Even as Man's output of Earth-warming CO2 has risen, so has the capacity of plants and the oceans to absorb it, scientists said Wednesday, but warned this may not last forever.

Carbon storage by land and sea, known as carbon sinks, has more than doubled in the past 50 years from about 2.4 billion tonnes in 1960 to some five billion tonnes in 2010, said a study in Nature.

At the same time, fossil-fuel CO2 emissions rose almost four-fold.

"The growth rate of atmospheric CO2 continues to rise because fossil fuel emissions are accelerating not because sinks are diminishing," researcher Ashley Ballantyne of the University of Colorado's geology department told AFP.

The finding was contrary to widespread expectations that carbon sinks were slowing their CO2 uptake.

"We were somewhat surprised by this result because several recent studies have been published showing that the land and oceans have been taking up less CO2," said Ballantyne.

"We discovered that the Earth continues to take up more CO2 every year and there is no indication that this uptake has weakened."

Ballantyne and colleagues used reported annual changes in atmospheric CO2 levels, from which they subtracted annual total man-made emissions to quantify Earth's uptake.

About half of man-made CO2 emissions caused by burning fossil fuels and land-use changes such as deforestation, are taken up by plants and the oceans.

CO2 can be stored away deep in the oceans for centuries. Plants and trees also use CO2 but later return it to the atmosphere through respiration or the burning of forests, for example.

"We don't expect this uptake to continue to increase indefinitely because increased temperature as a result of rising CO2 may limit the net uptake of CO2 by land and oceans," said Ballantyne.

In fact, carbon sinks may become new sources of CO2 within the next century.

"Obviously if the Earth suddenly stopped taking up as much CO2 this would have potentially catastrophic consequences for Earth's climate system."

Better understanding of these processes is crucial for climate change planning.

"It makes a big difference whether the extra carbon emitted is stored in reservoirs such as the deep oceans, where it could stay for hundreds or thousands of years, or whether it is taken up by the growth of new forests where it would stay for only a few years or decades," German scientist Ingeborg Levin said in a comment that accompanied the paper.