Top of sea warming 50% faster than thought

The top few hundred meters of the world's oceans have warmed 50 percent faster than previously thought during the past half century, a discovery that has solved an enduring puzzle about the world's rising sea levels. Sea-level rise is a key consequence of of climate change but the actual change has been higher than scientists had predicted. Now scientists believe they understand the rise in sea levels observed since 1961 and can link them to the expansion of the oceans as they warmed, along with melting of glaciers, ice caps and ice sheets. The new study by Australian and US climate researchers, published in the journal Nature, concludes that the upper 700 meters of the world's oceans warmed at a rate 50 percent faster in the last four decades of 20th century than documented in the 2007 Intergovernmental Panel on Climate Change Report, which produces a consensus view of scientists around the world. "For the first time, we can provide a reasonable account of the processes causing the rate of global sea-level rise over the past four decades -- a puzzle that has led to a lot of scientific discussion since the 2001 IPCC report but with no significant advances until now," says Dr. Catia Domingues of the Commonwealth Scientific and Industrial Research Organization (CSIRO), Hobart. The research gives significantly greater credibility to the way computer models simulate the degree of warming in the world's oceans, a key feature of sea-level rise and climate change. "Following the review of millions of ocean measurements, predominantly from expendable instruments probing the upper 700 meters of the ocean, we were able to more accurately estimate upper-ocean warming, and the related thermal expansion and sea-level rise. We show that the rate of ocean warming from 1961 to 2003 is about 50 percent larger than previously reported," says Dr. Domingues. "Our results are important for the climate modeling community because they boost confidence in the climate models used for projections of global sea-level rise resulting from the accumulation of heat in the oceans." Central to unlocking more accurate estimates of sea-level rise were ways of correcting small but systematic biases recently discovered in 70 percent of measurements in the global ocean observing system. Coauthor Dr. John Church, CSIRO Marine & Atmospheric Research, says: "Our estimates of ocean warming are in much better agreement with the ocean component of climate models than previously -- this is a strong confirmation of these models. "However, there is a suggestion that the models have slightly underestimated (30 percent in the upper 300m and 10 percent in the upper 700m) the amount of ocean warming." While scientists agree that sea levels rose by six inches over the course of the 20th century and are currently rising at the upper end of the IPCC projections, estimates of future rises remain alarmingly hazy. There are still major outstanding questions about how ice sheets will behave in a warmer world. "Unfortunately, our work does not resolve the major questions about future ice sheet contributions."