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

Matt McGrath BBC News 31 Mar 16;

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

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 study suggesting 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.

"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.

Sea levels set to 'rise far more rapidly than expected'
New research factors in collapsing Antarctic ice sheet that could double the sea-level rise to two metres by 2100 if emissions are not cut
Damian Carrington The Guardian 30 Mar 16;

Sea levels could rise far more rapidly than expected in coming decades, according to new research that reveals Antarctica’s vast ice cap is less stable than previously thought.

The UN’s climate science body had predicted up to a metre of sea level rise this century - but it did not anticipate any significant contribution from Antarctica, where increasing snowfall was expected to keep the ice sheet in balance.

According a study, published in the journal Nature, collapsing Antarctic ice sheets are expected to double sea-level rise to two metres by 2100, if carbon emissions are not cut.

Previously, only the passive melting of Antarctic ice by warmer air and seawater was considered but the new work added active processes, such as the disintegration of huge ice cliffs.

“This [doubling] could spell disaster for many low-lying cities,” said Prof Robert DeConto, at the University of Massachusetts Amherst, who led the work. He said that if global warming was not halted, the rate of sea-level rise would change from millimetres per year to centimetres a year. “At that point it becomes about retreat [from cities], not engineering of defences.”

As well as rising seas, climate change is also causing storms to become fiercer, forming a highly destructive combination for low-lying cities like New York, Mumbai and Guangzhou. Many coastal cities are growing fast as populations rise and analysis by World Bank and OECD staff has shown that global flood damage could cost them $1tn a year by 2050 unless action is taken.

The cities most at risk in richer nations include Miami, Boston and Nagoya, while cities in China, Vietnam, Bangladesh and Ivory Coast are among those most in danger in less wealthy countries.

The new research follows other recent studies warning of the possibility of ice sheet collapse in Antarctica and suggesting huge sea-level rises. But the new work suggests that major rises are possible within the lifetimes of today’s children, not over centuries.

“The bad news is that in the business-as-usual, high-emissions scenario, we end up with very, very high estimates of the contribution of Antarctica to sea-level rise” by 2100, DeConto told the Guardian. But he said that if emissions were quickly slashed to zero, the rise in sea level from Antarctic ice could be reduced to almost nothing.

“This is the good news,” he said. “It is not too late and that is wonderful. But we can’t say we are 100% out of the woods.” Even if emissions are slashed, DeConto said, there remains a 10% chance that sea level will rise significantly.

Active physical processes are well-known ways of breaking up ice sheets but had not been included in complex 3D models of the Antarctic ice sheet before. The processes include water from melting on the surface of the ice sheet to flow down into crevasses and widen them further. “Meltwater can have a really deleterious effect,” said DeConto. “It’s an attack on the ice sheet from above as well as below.”

Today, he said, summer temperatures approach or just exceed freezing point around Antarctica: “It would not take much warming to see a pretty dramatic increase [in surface melting] and it would happen very quickly.”

The new models also included the loss of floating ice shelves from the coast of Antarctica, which currently hold back the ice on land. The break-up of ice shelves can also leave huge ice cliffs 1,000m high towering over the ocean, which then collapse under their own weight, pushing up sea level even further.

The scientists calibrated their model against geological records of events 125,000 years ago and 3m years ago, when the temperature was similar to today but sea level was much higher.

Sea-level rise is also driven by the expansion of water as it gets warmer and in January scientists suggested this factor had been significantly underestimated, adding further weight to concerns about future rises.

Recent temperatures have been shattering records and on Monday, it was announced that the Arctic ice cap had been reduced to its smallest winter area since records began in 1979, although the melting of this already floating sea ice does not push up ocean levels.