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A continent in pieces

  • Written by  Mark Rowe
  • Published in Polar
A continent in pieces Shutterstock
01 Mar
2014
Temperatures are rising rapidly in parts of Antarctica, but the overall picture of the way in which the frozen continent and its inhabitants are responding to climate change is still incomplete

The science is emphatic – parts of Antarctica are among the most rapidly warming regions on Earth. According to scientists at the British Antarctic Survey (BAS), air temperatures at some weather stations are ‘rising at four to six times the global average rate’. On the Antarctic Peninsula, the air temperature has increased by as much as 3°C in the past 50 years, while a recently compiled temperature record for Byrd Station in West Antarctica revealed a linear increase in annual temperature between 1958 and 2010 that amounted to a total rise of about 2.4°C.

The impact of this temperature rise is being felt across the continent. On the small scale, it has led to an increase in the growth of moss and the activity of soil microbes on the Antarctic Peninsula. On a larger scale, it’s causing the volume of ice in the Antarctic Ice Sheet to decline.

The ice sheet losses are mainly from the northern Antarctic Peninsula and the Amundsen Sea sector of West Antarctica – primarily from the acceleration of outlet glaciers. In all, the average rate of ice loss from Antarctica has increased five-fold in the two decades, from 30 gigatonnes a year between 1992 and 2001, to 147 gigatonnes a year between 2002 and 2011 (some estimates put the current rate as high as 250 gigatonnes a year).

Scientists are now highly confident that the Antarctic Ice Sheet is in a state of net mass loss and that its contribution to sea level is also likely to have increased in the past two decades.

 

FLORAL ENCROACHMENT

One of the more unusual pieces of evidence that the Antarctic is changing can be seen on the Antarctic Peninsula, where Professor Lloyd Peck, individual merit scientist at the BAS, and colleagues have documented plants growing where there was ice 25 years ago. And looking out to sea, the picture becomes even more stark. Along the Antarctic Peninsula alone, Peck and colleagues estimate that an area of more than 5,100 square kilometres that was covered by sea ice before the 1980s is now open water in summer. The US National Snow and Ice Data Centre (NSIDC) has also noted similar decreases in sea ice around the Bellingshausen/Amundsen sea.

Elsewhere on the continent, 87 per cent of the 244 marine glaciers have retreated over the past 50 years, Peck says. ‘We know we can never convince the die-hard sceptic, and no one event can be directly attributed to climate change – but we can say we understand how the Southern Ocean works, we understand how the wind and weather patterns work, and the warming of the Antarctic Peninsula is entirely consistent with a global warming effect,’ he says.

The Pine Island Glacier on the West Antarctic Ice Sheet is also being followed closely. The glacier has sped up 73 per cent since 1974 and thinned throughout 1995–2008, with the ice shelf at the glacier’s end currently melting at a rate of about 80 cubic kilometres a year, 50 per cent faster than it was during the early 1990s.

The cause of the rapid melt is ‘grounding line retreat’. The grounding line is the point where the bottom of the glacier comes into contact with the ground, and recent studies suggest that warm ocean water is eating away at the glacier at this point, making it more unstable. There’s also evidence, from a study published a few years ago in Nature Geoscience, that an increase in the strength of ocean currents beneath the glacier is causing it to melt more rapidly.

The neighbouring Thwaites, Smith and Kohler glaciers are also speeding-up, thinning and contributing to increasing mass loss.

 

LIMITED DATA

And yet the picture across Antarctica isn’t straightforward, and nor is it complete. ‘It’s pretty obvious, but Antarctica doesn’t have the density of people you have in Europe, where people can walk down the street and see different butterflies that weren’t there before,’ says Peck. ‘We just don’t have the coverage to monitor species’ range within the sea, so we don’t have the data you can get in other parts of the planet.’

The pattern of warming across the Antarctic continent has been far from uniform. While West Antarctica has experienced rapid warming, there’s some evidence that parts of East Antarctica have showed some summer cooling.

And there are other anomalies. According to the NSIDC, Antarctic September sea ice has been increasing at 1.1 per cent per decade relative to the 1981–2010 average, mainly around the southern Indian Ocean and Ross Sea. Scientists are still unsure why this might be the case, with a number of different theories having been put forward.

Last year, a paper published in Nature Geoscience suggested that enhanced melting of the Antarctic ice sheet was the main factor behind the sea-ice expansion. A team of scientists led by Richard Bintanja of the Royal Netherlands Meteorological Institute in Utrecht used a combination of satellite and buoy observations of ocean temperature and salinity, and computer modelling to show that meltwater was forming a cool, freshwater cap on the ocean that facilitated the expansion of sea ice.

An earlier paper, by Paul Holland of the BAS and Ron Kwok from NASA’s Jet Propulsion Laboratory in California, also published in Nature Geoscience, pinned the blame on changing regional wind patterns, which the paper suggested were both physically moving the ice and changing the sea-surface temperature.

 

STRENGTHENING WINDS

The changes to the way in which air flows over Antarctica are having impacts elsewhere on the continent. The strengthening of westerly winds that force warm sea air over the Antarctic Peninsula is extending the region’s summer melt season. In a study published last year in the Journal of Geophysical Research, BAS scientists collated data from 30 weather stations on the peninsula. The results showed a significant increase in the length of the melting season over the past 60 years at most of the stations with the longest temperature records.

The scientists also created maps of the seasonal meltwater change between 1999 and 2009 using satellite data and found that unusually long melt seasons coincided with events of significant ice-shelf fragmentation. The results supported the theory that meltwater-caused cracks are the principle trigger for ice-shelf collapse.

According to NASA’s Earth Observatory, ice shelves frequently calve icebergs, and this is a natural process, not necessarily a sign of climate change. But the rapid disintegration and retreat of an ice shelf (such as the collapse of the Larsen B shelf in 2002) is seen as a warming signal. Although sea ice is too thin to physically buttress an ice shelf, intact sea ice may preserve cool conditions that stabilise an ice shelf because air currents passing over sea ice are cooler than those passing over open ocean.

 

CONSERVATION CONCERN

The knock-on effects of this melt appear to be having a considerable impact on the Antarctic food chain. BAS scientists have found that the density of krill is declining. Krill feed on phytoplankton and ice algae, which forms under the ice in winter. It’s thought that with less sea ice, there’s less algae and phytoplankton for the krill, which may in turn lead to a shortfall in food availability of seals and penguins.

The extinction of an emperor penguin colony on Dion Island, close to the peninsula, may be the first example of an Antarctic species losing a foothold in its native habitat because of climate change. ‘We believe the colony has now gone completely,’ says Dr Phil Trathan, head of conservation biology at the BAS. ‘If you lose sea ice, potentially many different things can happen in the ecosystem and in the marine food web. We believe that this has happened near this emperor colony and that climate change is the most likely cause.’

Trathan belives that the warming that has taken place along the peninsula is changing the nature of the snow that falls there. ‘You may get more wet snow than previously, and that means that penguin chicks may get wet and cold and maybe perish; in contrast, cold, dry snow just falls off their feathers and they don’t get cold or wet,’ he says.

Along the Antarctic Peninsula, populations of both chinstrap and Adélie penguins are now decreasing, and Trathan says that ‘a common contributing factor is likely to be climate change. I’m reasonably confident that along the west Antarctic Peninsula, ecological changes associated with climate change are some of the major influences on penguin populations,’ he says. ‘Elsewhere on the Antarctic continent, warming is less evident; also, in many places, penguin data are inadequate to categorically and statistically relate penguin population changes to climate change.’

The newly exposed Antarctic waters are also becoming home to new marine zooplankton and seabed communities, according to Peck. In time, this may well amount to a substantial and previously unquantified carbon sink that would act as a negative feedback to climate change.

This potential mitigating factor may be enough to save seabed organisms such as Antarctic sponges, shrimps and echinoderms. ‘If you try to keep common brittle stars at water temperatures above 2°C, they die,’ says Peck. ‘The water around the Antarctic Peninsula is around 1.5°C – if you start getting temperatures of 2°C–3°C, they’ll face a lot of problems.’

 

THE WARMER THE BETTER?

There will always be winners as well as losers with climate change, and at the moment, one winner appears to be the gentoo penguin, which is increasing in numbers and moving south. ‘They have a slightly wider diet than some other penguin species and eat both krill and fish,’ says Trathan. ‘They may just be more flexible.’

Antarctic fur seals are also being affected by climate change, according to Trathan. A recent study of fur seals at Bird Island in South Georgia found that breeding success declines when the weather warms. ‘We see strong climate variability signals affecting the marine system at South Georgia, with krill being less abundant in warmer years,’ he says.

Other studies suggest that some polar wildlife may already be adapting to climate change. ‘We’ve used satellite remote sensing to identify emperor penguin breeding colonies and we’ve recently found a few colonies breeding on ice shelves rather than on sea ice,’ says Trathan. ‘We don’t know if this is a new behaviour or if it’s just serendipitous that we’ve only now observed it. In general, we need to understand the behaviour of species better, as animals may be able to adapt in ways that we can’t imagine at the moment.’

The complexity of interactions among the various members of the Antarctic food chain also makes predictions difficult. For example, Trathan and others believe that populations of whales and seals may still be increasing after they were hunted almost to extinction in the 19th and early 20th centuries. The recovery of these species could have an impact on the marine ecosystem as they eat krill. ‘If there are more seals and whales eating krill, then there may be less krill available for penguins to eat, making it more difficult to understand changes in penguin populations,’ he says.

This phenomenon may well be influencing populations of macaroni penguins on South Georgia, which have dropped from 5.4 million pairs to one million pairs since the late 1970s. Nevertheless, it’s impossible to ignore the potential impact of climate change. ‘The implied resource competition and the observed population changes may also be exacerbated by recent reductions in Antarctic krill abundance, which have been linked with reductions in seasonal sea ice following recent, rapid, regional warming in the Antarctic,’ Thratham says.

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

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