Researchers claim to have found the first strong evidence that human activities are responsible for warming the world's oceans over the past 50 years.
In a paper published today (8 July) in Science, the researchers, led by Tim Barnett of the US-based Scripps Institution of Oceanography, rule out natural variations in the climate and the effects of volcanic or solar activity as causes for the oceans getting warmer.
Evidence that the Earth is getting warmer is abundant, and it is thought that the oceans have absorbed more than 80 per cent of the increased heat. But few studies have looked specifically at the oceans and attempted to discover why they have warmed up.
Barnett's team looked at each of the planet's oceans and focused on how increases in water temperature since 1960 has penetrated to different depths.
This allowed them to create 'fingerprints' for each ocean showing how the temperature has changed at varying depths and latitudes.
They compared the data with two independent computer models of climate change in an effort to identify causes of the observed warming.
The three potential triggers they looked at were the natural variations in the way the oceans and atmosphere interact; natural variability caused by solar or volcanic activity; and changes caused by human activity — principally, emissions of greenhouse gases.
Comparing the 'fingerprints' with the computer models, which predicted water temperatures with and without each of the possible causes, the researchers found that ocean warming "cannot be explained" by natural variations in the climate, or by solar or volcanic activity.
But the models incorporating greenhouse gas emissions closely matched the observed changes in temperature.
"The immediate conclusion is that human influences are largely responsible," for the warming of the oceans, write the researchers in Science.
Gabriel Hegerl of Duke University, United States, and Nathanial Bindoff of the University of Tasmania, Australia, agree.
Commenting on the research in an accompanying article in Science, they write that Barnett's team has "substantially" strengthened the evidence that human activities are warming the oceans. They also point out that two independent climate models agreed.
"This is perhaps the most compelling evidence yet that global warming is happening right now, and it shows that we can successfully simulate its past and likely future evolution," said Barnett in a press release.
"The statistical significance of these results is far too strong to be merely dismissed and should wipe out much of the uncertainty about the reality of global warming."
Hegerl and Bindoff say that at least 25 per cent of the increase in global sea levels since the 1950s is due to seawater expanding as it heats up.
Link to full paper by Barnett et al in Science
Link to article by Hegerl and Bindoff in Science
Read more about climate change in SciDev.Net's climate change dossier
Reference: Science 309, 284 (2005), Science 309, 254 (2005)