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The world's oceans have warmed 50 percent faster over the last 40 years than previously thought due to climate change,Australian and US climate researchers reported Wednesday．Higher ocean temperatures expand the volume of water,contributing to a rise in sea levels that is covering small island nations and threatening to destroy the low-lying,densely-populated low regions around the globe.

The study,published in the British journal Nature,adds to a growing scientific chorus of 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 UN Intergovernmental Panel on Climate Change(IPCC),according to the authors.

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 meters,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 up to now,there has been a puzzling gap between the projections of computer-based climate models,and the observations of scientists gathering data from the oceans.

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

What happens when the ocean's temperature rises?