The sleeping giant In Antarctica

The edge of Thwaites Glacier

Could it be that in 200 years the world’s coastlines are vastly different due to sea-level rise of several metres? Will millions have moved and hundreds of billions been spent on coastal protection? Would human-induced climate change be to blame and could the problem have come from Antarctica?

If it does happen, and it might, we may have started this ball rolling or we may have not for whatever we have done we should take notice of the changes occurring in the West Antarctic Ice Sheet (WAIS) which is receding and under attack from below by warm water and geothermal heating. As it is the WAIS is responsible for almost all of the Antarctic’s current contribution to global sea level rise.  How soon will this sleeping giant awaken? The past holds many, sometimes contadictory, clues.

The WAIS has 2.1 million cubic kilometres of ice above sea level and 1 million below. It arouses fear because of the ice that is sitting on bedrock below sea-level. As the glacier retreats the grounding line will eventually reach these areas and cause the destruction of the glacier from the base upward.

There has been much hype about the collapse of the West Antarctica Ice Sheet.  Particularly Pine Island and Thwaites Glaciers. But what if it collapsed? It terrifies some. One climate scientist and video blogger recently said that the “science is genuinely scary,” and said she felt physically sick and it made her cry, twice, adding that if it collapses it could result in 3-5 m of global sea-level rise in 3-500 years.

She was referring to Naughton et at in Nature Climate Change which contends that increased melting of the WAIS may be unavoidable whatever we do though I suspect given some of the research described here it would be the same even if there was no anthropogenic influence on the climate. But the claim it could happen in as little as 3 years is exaggeration.

To put the warm water melting the ice from below into context the adjacent Amundsen Sea is believed to have warmed in the 20th century but since observations only started in 1994 it is too soon to detect a long-term trend.

Models suggest that it has not reached a tipping point and is a long way from doing so. Some estimates that lies between 300-500 years to 10,000 years in the future. Recent simulations also suggest that is it does start to melt it will take many centuries, or longer, to fully respond.

Rapid changes

Whilst this is far in the future it is also clear that big changes can occur, very rapidly. A new study shows it was at its recent warmest some 4100 years ago (see Fig 1) and has been cooling ever since. The researchers looked at long-term temperature proxies - water isotopes - in the WAIS Divide Ice Core over the last 11.000 years. They found a regional Holocene thermal maximum of  4-5,000 BP that was 1.1 deg C warmer than today and reflects changes in maximum summer radiation caused by orbital effects. It also has data indicating the past lowering of the WAIS which was less than 162 m, and probably less than 50 m which is consistent with geological constraints.

Fig 1

Overall the researchers say their work provides a cautionary tale as the WAIS’s temperature history reflects many different controlling factors of summer and winter temperature whose importance varies in time. In the future it implies human influence will not be the entire story.

Inside the Thwaites Glacier drilling tent. CAM/BAS

That it doesn’t need us to induce major changes in the WAIS is also shown by recent research carried out by the British Antarctic Survey and Cambridge University that reveals that only 8,000 years ago it shrank dramatically, losing 450 m in height in less than two years.

Even biological data shows how unstable the WAIS can be. Using genetic analysis of a type of circum-Antarctic octopus researchers show that the WAIS collapsed completely during the last interglacial period, when global sea levels were 5 to 10 metres higher than today and global temperatures were up to 1.5 deg C warmer than preindustrial. As a result they suggest that the WAIS is close to a tipping point.

But things are happening now Grieman et al. in Nature Geoscience investigate the retreat, grounding and buttressing of the Ronnie Ice Shelf which moderates WAIS stability shows evidence of rapid loss in the past and again shows how fast the WAIS can change. Miles and Bingham look at more recent data showing the  unanchoring of the WAIS since 1973 with its ice loss driven by the thinning of floating ice shelves that reduces their buttress potential and allows ice to accelerate towards the sea. They find that in the 1973 – 1989 period there was only local ice sheet thinning but since then it has spread rapidly.

The West Antarctic Ice Sheet will be a problem humanity may have to face someday, but there is no need to cry just yet.