Each year, human beings put vast amounts of carbon dioxide into the atmosphere through processes like the combustion of fossil fuels or the clearing land for agriculture. Thankfully, the majority of it doesn't stay there, as there are a number of significant carbon sinks that pull somewhere around 60 percent of human emissions back out of the air, dissolving it into the oceans and sequestering it in growing forests. One of the worries about our continued carbon emissions is that these sinks could eventually start to fill, increasing the challenge involved in limiting the levels of atmospheric carbon. Two new studies have looked at the issue, and they come to what appear to be very different conclusions.

Any process that removes carbon from the atmosphere can act as a carbon sink. These include basic processes like having the gas dissolve into the ocean, to more complex ones, like the sequestration that appears to take place in mature forests. The cumulative impact, however, is huge; carbon sinks are estimated to remove about 60 percent of the CO 2 that human activity puts in the atmosphere annually. (The remaining 40 percent is termed the airborne fraction.)

There are a number of factors that may change the ability of many of these sinks to continue absorbing. On the plus side, agricultural efficiency has allowed extensive reforestation in the industrialized world, and the restored forests should be acting as new carbon sinks.

Obviously, however, not everything is a positive. In the oceans, increasing temperatures limit the amount of gas that can dissolve, which should slow the oceanic sink. On land, the fertilization effect of additional carbon dioxide will eventually tail off as other factors, like water or trace nutrients, begin to limit plant growth. The uncertainty about when and to what degree the sinks would start to lose capacity has made them an active area of study. So it's no surprise that two papers that attempt to track the behavior of the sinks were published in rapid succession, one in Geophysical Research Letters, the second in Nature Geoscience. The surprise may be that they're being promoted as if they contradict each other.

Both papers use a similar methodology: total up the estimates of global emissions from various sources, and figure out what the airborne fraction is. The remainder has to be going into carbon sinks. But the two operate on very different time scales. The GRL paper starts all the way back in 1850, and comes to the conclusion that the airborne fraction has been increasing, but only by about 0.7 percent a decade; the uncertainties are nearly twice that, making it statistically indistinguishable from zero.

In contrast, the Nature Geo paper starts its analysis in 1960, when the post-war boom in carbon emissions was really kicking in. It sees a more pronounced trend, one of 0.3 � 0.2 percent a year, which they say has a 90 percent probability of being statistically significant.

Why the difference? The rate of human CO 2 emissions rises dramatically starting in 1960. By lumping those later years with data that extends back to 1850, the GRL paper runs the risk of having earlier data swamp more recent (and, arguably, relevant) trends. In addition, it necessarily extends into years where reporting on things like land use was sketchy or nonexistent, creating more significant uncertainties.

There's also a difference in interpretation; the author of the GRL paper actually sees what appears to be an increase in the airborne fraction after the year 2000. But he ascribes this to differences in how land use changes are accounted for (in one scenario, he estimates that land use emissions figures are off by over 80 percent). He also repeats his analysis using approaches employed in other publications, and does see a significant trend for the entire time period.

To an extent, the authors of the Nature Geosciences paper agree with him, as they find land use trends in recent years to be problematic. In their case, they suggest that singular events in recent years have swamped some trends. Burning of forests shot up in Indonesia during an El Ni�o inflicted drought, then plunged as rainier conditions returned. Deforestation of the Amazon has plunged in the last year, for reasons that are unclear. Nevertheless, they see a trend that's distinct from these one-time events over longer periods.

So, are carbon sinks beginning to reach their limits? Given the two papers, I have to admit I lack the expertise to judge.

What is clear, however, is that two extremely cautious and technical papers have been handled awkwardly from a media perspective. The GRL paper was heralded with a press release that touted it as "Controversial new climate change data," even though it didn't directly address climate change, and actually applies new methods to existing data sets. Two articles removed from the press release, and you end up with an article that claims "new research shows that atmospheric levels of CO2 have effectively remained unchanged since the advent of the industrial revolution." It's hard to imagine anyone getting it so badly wrong.

This seems to have prodded Nature publishing to respond; normally, Nature Geoscience papers are released on Sundays. An exception was made to release this one on a Tuesday, accompanied by a press conference. One of the universities involved also felt compelled to issue a press release. The title on this one was at least accurate, but focused on carbon emissions when these figures are widely estimated and reported already.

These are two highly technical papers that use different data sets and different methods; it's no surprise that they've reached different conclusions, and it will probably take the scientific community a few months of digesting them and comparing them to previously published work in order to reach a consensus on which one (if either) is likely to better reflect reality. Using them to generate competing coverage in the popular press doesn't do a public that understands climate science poorly any favors.

Geophysical Research Letters, 2009. DOI: 10.1029/2009GL040613 Nature Geoscience, 2009. DOI: 10.1038/ngeo689