A decade-long study of the oceans has shown they are soaking up less and less carbon dioxide.
The oceans' ability to absorb the greenhouse gas halved between the mid 90s and the first five years of this century, scientists said.
The team of researchers, based at the University of East Anglia, carried out the study using automatic sensors attached to merchant ships crossing the North Atlantic. The findings are based on more than 90,000 measurements.
Whatever the cause, the team says the findings are worrying because the oceans are one of the planet's biggest carbon sinks, absorbing anywhere up to a quarter of our carbon emissions.
The team told the BBC the research suggests that in time the oceans could become saturated. If the seas are starting to become saturated, the CO2 they would have absorbed will remain in the atmosphere instead, accelerating global warming. The possible effect on marine life is unclear.
This work follows earlier research from the university which revealed the Southern Ocean is becoming saturated with carbon. In that instance, the team was confident in pointing the finger of blame at climate change, noting that variation in the region's winds, triggered by ozone depletion and greenhouse gas emission, were behind a release of previously stored CO2
The work is published in the Journal of Geophysical Research. ®