Ice melt, sea level rise, to be less severe than feared: study

Alister Doyle PlanetArk 15 May 13;

A melt of ice on Greenland and Antarctica is likely to be less severe than expected this century, limiting sea level rise to a maximum of 69 cm (27 inches), an international study said on Tuesday.

Even so, such a rise could dramatically change coastal environments in the lifetimes of people born today with ever more severe storm surges and erosion, according to the ice2sea project by 24, mostly European, scientific institutions.

Some scientific studies have projected sea level rise of up to 2 meters by 2100, a figure that U.N. Secretary-General Ban Ki-moon has called a worst case that would swamp large tracts of land from Bangladesh to Florida.

Ice2sea, a four-year project to narrow down uncertainties of how melting ice will pour water into the oceans, found that sea levels would rise by between 16.5 and 69 cm under a scenario of moderate global warming this century.

"This is good news" for those who have feared sharper rises, David Vaughan, of the British Antarctic Survey who led the ice2sea project, told Reuters in a telephone interview.

"But 69 cm is a very real impact ... it changes the frequency of floods significantly," he said. And seas would keep rising for centuries beyond 2100, in a threat to coastal cities and low-lying islands such as the Maldives or Tuvalu.

Ice2sea said a thaw of Antarctica, Greenland and glaciers from the Alps to the Andes would contribute between 3.5 and 36.8 cm to sea level rise this century. The fact that water expands as it warms would add another 13 to 32 cm, Vaughan said.

Some other scientists disputed ice2sea's projections.

"I think the numbers are too low," Dorthe Dahl-Jensen, an ice expert and professor at the Niels Bohr Institute in Copenhagen, told Reuters. She said ice2sea wrongly assumed a slowdown in the rate of ice discharge from Greenland.

ANTARCTIC SNOW

Sea levels rose by 17 cm last century and the rate has accelerated to more than 3 mm a year. A third of the current rise is from Antarctica and Greenland - equivalent to emptying 138 million Olympic-sized pools into the sea every year.

One factor likely to offset sea level rise, ice2sea said, is that warmer temperatures will result in more snow, especially over Antarctica, locking in the moisture on land. It also played down worries of a runaway melt of Greenland, and of the breakup of major Antarctic ice shelves.

Governments want to know future sea levels to plan sea barriers and regulations for everything from vacation homes to nuclear power plants by the coast. And every extra centimeter means big costs.

A Dutch commission planning to bolster sea defenses, for instance, has advised spending more than 100 billion euros ($130 billion) by 2100 to strengthen dykes and other barriers for a worst case scenario of a 1.2 meter North Sea rise by 2100.

The ice2sea study also said that a survey of experts' opinions showed there was a less than one-in-20 risk that melting ice sheets would contribute more than 84 cm to sea level rise this century. Taken with thermal expansion, that would mean a sea level gain of just over a meter, Vaughan said.

Almost 200 governments have agreed to work out a U.N. deal, by the end of 2015, to combat global warming that would help limit temperature rises and rising seas.

A leaked report by a U.N. panel of climate scientists, due for release in September and drawing on ice2sea data, estimates sea level rise at between 29 and 82 cm by the late 21st century, above the estimates in its last report in 2007 of between 18 and 59 cm.

Many studies since 2007 have had higher upper numbers, including by the World Bank, the U.S. National Oceanic and Atmospheric Administration (NOAA) and a report for the Arctic Council. NOAA put the upper limit at 2 meters.

(Editing by Alison Williams)

'Best estimate' for impact of melting ice on sea level rise
Matt McGrath BBC News 14 May 13;

Researchers have published their most advanced calculation for the likely impact of melting ice on global sea levels.

The EU funded team say the ice sheets and glaciers could add 36.8 centimetres to the oceans by 2100.

Adding in other factors, sea levels could rise by up to 69 centimetres, higher than previous predictions.

The researchers say there is a very small chance that the seas around Britain could rise by a metre.

The last Intergovernmental Panel on Climate Change (IPCC) report was highly detailed about many aspects of Earth's changing climate in the coming decades,
Advanced models

While they estimated that sea levels could rise by 18-59 centimetres by 2100, they were very unsure about the role played by the melting of ice sheets and mountain glaciers.

To fill the void, the EU funded experts from 24 institutions in Europe and beyond to try and come up with more accurate figures on the melting of ice sheets and glaciers in Antarctica and Greenland and how this might swell the oceans.

Called Ice2sea, the group of scientists have made what they term the "best estimate" yet of the impact of melting based on a mid-range level of carbon emissions that would increase global temperatures by 3.5C by the end of this century.

"For that one scenario we have an ice sheet and glaciers contribution to sea level rise of between 3.5 and 36.8 cm by 2100," said Prof David Vaughan from the British Antarctic Survey who is the co-ordinator of the Ice2sea programme.

While the range is wide, the scientists say it is a relatively robust calculation and based on several advances in their modelling since the last IPCC report.

"In order to be able to model the ice sheet properly you need to be able to resolve things down to hundreds of metres," says Prof Tony Payne from the University of Bristol.

"That's quite a task when an ice sheet is a thousand kilometres or more in size, that's a very demanding computational task. What we found is that the Pine Island glacier, the poster child of sea level rise in the Antarctic, that will continue through to the end of the century and very little else happens."

Despite the improvements, there are still many factors that are difficult to include in models. To get around this, the leading researchers were asked to estimate the worst-case scenarios.

They concluded there was a one in 20 chance that the melting ice would drive up sea levels by more than 84 centimetres, essentially saying there's a 95% chance it wouldn't go above this figure.

While ice melt is a major contributor to the height of the seas, there are other important factors especially thermal expansion caused by the warming of the waters.

This is estimated to be raising sea levels by 3 millimetres every year. Taken together with the ice melt estimate, the scientists say the overall, maximum impact on the seas by 2100 will be a rise of 69 centimetres - just ten centimetres higher than the IPCC projection in 2007, termed AR4.

"What we are talking about is a reduction in uncertainty - we find we haven't changed the number enormously compared to AR4, we've added maybe another 10 centimetres but the level of certainty we have around that, is actually higher than it was in the AR4," said Prof Vaughan.

The researchers also included projections for sea level rise in Europe that includes the effects of thermal expansion, ice melt and storm surges. In these scenarios, the British Isles could face an increase of slightly over a metre by 2100. Enough to overwhelm the Thames Barrier and see London flooded once every ten years.

But the scientists stress that there's a 95% chance that these numbers will not be reached.

"The previous IPCC identified this gap in our knowledge, we've addressed that gap and what we've found is not scary," said Prof Payne.

"We're always talking about tens of centimetres, maybe a metre tops, none of the experiments are suggesting 2,3,4 metres and that's different to the literature that existed before Ice2sea."

However the scientists stressed that sea level rise in line with their projections could still make some islands in the Pacific uninhabitable. And if global emissions of carbon dioxide are not curtailed then the actual level of the sea by 2100 could be significantly higher than the Ice2Sea estimates.