impact on the first-year ice.

Another consequence of these developments is the albedo feedback effect. Loss of the sea ice produces larger areas of open water. These darker areas absorb more solar radiation than the more reflective white ice, which in turn warms up the surface waters – which increases the melt-rate of sea ice, as well as delaying when water starts to freeze again. ‘The sea ice has an albedo of 0.8 – which means it reflects 80 per cent of solar radiation back into space,’ says Wilkinson. ‘But water has an albedo of 0.2, so it absorbs 80 per cent of the radiation.’

 

GREENLAND MELT

Away from the sea ice, ice caps and glaciers across the Arctic are also retreating. Indeed, a study published in 2011 found that between 2004 and 2009, Canada’s Arctic glaciers lost the equivalent of about three quarters the volume of Lake Erie, while the situation is Greenland has been even more alarming. ‘Last summer [2012] was the first time when the entire surface of the Greenland ice sheet was in melt for a few days,’ says Professor Peter Wadhams, head of the Polar Ocean Physics Group at the University of Cambridge.

The summer melt of the Greenland ice sheet is, according to Wadhams, the direct result of climate change and higher temperatures and warm air masses that now pass over it. ‘There was no observed melting of the ice sheet until the early 1980s, but the mass loss from the ice sheet is going up year on year and has become the main single contributor to sea-level rise,’ he says.

NASA’s scientists, too, have been struck by the melting, particularly of the Baffin Island ice caps. ‘These aren’t glaciers; they don’t move,’ says Dr Gavin Schmidt, deputy chief at NASA’s Goddard Institute of Space Studies, ‘but if you go to their edges in summer, you now see plants that have been frozen, preserved under the ice. Scientists are finding plants that have no radiocarbon in them, which puts them at around 125,000 years old. If people want evidence that the world is warming, and something important is happening on a geological scale, that is it.’

 

HUMAN INFLUENCE

Even though evidence of a melting Arctic is irrefutable, nailing down the cause is more difficult. ‘You can’t directly attribute things to a smoking gun next to melting particles of ice –
it’s mostly a process of elimination,’ says Dr Ted Scambos, lead scientist at the NSIDC. ‘People look at other possible causes – soot, solar irradiation or past volcanic activity – but these aren’t enough by themselves. The Arctic is warming and melting too rapidly for those. It’s so abrupt. The pace of melt of Arctic snow cover has accelerated dramatically over the past 40 years. We’ve seen a tremendous loss of sea ice. Every glacier system is in retreat.’

NASA is certain humans are behind the melting Arctic. ‘Current climate models are highly unlikely to produce this distinctive signal pattern by internal variability alone, or in response to naturally forced changes in solar output and volcanic aerosol loadings,’ says Schmidt. ‘We detect a human influence signal in all cases, even if we test against natural variability estimates with much larger fluctuations in solar and volcanic influences than those observed since 1979. When you consider ocean variability and the sun, the fingerprints of anthropogenic climate change are all over this.’

 

FURTHER EMISSIONS

Documented increases in offshore methane emissions are another pointer towards the real and current effects of climate change. ‘It has only been picked up on for a few years, but the observations in summer show it’s increasing quite quickly,’ says Wadhams.

The methane is stored as methane hydrate or clathrate, essentially a type of ice that’s only stable at certain combinations of temperature and pressure. Warming of Arctic waters is causing offshore permafrost to thaw, allowing the clathrate to melt and releasing methane into the water and thence into the atmosphere. This is of particular concern because methane is more than 30 times as potent at trapping heat as carbon dioxide.

According to a study published last year, the East Siberian Arctic Shelf (ESAS) has already started releasing more than twice as much methane as was previously thought, amounting
to at least 17 million tonnes of the gas entering the atmosphere each year. Although this isn’t a significant amount (globally, ruminant livestock produce about 80 million tonnes of methane a year), estimates suggest that at least 1,400 gigatonnes of carbon is stored in the ESAS, and there are fears that if temperatures rise much further, the shelf could release as much as 50 gigatonnes of methane over a relatively short period, which would wreak havoc on the world’s climate.

On land, several scientists, says Scambos, ‘are seeing a reduction of permafrost across Siberia and even Mongolia’. This permafrost, which represents thousands of years’ of frozen carbon, is another methane source. It’s thought that from 1975 to 2005, permafrost up to 15 metres thick in the Russian European north completely thawed.

The loss of the Arctic sea ice is also accelerating the rate at which permafrost is disappearing. Work by the German Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research has found that the high cliffs of Eastern Siberia, which mainly consist of permafrost, have retreated on average 2.2 metres a year in the past 40 years.

The institute’s scientists concluded that if average temperatures rise by 1°C in the summer, erosion accelerates by 1.2 metres a year. According to the institute’s Paul Overduin ‘during the past two decades, there were, on average, fewer than 80 ice-free days in this region per year. During the past three years, however, we counted 96 ice-free days on average.’ This means, he says, that waves ‘can nibble’ at the permafrost coasts for about two more weeks each year.

 

MORE IN STORE

And the permafrost has recently begun to unveil more unsettling surprises, following the discovery that it’s releasing methane in winter, as well as summer. ‘In the early years, scientists were only looking at the permafrost in summer,’ says Professor Bruce Forbes of the Arctic Centre at Lapland University. ‘In recent years, they’ve noticed that methane is released in winter and sits under the lake ice until it melts in the summer. The speed at which this is happening is quicker than we’ve ever seen. We’re squeezing this into a few decades. It has probably happened before, but then it took centuries.’

Forbes studies changes to the permafrost across a vast area from Scandinavia to northwest Siberia, and he has observed numerous changes across the region during his time there. ‘We’re seeing an earlier start for the growing season, later freezing in autumn, increased growth of shrubs,’ he says. ‘There are areas [of the Arctic] that were carbon and methane sinks in the 1960s and ’70s, but are now sources.’

A recent study by scientists from the University of Oxford backs up Forbes’s observations of a greening Arctic. Looking across the northwest Eurasian tundra, a vast area of 100,000 square kilometres that stretches from western Siberia to Finland, they found that willow and alder shrubs have grown into trees more than two metres in height in the past 30–40 years. This can further alter the albedo, because shrubs tend to get covered in white, light-reflecting snow, but trees are tall enough to rise above the snowfall, presenting a dark, light-absorbing surface.

That particular study drew heavily on observations from indigenous reindeer herders, a source that scientists are increasingly utilising to reinforce the case that climate change is happening. Wilkinson admits that scientists haven’t always done enough of this in the past. ‘There’s a lot to be learned from the changes they report – they see it day in, day out, in a way that scientists can’t,’ he says.

Wilkinson and colleagues have placed sensors on the sleds of Inuit hunters to measure the thickness of the ice. ‘They say that the ice is thinning, that they lose hunters through the ice and that they can’t get to hunt walruses, and that weather patterns have changed,’ he says.

 

BEARING THE BRUNT

Walruses are also responding to the lack of sea ice in different ways – none of which suggest a beneficial impact from climate change. ‘The walruses in the Pacific Arctic are having to haul out on land because there’s no ice, so you get 40,000 huge animals stampeding and pushed together – the younger animals get crushed,’ says Professor Tom Arnbom of WWF International. ‘We did think a lack of sea ice might help walruses, as it might make it easier for them to find mussels, but they need the ice to haul out on and rest. Now they find it difficult to forage the long distances for food.’

However, the biggest climate change casualties by numbers appear to be harp seal pups. According to WWF, data from Canada’s Department of Fisheries show that 500,000 harp seal pups have drowned in each of the past three years because the sea ice is melting prematurely. ‘There are ten million harp seals, so the population can withstand it for now,’ says Arnbom, ‘but it’s the most dramatic example of climate change that I’ve seen.’

This story was published in the March 2014 edition of Geographical Magazine

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