The Earth's ability to soak up man-made carbon dioxide emissions is a crucial yet poorly understood process with profound implications for climate change.
Among the questions that have vexed climate scientists is whether the planet can keep pace with humanity's production of greenhouse gases. The loss of this natural damper would carry dire consequences for global warming.
A study published in Thursday's edition of the journal Nature concludes that these reservoirs are continuing to increase their uptake of carbon — and show no sign of diminishing.
In an accounting of the global "carbon budget," researchers calculated that Earth's oceans, plants and soils had almost doubled their uptake of carbon each year for the last half-century. In 1960, these carbon sinks absorbed around 2.4 billion metric tons of carbon; in 2010, that figure had grown to around 5 billion metric tons.
"Since 1959, approximately 350 billion [metric tons] of carbon dioxide have been emitted by humans to the atmosphere, of which about 55% has moved into the land and oceans," they reported.
Carbon dioxide is a greenhouse gas because it traps heat — in the form of infrared energy — in the atmosphere. The more that's added to the atmosphere, the hotter it gets.
But although it appears that the planet is absorbing more carbon dioxide than expected, it does not mean humanity can breathe easier, the researchers said.
Study leader Ashley Ballantyne, a biogeochemist at the University of Colorado, noted that while global carbon sinks have roughly doubled their capacity over the last 50 years, fossil fuel emissions have nearly quadrupled during that period.
"One interpretation that will undoubtedly emerge in the blogosphere is that we don't have to worry," Ballantyne said. But that would be wrong, he said.
The study was not the first to assess the state of the Earth's carbon sinks, but it was different in that it avoided the use of complex models in favor of a relatively simple approach.
The scientists calculated how much carbon dioxide humans released into the atmosphere each year through the burning of fossil fuels, deforestation and other means. Then they measured carbon dioxide levels in the atmosphere. Since the atmospheric levels were less than what they would have been if all man-made gases had been added, they surmised that the difference was absorbed by carbon sinks.
Some scientists saw the study as an elegantly simple approach to a hotly debated topic. Writing in the same issue of Nature, Ingeborg Levin of Heidelberg University's Institute for Environmental Physics said the study made a strong case that the planet's carbon sinks were still going strong.
However, others said the study was overly simplistic.
"Frankly, it's a bit limited, this analysis," said Corinne Le Quere, director of the Tyndall Center for Climate Change Research at the University of East Anglia in England. "From this data, it's just not clear-cut enough to say if the sinks are weakening or not." More complex models would be needed to make such a conclusion, she said.
Ballantyne said that he and his colleagues attempted to account for as many uncertainties as they could think of, including events like El Niño.