31 August 2010, by Sara Coelho
The West Antarctic Ice Sheet (WAIS) may be vulnerable to collapse if temperatures continue to rise. Now a new report suggests that this has already happened, and not long ago. The findings support the idea that this ice sheet is highly unstable and vulnerable to minor changes in climate.
The idea of a trans-Antarctic seaway linking the Weddell Sea to the Ross Sea is not new, but the study published in Global Change Biology reports the first evidence that such a seaway opened in response to a collapse of the WAIS as recently as 125,000 years ago.
The conclusions are based on observations of bryozoans, a group of unassuming marine creatures that build coral-like structures up to a metre across.
'Bryozoans are spectacularly diverse around Antarctica,' says biologist Dr David Barnes, from the British Antarctic Survey (BAS).
'We know of at least 500 Antarctic species, some of them endemic to the area.' Bryozoans live attached to the ocean floor and don't travel far when they're larvae. This means that every region of the Southern Ocean has its own distinct collection of bryozoan species.
Barnes and geologist Dr Claus-Dieter Hillenbrand analysed the bryozoans found around much of the Antarctic continental shelf and the deep waters beyond, cataloguing species and defining geographical ranges as part of the Census of Antarctic Marine Life.
Having found which bryozoans appear where, 'we then used statistical methods to compare the degree of similarity between the assemblages in different areas,' explains Barnes.
The results were striking: the most similar pair of areas around Antarctica in terms of bryozoan assemblages is formed by the Weddell Sea and the Ross Sea, which are currently separated by the WAIS and by the islands buried under this large ice mass. Geography might place them a world apart, but the two seas share 77 per cent of their bryozoan fauna.
This similarity is more than between any other two regions, even those which are closest neighbours with directly-linked continental shelves.
The authors realise the significance of their study: 'Our data implies the existence of a seaway between the Weddell and the Ross Sea during the recent past,' they write. 'This is only possible without much of the West Antarctic Ice Sheet in the way, and this means global sea level rise up to five metres'.
'Clearly the WAIS is very vulnerable to changes in temperature,' Barnes says. 'It has collapsed in the past, under climate conditions similar to the ones we're observing today.'
The conclusions have important implications for studies on the stability of the ice sheet, but Barnes is confident of his findings. 'This by far the simplest explanation of our results,' he says.
The similarity between the bryozoans of the Weddell and the Ross Sea is unlikely to be explained by similar environments. The two seas have different local conditions, and their ecology is closer to their neighbours than to each other.
The two seas are also very unlikely to have been colonised from subpolar islands or the deep sea because their faunas are so different, and because few bryozoan larvae travel far, while adults do not move at all.
With these alternative hypothesis eliminated, Barnes concludes that the bryozoans must have mixed between the Weddell and the Ross Sea when the seaway was open.