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A dramatic decline in the ability of the earth to soak up man-made emissions of carbon dioxide and a corresponding acceleration in the rate of increase of greenhouse gas in the atmosphere have been detected for the first time.
The discovery that more carbon dioxide from human activities is lingering in the air rather being absorbed by the world's forests and oceans has alarmed scientists, who believe that it signals a potentially dangerous turn of events.
They fear that a much-anticipated "feedback" in the global climate - when increases in carbon dioxide in the air trigger further increases in atmospheric concentrations of the gas - has already begun to occur decades before many predicted.
"We always said that these feedbacks would happen in the future, but what this study shows is that these feedbacks are happening right now," said Josep Canadell, executive director of Canberra's Global Climate Project and lead author of the study.
About half of the carbon dioxide emissions resulting from human activities are absorbed by natural "sinks" on land and the oceans but the new study shows that the efficiency of these sinks has fallen significantly in the past half-century.
"What we are seeing is a decrease in the planet's ability to absorb carbon emissions due to human activity. Fifty years ago, for every tonne of CO2 emitted, 600kg were removed by natural sinks. In 2006, only 550kg were removed per tonne and that amount is falling," Dr Canadell said.
The study also found that the amount of CO2 released into the air from human activities has accelerated in recent years not just because of the growth of the global economy but because, for the first time in a century, the efficiency with which fossil fuels are used has stagnated.
The study, published in the journal Proceedings of the National Academy of Sciences, estimated that the inefficiency in the use of fossil fuels over the past six years has increased levels of atmospheric CO2 by 17 per cent, while 18 per cent came from the decline in the efficiency of natural sinks.
Corinne Le Quere, a climate researcher at the British Antarctic Survey in Cambridge, said that stronger winds in the southern ocean, caused by global warming and the loss of the ozone layer, had resulted in more dissolved carbon dioxide in the deep sea being brought to the surface, and consequently less carbon dioxide being absorbed from the atmosphere.
"It is bad news because we can't do much about these natural carbon sinks but the good news is that we can increase the efficiency of fossil fuel use. Things are happening much faster than we expected."
