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  BBC 14 Dec 06
Sea level rise 'under-estimated'

Yahoo News 14 Dec 06
Global warming may raise sea levels faster than previously expected

PlanetArk 15 Dec 06
Oceans May Rise up to 140 cms by 2100 Due to Warming
Story by Alister Doyle, Environment Correspondent

National Geographic 14 Dec 06
Sea Level May Rise 40 Percent Higher Than Predicted, Study Says
John Roach for National Geographic News

Global warming could push sea levels about 40 percent higher than current models predict, according to a study that takes a new approach to the calculation.

Most sea level models predict changes based on what we know about how ice sheets melt and warmer waters expand. These models suggest that by 2100 sea level will be between 4 and 35 inches (9 and 88 centimeters) higher than it was in 1990.

But the physics of how ice sheets melt and how the oceans will expand in a warmer world is still poorly understood.

So Stefan Rahmstorf, an ocean physicist at Potsdam University in Germany, took a different approach: He used studied actual observations of changes in sea level collected in the 20th century to make predictions for the 21st century.

Current models don't jibe with actual sea level rise during recent decades, Rahmstorf says. So he crafted a formula based on a relationship between global temperature and sea level seen during the past hundred years.

"The more the temperature rises, the faster the sea level rises," he said. In a paper published today in the online advance edition of the journal Science, Rahmstorf applied his formula to 21st-century warming scenarios from the Intergovernmental Panel on Climate Change (IPCC).

His results predict that by the end of the century sea level will rise between 20 and 55 inches (50 and 140 centimeters) above 1990 levels.

"We have much larger uncertainty than we previously thought about the sea level," Rahmstorf said.

Unforeseen Factors

Rahmstorf added that the actual range of uncertainty is probably larger than his calculations suggest. The IPCC numbers are based on an older assumption that the ice sheets over Greenland and Antarctica will melt by a steady amount over time.

Recent research suggests, however, that ice sheets are melting faster. "If something dramatically new happens--something we haven't foreseen--then of course the whole approach [of using observations to make predictions] breaks down," Rhamstorf said. "We may end up with more sea level rise."

Konrad Steffen is a professor of geography at the University of Colorado at Boulder who studies how melting ice sheets and glaciers contribute to sea level. He said one wild card that could impact predictions is the so-called dynamic response of the ice sheets to warming.

In the last five to eight years, he noted, the speed at which Greenland's glaciers move toward the sea has sped up dramatically. Scientists think that meltwater, which pools up on the ice, funnels down to the glacier bed. There, the water acts as a lubricant, allowing the ice to slip seaward more quickly. The process may last five or ten years, or it may last decades, Steffen said.

"We have hypotheses on what is happening, but we can't model it for the future," he said. "That is where [Rahmstorf] is correct."

High Water Risk

Study author Rahmstorf notes in Science that a sea level rise of 39 inches (1 meter) is plausible if the 20th-century relationship between temperature and sea level holds true in the 21st century.

That much sea level rise would expose major coastal cities such as London and New York to greater storm surges, threatening life and property.

Reducing greenhouse gas emissions can impact such sea level rise, he noted. "By implementing effective climate policy," he said, "we can stay below the lower end of my range [around 20 inches, or 50 centimeters]."

PlanetArk 15 Dec 06
Oceans May Rise up to 140 cms by 2100 Due to Warming
Story by Alister Doyle, Environment Correspondent

NORWAY: December 15, 2006 OSLO - The world's oceans may rise up to 140 cms (4 ft 7 in) by 2100 due to global warming, a faster than expected increase that could threaten low-lying coasts from Florida to Bangladesh, a researcher said on Thursday.

"The possibility of a faster sea level rise needs to be considered when planning adaptation measures such as coastal defences," Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research wrote in the journal Science.

His study, based on air temperatures and past sea level changes rather than computer models, suggested seas could rise by 50-140 cms by 2100, well above the 9-88 cms projected by the scientific panel that advises the United Nations.

A rise of one metre might swamp low-lying Pacific islands such as Tuvalu, flood large areas of Bangladesh or Florida and threaten cities from New York to Buenos Aires.

"The computer models underestimate the sea level rise that has already occurred," Rahmstorf told Reuters of a rise of about 20 cms since 1900. "There are aspects of the physics we don't understand very well."

Sea level changes hinge on poorly understood factors such as the pace of the melt of glaciers and of ice sheets in Greenland and Antarctica. Water also expands as it gets warmer but the rate of penetration of heat to the depths is uncertain.

"My main conclusion is not that my forecast is better but that the uncertainty is much larger because of the different results you get with reasonable methods," he said.

Almost all climate scientists reckon the world is warming because of emissions of greenhouse gases from human use of fossil fuels in factories, power plants and cars. Rising temperatures could bring more droughts, floods and heatwaves.

TIDES

Rahmstorf likened his approach to predicting the height of tides along a coast, largely based on past observations.

He said seas were 120 metres below present levels during the last Ice Age 20,000 years ago and 25-35 metres higher than the present in the Pliocene epoch 3 million years ago. In the Ice Age temperatures were 4-7 Celsius (7.2-12.6 Fahrenheit) cooler than today and 2-3 C (3.6-5.4F) warmer in the Pliocene.

That suggested sea levels change 10-30 metres per rise or fall per degree Celsius (1.8F), over thousands of years.

The UN climate panel has projected temperatures will rise by 1.4-5.8C (2.5-10.4F) by 2100, mainly because of human influences.

"Sea level is a very slow component of the climate system so what we see by the year 2100 is just a small percentage of the total we are causing," Rahmstorf said.

There was still time for the world to cut greenhouse gas emissions but he said the slow pace of UN talks on extending the UN's Kyoto Protocol beyond 2012 "gives you the impression that governments are not very well aware of how urgent the whole problem has become".

Coastal cities in the North Atlantic -- from New York to London -- could be especially vulnerable because a possible slowdown of ocean currents could also raise sea levels in the North Atlantic and lower them in the southern hemisphere.

"Any time you change ocean currents you change the sea surface...if you slow down the North Atlantic current you get a rise in the North Atlantic," Rahmstorf said.

Yahoo News 14 Dec 06
Global warming may raise sea levels faster than previously expected

CHICAGO (AFP) - Global warming may raise sea levels faster than previously expected this century, said a study that suggests the impact of climate change on the oceans may have underestimated.

The study says rising temperatures could boost sea levels by as much as 1.4 meters (4.6 feet) by 2100, almost twice the rate previously forecast, increasing the flood risk in low-lying areas, and the threat of storm surges to cities such as New York and London.

Climatologists so far agree that sea levels will increase 9-88 centimeters (4-35 inches) over 1990 levels by the end of the century, but in the paper published in the journal Science, a German researcher suggests the range could be much higher, 50-140 centimeters (20-55 inches).

Stefan Rahmstorf, professor of ocean physics at Potsdam University, said current working assumptions are unreliable because the computer models that generated them significantly underestimate the rise in sea levels already seen.

"In the past 40 years, sea levels have increased about 50 percent more than the climate models predicted. That tells us that we haven't understood the problem of rising sea levels yet," said Rahmstorf.

Current computer models used by the Intergovernmental Panel on Climate Change, an UN-sponsored body that advises policymakers on the effects of climate change, considers a number of factors including thermal expansion and the effect of melting ice sheets.

For his analysis, Rahmstorf examined the relationship between rising sea levels and increases in global average near-surface air temperatures. He concluded that the rate of increase in sea levels was proportional to rising temperatures and accounted for the changes seen in the 20th century.

Rahmstorf said the higher range has serious implications, not just for low-lying areas which have already been identified as global warming-related flood risks, but also for some major cities in the Western hemisphere.

In a study published last year, Rahmstorf and his team at the Potsdam Institute for Climate Impact Research argued that global warming could increase sea levels in the North Atlantic by shutting down or weakening an ocean current called the overturning circulation or conveyor belt.

Under that scenario, regional sea levels could rise by up to a meter (yard), and once the greenhouse gas effect is factored in, the increase could be as much as two meters, which could expose London and New York to potentially "devastating storm surges," Rahmstorf said.

At the very least, Rahmstorf says his study shows just how wide the margin of error is when it comes to forecasting climate-related change in sea levels.

"The fact that we get such different estimates using different methods shows how uncertain our sea level forecasts still are."

BBC 14 Dec 06
Sea level rise 'under-estimated'

The team from Germany and the US found that for the timescale relevant to human-induced climate change, the observed rate of sea level rise through the 20th Century held a strong correlation with the rate of warming.

When applied to the possible scenarios outlined by the Intergovernmental Panel on Climate Change (IPCC), the researchers found that in 2100 sea levels would be 0.5-1.4m above 1990 levels.

This projection is much greater than the 9-88cm forecast made by the IPCC itself in its Third Assessment Report, published in 2001.

Troubled waters

The paper's lead author, Stefan Rahmstorf, from the Potsdam Institute for Climate Impact Research, Germany, acknowledges that projecting future outcomes is a challenge. He writes in Science Express: "Understanding global sea level changes is a difficult physical problem, as a number of complex mechanisms with different timescales play a role."

These include: thermal expansion of water through heat absorption water entering the oceans from glaciers and ice sheets increased ice flows after the removal of buttressing ice shelves Professor Rahmstorf said he decided to use observational data because computer models of climate significantly under-estimated the sea level rise that had already occurred.

"The fact that we get such differences using different methods shows how uncertain our sea level forecasts still are," he said. He added that the main uncertainty was the response of large ice sheets in Greenland and Antarctica to rising temperatures, which was difficult to predict.

Simon Holgate, from the UK's Proudman Oceanographic Laboratory, said fellow scientists would have to take a closer look at Professor Rahmstorf's findings; until then, the jury would remain out.

"Normally, you would use the heat content of the surface layer of water rather than just using the surface temperature because the surface temperature is affected by a lot of other factors," Dr Holgate observed. "When you try to do a similar exercise as Professor Rahmstorf but using heat content you do not get such a good correlation."

He added that only 40% of observed sea level rise was believed to a result of thermal expansion. Dr Holgate said the next assessment report from the IPCC would include data based on more robust modelling, thereby reducing uncertainties surrounding models on sea level rises. The IPCC is set to publish its much anticipated Fourth Assessment Report in February 2007.
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