The ocean is warming about 50 per cent faster than reported two years ago, according to an update of the latest climate science.
A report compiling research presented at a science congress in Copenhagen in March says recent observations are near the worst-case predictions of the 2007 report by the United Nations' Intergovernmental Panel on Climate Change.
In the case of sea-level rise, it is happening at an even greater rate than projected - largely due to rising ocean temperatures causing thermal expansion of seawater.
Released last night at the European Policy Centre in Brussels, the report says ocean temperatures are a better indicator of global warming than air temperature as the ocean stores more heat and responds more slowly to change.
Report co-author Will Steffen, executive director of the Australian National University's Climate Change Institute, said the top 700 metres of the ocean had warmed by about 0.1 degrees over the past half-century.
"While that looks like a modest figure, that would correspond to something like 15 to 20 times more heat going into the ocean than has gone into the atmosphere," Professor Steffen said.
"Well over half of the increase in ocean temperature occurred in the last 10 years, so the system is accelerating."
The report, titled Climate change: Global risks, challenges & decisions, says greenhouse gas emissions needed to peak within the next six years for the world to give a chance of limiting global warming above pre-industrial levels to about two degrees.
But it warms that even a two-degree rise in temperature would lead to significant risks, including loss of water storage capacity in the Himalayan glaciers and the melting of the Greenland ice sheet.
Ice sheet melting could be locked in for centuries before it is felt.
Other findings in the report include that:
* Sea level is predicted to rise by about a metre by 2100, though it notes models of the behaviour of polar ice sheets are in their infancy.
* Summer Arctic sea ice is reducing dramatically, with the decrease in 2008 almost as great as the record loss in 2007. As ice and snow reflect the sun, loss of sea ice will lead to more rapid warming as heat is instead absorbed by seawater.
* Carbon dioxide concentrations in the atmosphere have not been substantially higher than now for at least the last 20 million years.
* Global average surface temperature will hardly drop in the first thousand years after greenhouse gas emissions are cut to zero.