Using recent detailed data, scientists said the amount absorbed is also fluctuating each year, making it hard to predict how and whether the trend will continue and if oceans will be able to perform their vital balancing act in the future.

Oceans soak up around a quarter of annual CO2 emissions, but should they fail to do so in the future the gas would stay in the atmosphere and could accelerate the greenhouse effect, a prospect project director Christoph Heinze called "alarming".

Oceans are like a "slow-mixing machine". Carbon absorbed in the North Atlantic takes around 1,500 years to circulate around the world's seas. This means changes to their fragile balance could be felt long into the future, Heinze said at a geoscience conference in Vienna.

Scientists are still debating the reasons why oceans are absorbing less carbon dioxide. While some point to CO2 saturation, others say it could be caused by a change in surface water circulation, triggered by changes in weather cycles.

Heinze described a "bottleneck effect" because of the large amount of manmade carbon dioxide oceans already store.

"The more CO2 the oceans store, the more difficult it will be for them to take up the additional load from the atmosphere and carbon absorption will stagnate even further," Heinze said.

Some forms of sea life have suffered from the large amounts of CO2 absorbed, because of changes in acidity levels.

"The seafloor is becoming an increasingly hostile environment," said Marion Gehlen, from the Laboratory of Climate and Environment Science in France.

"This corrosive water means mollusc organisms have a hard time making their shells and eventually they might not be able to do it at all."

For the scientists there is only one thing humans can do to resolve the problem -- reduce emissions by at least 75 percent.

"We must act now. The good news is that while the negative effects can last a long time, the good things we do will also have an effect for the next 1,500 years," Heinze said.