'Drastic' Antarctic melt could double global sea-level rise

Global sea levels could rise by more than double the current best estimate, according to a new analysis of climate change in Antarctica.

The latest model suggests that Antarctic melting could double projections of sea-level rise

The modelling assessment says that Antarctic melting alone could contribute more than a metre to sea level by the end of this century.

By 2500, according to the study, the same source could cause levels across the world to rise by 13m.

The authors say that rapid cuts in carbon emissions could limit this risk.

Competing ideas

In 2013, the Intergovernmental Panel on Climate Change (IPCC) predicted that, without any restrictions on carbon emissions, the seas around the world likely rise by up to 98cm by 2100.

However, the IPCC estimates contained a minimum contribution from Antarctica.

Other analyses since then have projected bigger increases, with a recent studysuggesting that the oceans were rising faster than at any time in the past 2,800 years and by 2100 they could be up to 1.31m higher.

The exact level of Antarctica's impact on these projections has been vigorously debated. Late last year, a research paper suggested that projections of a contribution of a metre or more were not plausible.

But this new study argues that by 2100 the world could see 1.14m of sea-level rise from Antarctica alone.

Additions to the model

The scientists say that their model is able to provide a more accurate prediction because it incorporates the impacts of some physical processes for the first time.

While other models have focussed on the impact of warmer waters melting the ice shelves from below, this new study also includes the effect of surface melt-water and rain trickling down from above and fracturing supporting ice, hastening its slide to the sea.

The model also calculates the impact of the disintegration of floating ice shelves. If this happens, it will reveal walls of ice so tall that they cannot support their own weight.

The scientists involved expect that these extra factors will kick in over the coming decades, as warming from the atmosphere (not just from warmer waters below) becomes the dominant driver of ice loss.

The collapse of ice cliffs could be a significant factor in the new projection

"One reason that other models didn't include the atmospheric warming is because it hasn't started to happen just yet," said co-author Dr David Pollard from Penn State University, US.

"In Antarctica, around the edges at sea level, it's just beginning to get up to the melt point in summer.

"With that warming, the flanks of Antarctica will start to melt drastically in about 50 to 100 years - and then it will start to kick in according to our model."

The authors believe that they have demonstrated the accuracy of the new model by correctly replicating sea-level rise in warm periods, millions of years into the past.

"Recently, we looked at the long-standing problem posed by geological evidence that suggests sea level rose dramatically in the past, possibly up to 10 to 20 metres around 3 million years ago, in the Pliocene," said Dr Pollard.

"Existing models couldn't simulate enough ice-sheet melting to explain that."

'Right questions'

If the world continues to emit "business as usual" levels of carbon dioxide over the coming decades, the scientists argue that sea-level rise will be double what has already been estimated for the coming 100 years.

"If these processes do kick in and they end up being as important as we think that they could be, then they really do have a big impact," said Prof Robert DeConto from the University of Massachusetts, Amherst.

"West Antarctica is responding very soon in these simulations and that ends up having a big impact on North America in particular."

Other researchers have praised the development of the new model for including impacts such as surface melt water and ice-cliff collapse, but they are uncertain about the conclusions.

"I have no doubt that on a century to millennia timescale, warming will make these processes significant in Antarctica, as well as Greenland, and drive a very significant Antarctic contribution to sea-level rise," commented Prof David Vaughan of the British Antarctic Survey.

"The big question for me is, how soon could this all begin, and could it be early enough to drive substantially higher sea levels by 2100? I'm not sure, but these guys are definitely asking the right questions."

The authors believe that there is "good news" in their report. If global emissions of carbon are curtailed significantly then the extra factors that substantially boost Antarctic melting will be avoided.

Seas will continue to rise, but not at the runaway rates suggested by this paper,which has been published in the journal Nature.

From BBC news-Science/ Environment

How Arctic ozone hole was avoided by Montreal Protocol

The Antarctic ozone hole would have been 40% bigger by now if ozone-depleting chemicals had not been banned in the 1980s, according to research.

Arctic ozone without the Montreal Protocol (left) and following its implementation (right)

Models also show that at certain times, a large hole would have opened up at the other end of the globe.

The Arctic hole would have been large enough to affect northern Europe, including the UK, scientists say.

The Montreal Protocol is regarded as one of the most important global treaties in history.

It was signed in 1987 after the discovery of a hole in the ozone layer above the Antarctic, the part of the upper atmosphere where ozone is found in high concentrations.

Ozone absorbs ultraviolet radiation, preventing most of it from reaching the ground.

Concerted international action led to the signing in Montreal of a UN agreement which phased out ozone-depleting chemicals, including chlorofluorocarbons (CFCs) - once widely used in fridges and spray cans.

The new research, led by scientists at the University of Leeds, simulated what the ozone hole would have been like today if nothing had been done.

The largest hole in the ozone layer appears over Antarctica

Lead researcher, Martyn Chipperfield, said: ''We would be living in an era of having regular Arctic ozone holes.

''The Antarctic ozone hole which was discovered before 1987 would have got bigger and we would also have a fairly significant ozone depletion over mid-latitudes where there are high populations, including parts of Europe.''

Foresight

The study found that the Antarctic ozone hole would have grown in size by 40% by 2013, and the ozone layer would be thinner over middle latitudes of the northern hemisphere.

There would also have been a hole over the Arctic at times, which would have rivalled that of the Antarctic and would have affected northern Europe, including the UK.

The ozone loss would have led to increases in UV levels of about 10% in Australia, New Zealand and the UK, leading to more skin cancers, the study concludes.

Jonathan Shanklin of the British Antarctic Survey is one of three Cambridge scientists who discovered the ozone hole 30 years ago this month.

He said the Montreal Protocol was the UN's most successful treaty to date and observations from the British Antarctic base were showing signs of a ''recovery'' in ozone levels.

''The protocol provides a lesson for the future and we must hope that the coming climate change talks show the same foresight and result in a treaty that will benefit the whole planet,'' said Dr Shanklin.

Since the Montreal Protocol came into force, levels of chlorine and bromine containing ozone depleting chemicals have peaked and then declined.

The new research is published in the journal Nature Communications.

From BBC Science/Environment

Antarctic coastal waters 'rising faster'

Melting ice is fuelling sea-level rise around the coast of Antarctica, a new report in Nature Geoscience finds.

Parts of West Antarctica are undergoing significant change

Near-shore waters went up by about 2mm per year more than the general trend for the Southern Ocean as a whole in the period between 1992 and 2011.

Scientists say the melting of glaciers and the thinning of ice shelves are dumping 350 billion tonnes of additional water into the sea annually.

This influx is warming and freshening the ocean, pushing up its surface.

"Freshwater is less dense than salt water and so in regions where an excess of freshwater has accumulated we expect a localised rise in sea level," explained Dr Craig Rye from the University of Southampton, UK, and lead author on the new journal paper.

Globally, sea levels are going up, in part because of the contribution of the world's diminishing ice fields. This is well known.

But the Nature Geoscience report is the first to show the direct consequences to sea surface height (SSH) around Antarctica itself.

While the satellite data record indicates there has been a general upward trend in SSH in the Southern Ocean south of 50 degrees of up to 2.4mm per year, those satellites also indicate a more rapid rise in waters sitting on the continental shelf.

Mass losses

Modelling by Dr Rye's team suggests that this additional 2mm per year can be attributed almost exclusively to freshwater runoff from Antarctica, and not to some climatic oscillation that might make sea levels "breathe" up and down on decadal timescales.

"We can estimate the amount of water that wind is pushing on to the continental shelf, and show with some certainty that it is very unlikely that this wind forcing is causing the sea level rise," Dr Rye told BBC News.

"And because we can model the freshwater forcing, and the fact that this is so much more like what we see in the real world, we can come to the confident conclusion that the signal is driven by freshwater forcing."

Recent satellite studies have underlined the increased mass losses occurring in Antarctica.

Much of this accelerated ice discharge is occurring in the continent's western sectors, particularly in the Amundsen Sea Embayment and along the Antarctic Peninsula.

In contrast to land ice, the sea ice around Antarctica stands at record levels and is becoming more extensive.

The growth is small and very regional in character, but nonetheless significant.

Quite how the freshening of surface waters might be influencing this phenomenon is now a target for study.

"That's a really interesting question which I'd like to look into with further research," Dr Rye said.

From BBC Sci/Environment