The amount of carbon dioxide being absorbed by the world's oceans has reduced, scientists have said.
University of East Anglia researchers gauged CO2 absorption through more than 90,000 measurements from merchant ships equipped with automatic instruments.
Results of their 10-year study in the North Atlantic show CO2 uptake halved between the mid-90s and 2000 to 2005.
Scientists believe global warming might get worse if the oceans soak up less of the greenhouse gas.
Researchers said the findings, published in a paper for the Journal of Geophysical Research, were surprising and worrying because there were grounds for believing that, in time, the ocean might become saturated with our emissions.
BBC environment analyst Roger Harrabin said: "The researchers don't know if the change is due to climate change or to natural variations.
"But they say it is a tremendous surprise and very worrying because there were grounds for believing that in time the ocean might become 'saturated' with our emissions - unable to soak up any more."
He said that would "leave all our emissions to warm the atmosphere".
Of all the CO2 emitted into the atmosphere, only half of it stays there; the rest goes into carbon sinks.
There are two major natural carbon sinks: the oceans and the land "biosphere". They are equivalent in size, each absorbing a quarter of all CO2 emissions.