Earth absorbs more CO2 emissions

2012-08-02 12:10

Dangerously hot summer days have become more common across the US Midwest. (AP)

Paris - Even as Man's output of Earth-warming carbon dioxide (CO₂) has risen, so has the capacity of plants and the oceans to absorb it, scientists said, 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 tons in 1960 to some five billion tons 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," said researcher Ashley Ballantyne of the University of Colorado's geology department.

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

Deforestation

"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.

- AFP