An international team of scientists, reporting their findings on Thursday in the journal Nature, looked at millions of ship-based measurements taken since 1950, but particularly from 1960, and revealed an error in data from a common probe called an XBT.
Correcting the error in data running over decades as well as applying a complex statistical analysis to sea temperature data, the team came up with a global estimate of ocean warming in the top layers down to 700 metres (2,300 feet) as well as how fast oceans are rising.
"We show that the rate of ocean warming from 1961 to 2003 is about 50 percent larger than previously reported," said team member Catia Domingues, from the Centre for Australian Weather and Climate Research.
Fellow report author John Church said he had long been suspicious about the historical data because it did not match results from computer models of the world's climate and oceans.
"We've realigned the observations and as a result the models agree with the observations much better than previously," said Church, a senior research scientist with the climate centre.
"And so by comparing many XBT observations with research ship observations in a statistical way, you can estimate what the errors associated with the XBTs are."
This was crucial because the oceans store more than 90 percent of the heat in the planet's climate system and can act as a buffer against the effects of climate change, Domingues said.
Water also expands the warmer it becomes, pushing up sea levels, in addition from run-off from melting glaciers and ice sheets in Greenland and parts of Antarctica.
Church said the global average surface warming between 1961 to 2003 was about about 0.4 degrees Celsius according to his team's estimates and that seas rose on average 1.6 millimetres a year during this period.
But Church said that since 1993, sea levels had been rising more than 3 mm a year as the world consumes ever greater amounts of fossil fuels.
XBTs were widely used by commercial vessels but have since been largely replaced by satellites and permanent probes in the ocean. The disposable XBTs were thrown over the side with a wire attached to measure temperatures as it sank.
"If you miscalculate how quickly the instrument falls through the water column, you miscalculate the depth and therefore the temperature at that depth and that's the prime source of error," said Church.
So a colleague, Susan Wijffels and other associates, figured out a mathematical formula to correct the error.
That, combined with a wider statistical analysis of global ocean temperature data, revealed a clearer picture that better matched widely used computer models that project how the climate and oceans behave because of global warming.
"Now we see a more steady rate of warming and an increased trend in that warming," Church told Reuters.
"It builds confidence in the models that we use for projecting the future," adding that observations also indicated that the actual sea level rise was tracking on the upper end of those projections.
The UN Climate Panel's latest global assessment last year estimated sea levels could rise by up to 80 cm by the end of 2100 unless carbon dioxide levels were reined in.
Oceans Warming Faster Than Realized
Discovery.com, June 18, 2008
The world's oceans have warmed 50 percent faster over the last 40 years than previously thought due to climate change, Australian and U.S. climate researchers reported Wednesday.
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 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 (23 feet), 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 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.
The new study, led by Catia Domingues of the Center 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 meters (2,300 feet) 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.
"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 minimize impacts, and in developing adaptation strategies," she added.
The IPCC report was criticized 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 percent of the heat in the Earth's climate system and act as a temporary buffer against the effects of climate change.