SINGAPORE (Reuters) - Scientific detective work has uncovered a decades-old glitch in ocean temperature measurements and revealed that the world's seas are warming and rising faster than previously reported.
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 meters (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 0.4 degrees Celsius according to his team's estimates and that seas rose on average 1.6 millimeters 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 U.N. 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.
Editing by David Fox