Getting to the Bottom of Sea-level Rise

 Sea level rise will displace millions across the world.
Sea level rise will displace millions across the world.  Photo: Shamsuddin Ahmed/IRIN.

JOHANNESBURG, 10 March 2009 (IRIN) — In the past few months, newspapers across the globe have been flooded with a debate over new studies projecting a higher and faster sea-level rise by the next century, which would sound the death-knell for low-lying countries and coastal cities.

The debate has been fuelled by varying interpretations of the impact of melting ice, and by a new projection of up to 1.4m in sea-level rise by 2100, rather than a 2007 projection by the authoritative Intergovernmental Panel for Climate Change (IPCC) of between 18cm and 59cm by that time, depending on a range of greenhouse-gas emission scenarios.

The new projections in sea-level rise, caused by accelerating rates of loss from ice sheets in Greenland and Antarctica on account of higher global temperatures, even prompted the UN Environment Programme (UNEP) Year Book 2009 to warn that important tipping points leading to irreversible changes in major earth systems “may already have been reached or passed”.

A three-day gathering of climate change researchers in the Danish capital, Copenhagen, starting on 10 March, hopes to settle the debate and come up with an updated volume of scientific climate-change findings to inform the UN talks on climate change to be held in the city in December.

“This conference will bring 2,000 scientific researchers and stakeholders together, and they will also debate on how you should interpret scientific results,” said Jasper Winkel, spokesman for the University of Copenhagen.

The university is hosting the congress, which is the result of cooperation between ten of the world’s leading universities in the International Alliance of Research Universities.

Impacts Now

The sea level is rising because of melting land ice, glaciers and ice-shelves, and the thermal expansion of water — warm water occupies more volume than cold.

Sea levels rose 2cm in the 1700s, 6cm in the 1800s and 19cm in the 1900s, according to the UNEP Year Book.  As ice sheets disintegrated at the end of the last ice age, around 10,000 years ago, sea levels rose by between 70cm and 1.3m per century, said the Year Book, citing new studies.

A one-metre rise in the sea level worldwide would displace around 100 million people in Asia, mostly eastern China, Bangladesh and Vietnam; 14 million in Europe; and eight million each in Africa and South America.

Based on IPCC’s findings a sea-level rise of 50 cm projected for the next 100 years is expected to occur mostly in the second half of the next century, according to Proudman Oceanographic Laboratory of UK’s Natural Environment Council.  “Consequently, rises of level for the next 20-30 years (your remaining lifetime) can be expected to be similar to those for the past 30 years (of the order of 10cm)”.

The impact of the sea-level rise is already unfolding.  Island states such as the Papua New Guinea are already feeling the impact: in 2005, 1,000 residents on its Carteret atoll had to be evacuated as the rising sea level was slowly drowning their land.

“We will also see an increase in storm surges,” said Robert Bindschadler, chief scientist at the Hydrospheric and Biospheric Sciences Laboratory of NASA’s Goddard Space Flight Center.

Stefan Rahmstorf, a climate scientist and oceanographer at the Potsdam Institute for Climate Impact Research, in Germany, who made the 1.4m projection and is attending the Copenhagen conference, said it was “very critical” that governments take into account the new findings “because of the long time-scale of sea-level rise”.

“Once set in motion, sea-level rise is impossible to stop.  The only chance we have to limit sea-level rise to manageable levels (say, one metre, which is severe enough) is to reduce emissions very quickly, early in this century.  Later it will be too late to do much,” Rahmstorf commented.

But scientists such Monirul Qader Mirza, an author of IPCC reports, have pointed out that even with conservative estimates such as a 32cm rise, 84 percent of the Sundarbans, the world’s largest mangrove swamp, located between Bangladesh and India, would be inundated by 2050.

Impacts Sooner or Later

And there is more bad news. Rahmstorf noted that the Dutch Delta Commission of 20 international experts, called in by the Dutch government, also projected a high-end estimate of between 0.55m and 1.10m rise in sea level by 2100.  Alarmingly, they said sea-level rise would continue, and concluded that by 2200 the sea level would be between 1.5m and 3.5m higher unless global warming was stopped now.

Global temperatures are rising because of the high concentration of atmospheric carbon dioxide (CO2).  In a 2008 study, Jim Hansen, the head of NASA’s Goddard Institute for Space Studies, pointed out that if atmospheric CO2 concentrations were not kept below 350 parts per million (ppm) the results could be disastrous.

The current level of atmospheric CO2 is 385ppm, and could exceed 450ppm; if left at that level, all the ice will melt — “that’s a sea-level rise of 75 metres,” Hansen said.  The European Union has set a target of 550ppm.

The IPCC had not taken into account the impact of ice melt in the polar regions in its projections, as it was “unable to formally attribute to human influence the climate changes observed in the polar regions because of the regions’ natural variability and insufficient coverage,” the UNEP Year Book said.

The IPCC “had kind of left it to the scientific community to develop accurate climate models” that would enable accurate projections of the impact of global temperatures on ice melt and the subsequent sea-level rise, said Bindschadler.

He is leading a team that will develop models to help inform the IPCC’s fifth assessment report, which is expected in 2014.  In the meantime, empirically based estimates of the ice melt, such as the one by Rahmstorf, have surfaced.

“I like . . . these approaches in lieu of a quantitative understanding of the physical processes responsible for the witnessed recent increase in ice flow,” said Bindschadler.

The UNEP Year Book reported that 2008 was the second year in a row when there had been an ice-free channel in the Northwest Passage through the islands of northern Canada, “but this year [2008] also saw the opening of the Northern Sea Route along the Arctic Siberian coast.  The two passages have probably not been open simultaneously since before the last ice age, some 100,000 years ago.”

It is difficult to predict future sea-level rise.  “It cannot be a smooth curve — we don’t know how individual sea basins will react — but what we know for a fact is: we are on a path towards less ice and more water in the oceans,” said Bindschadler.

“We know it [ice-melt] is accelerating — the rate is between ‘fast and faster’ — and the time to take action is now, and for any decision-maker to ignore this warning would be wrong.”

This article was published by IRIN on 10 March 2009.