The findings assume significance as the ocean's ability to soak up less of the greenhouse could worsen the global warming scenario.
The study authors said the findings were worrying, as there were grounds to believe, that in time, the ocean might become saturated with emissions.
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,' the BBC quoted him as saying.
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 and absorb a quarter of all CO2 emissions each.
The study appears in the Journal of Geophysical Research.