Global oceans are soaking up less carbon dioxide, a development that could speed up the greenhouse effect and have an impact for the next 1,500 years, scientists said.

Research from a five-year project funded by the European Union showed the North Atlantic, which along with the Antarctic is of the world's two vital ocean carbon sinks, is absorbing only half the amount of CO2 that it did in the mid-1990s. 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, Prof. 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.