One of the uncertainties in predicting how the climate will respond to all the greenhouse gases we're belching up into the atmosphere is what will happen with the world's carbon sinks. Trees, ocean, and even the soil all absorb a huge fraction of the industrial carbon-dioxide we produce each year. But now a major new study from researchers at the British Antarctic Survey, led by the University of East Anglia's Corinne Le Quéré, has discovered that those sinks may be losing their ability to pull CO2 out of the air:

By studying 50 years of data on carbon emissions and combining with estimates of human carbon emissions and other sources such as volcanoes, the team was able to estimate how much CO2 is being absorbed naturally by forests, oceans and soil.

The team conclude in the journal Nature Geoscience that those natural sinks are becoming less efficient, absorbing 55% of the carbon now, compared with 60% half a century ago. The drop in the amount absorbed is equivalent to 405m tonnes of carbon or around 60 times the annual output of Drax coal-fired power station, which is the largest in the UK.

This has been something that's been predicted by climate models for awhile—as the ocean absorbs more CO2, it becomes more acidic and loses its ability to pull CO2 out of the air—but it hasn't really been observed until now. Likewise, another study published this week in Nature by Columbia oceanographer Samar Khatiwala and colleagues finds that, indeed, the oceans do appear to be absorbing less and less CO2. (The study was the first time anyone's measured the build-up of man-made carbon in the ocean since the Industrial Revolution, a quantity that's hard to separate from all the background CO2 out there.) Granted, not all climatologists are so certain that carbon sinks are losing their powers—the Guardian nicely captures some of the back and forth:

But Le Quéré's conclusion on the decline of the world's carbon sinks is not universally accepted. Wolfgang Knorr of the University of Bristol recently published a study in Geophysical Research Letters, using similar data to Le Quéré, where he argued that the natural carbon sinks had not noticeably changed. "Our apparently conflicting results demonstrate what doing cutting-edge science is really like and just how difficult it is to accurately quantify such data," said Knorr.

The amount of CO2 that natural carbon sinks can absorb varies from year to year depending on climactic and other natural conditions, and this means that overall trends can be difficult to detect. Le Quéré said her team's analysis had been able to remove more of the noise in the data that is associated with the natural annual variability of CO2 levels due to, for example, El Niño or volcanic eruptions. "Our methods are different – Knorr uses annual data, we use monthly data and I think we can remove more of the variability."

Jo House of the University of Bristol, who worked on the Nature Geoscience paper, said: "It is difficult to accurately estimate sources and sinks of CO2, particularly in emissions from land use change where data on the area and nature of deforestation is poor, and in modelled estimates of the land sink which is strongly affected by inter-annual climate variability. While the science has advanced rapidly, there are still gaps in our understanding."

So the jury's still out, but this is a pretty crucial issue. The study from the British Antarctic Survey—i.e., the one finding that carbon sinks are losing their ability to suck up our carbon—predicts that, as a result, the world is currently on pace to warm some 6°C (10.8°F) by the end of the century. And that, unless the world agrees to some sort of hard limits soon, emissions will likely fall briefly in 2009 as a result of the recession and then go up, up, up in 2010 and beyond.

(Flickr photo credit: hb19)