A flooded street in Dhaka, Bangladesh. New research suggests that oceans have warmed much faster than previously thought, contributing to the rise in sea levels threatening low-lying regions around the globe.

Credit: Mufty Munir/AFP

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PARIS: The world's oceans have warmed 50 per cent faster over the last 40 years than previously thought due to climate change, Australian and U.S. climate researchers reported yesterday.

Higher ocean temperatures expand the volume of water, contributing to a rise in sea levels that is submerging small island nations and threatening to wreak havoc in low-lying, densely-populated delta regions around the globe.

The study, published in the British journal Nature, adds to a growing chorus of scientific warnings about the pace and consequences rising oceans. It also serves as a corrective to a massive report issued last year by the Nobel-winning U.N. Intergovernmental Panel on Climate Change (IPCC), according to the authors.

Heat and ice

Rising sea levels are driven by two things: the thermal expansion of sea water, and additional water from melting sources of ice. Both processes are caused by global warming.

The ice sheet that sits atop Greenland, for example, contains enough water to raise world ocean levels by seven metres, which would bury sea-level cities from Dhaka to Shanghai.

Trying to figure out how much each of these factors contributes to rising sea levels is critically important to understanding climate change, and forecasting future temperature rises, scientists say.

But until now, there has been a perplexing gap between the projections of computer-based climate models, and the observations of scientists gathering data from the oceans.

"The numbers didn't add up," said Peter Geckler, a co-author of the study and a researcher at the Program for Climate Model Diagnosis and Intercomparison at the Lawrence Livermore Laboratory in California.

"When previous investigators tried to add up all the estimated contributions to sea level rise" – thermal expansion, melting glaciers, ice caps and ice sheets, along with changes in terrestrial storage – "they did not match with the independently estimated total sea level rise," he said.

Reconciling models

The new study, led by Catia Domingues of the Centre for Australian Weather and Climate Research is the first to reconcile the models with observed data. Using new techniques to assess ocean temperatures to a depth of 700 metres from 1961 to 2003, it shows that thermal warming contributed to a 0.53 millimetre-per-year rise in sea levels rather than the 0.32 mm rise reported by the IPCC.

"Our results are important for the climate modelling community because they boost confidence in the climate models used for projections of global sea-level rise resulting for the accumulation of heat in the oceans," Domingues said in a statement. "The projections will in turn assist in planning to minimise impacts, and in developing adaptation strategies," she added.

The IPCC report was criticised for including only the impact of thermal expansion in its projections of sea level rises over the next century, despite recent studies showing that melting ice is a significant – and growing – factor.

The planet's oceans store more than 90 per cent of the heat in the Earth's climate system and act as a temporary buffer against the effects of climate change.