Duds: a skeptic bombshell that never went off

Climate skeptics are having a field day with a new study on the earth's natural ability to absorb excess carbon dioxide (Knorr, W. (2009), Is the airborne fraction of anthropogenic CO₂ emissions increasing?). Influential skeptic and TV weatherman Anthony Watts calls it a "bombshell" (for more info on Watts, see "You're not a scientist, but you play one in real life"). Essentially the skeptic claim is this: “natural carbon sinks are soaking up human CO2 emissions at a faster rate than previously thought, which suggests that global warming will occur slower than models suggest. The science is not settled!” But is this an accurate interpretation of the study?

Here’s the story in a nutshell. Humans emit a lot of CO2, but only about 40% of that remains in the atmosphere (the airborne fraction, or AF). The other 60% is absorbed by natural carbon sinks, such as oceans, soil, and plants. As CO2 concentrations have increased, the AF has remained constant, meaning that carbon sinks’ ability to absorb CO2 is increasing in line with emissions. However, basic physics predict that eventually carbon sinks will begin to “fill up” with CO2, losing their ability to absorb more. When this happens, the AF will begin to increase exponentially, as carbon sinks absorb less and less CO2—a positive carbon cycle feedback.

The AF has remained constant for decades, but a few studies—Canadell et al (2007), Le Quere et al. (2007), and Schuster and Watson (2007)—have found signs that this carbon cycle feedback was starting to get underway. Canadell, for example, had found that the AF had increased from 40% in 1960 to 45% in 2007, which would signal that less carbon was being absorbed by the biosphere. The climate science website RealClimate notes:

There are uncertainties and caveats associated with each study, but taken as a whole, they provide convincing evidence that the hypothesized carbon cycle positive feedback has begun…

But then came the “bombshell” from Knorr (2009), who found that the AF has not, in fact, increased. The full study is behind a paywall, but you can read the abstract here. If Knorr is correct, it could mean that the studies from 2007 were premature in concluding that carbon cycle feedbacks have begun. The real question is, does this matter? If Knorr is right, does this cast doubt on the consensus view that greenhouse gas emissions from fossil fuels are causing global warming?

The short answer is no. I don’t have the scientific training to figure out which study is correct, but I do understand logic, and here’s what you need to know about carbon cycles.

First, the study's conclusion makes no claims about FUTURE carbon cycle feedbacks—it simply finds that carbon sinks' ability to absorb CO2 has not declined in the PRESENT. Claiming that Knorr casts doubt on models predicting accelerating future growth in CO2 concentrations makes the logical fallacy of extrapolating future trends from current results—the same error that led financial firms to conclude that housing prices would always increase.

Second, keep in mind that the current climate consensus is summarized within the 2007 IPCC report. On the other hand, the studies that Knorr critiques were published AFTER the IPCC report came out; therefore, if Knorr is correct in proving these studies wrong, his findings cannot logically have any bearing on the accuracy of the IPCC’s conclusions. At worst, Knorr simply returns us to the state of science when the IPCC report was written. In other words, skeptics are attacking a straw man.

Let me elaborate. For the Knorr study to make a dent in the armor of global warming theory, the findings it attacks would have to be included in, and critical to, the most widely accepted climate models. This is not the case. Studies like Canandell cannot have been included in the models on which the consensus is based, since they were published AFTER those models were created.

Moreover, while these studies gave us reason to believe global warming could occur faster, no one claimed they were certain, so it is inaccurate to claim that Knorr is throwing a bomb at the consensus. RealClimate, for one, emphasizes the difficulties with modeling carbon cycle feedbacks, because there are multiple physical processes that work in opposite directions. For example, as CO2 concentrations increase, the ocean’s ability to absorb CO2 also increases. At the same time, rising temperatures decrease the ocean’s ability to absorb CO2 (think of a warm soda can losing its carbonation). So the trick is to figure out when one process begins to dominate the other, and that’s hard to do:

We have to keep in mind that it is a tricky business to invert the atmospheric CO₂ concentration to get sources and sinks. The history of this type of study tells us to wait for independent replication before taking this result to the bank… I guess it’s fair to say that models are not decisive in their assessment about which of these two factors should be dominating at present.

In other words, none of these findings had been independently verified, and certainly not risen to the level of consensus. In fact, before Canandell, most models apparently predicted a declining airborne fraction, meaning that Knorr’s study is actually in line with IPCC models:

Carbon cycle models (13 of them, from the SRES A2 scenario) also predict that the atmospheric fraction should increase, but not yet. For the time period from 1960 to 2000, the models predict that we would find the opposite of what is observed: a slight decrease in the atmospheric fraction, driven by increasing carbon uptake into the natural world. Positive feedbacks in the real-world carbon cycle seem to be kicking in faster than anticipated, Canadell et al conclude.

In any case, according to Jones et al (2007), it is incorrect to conclude that a constant AF disproves carbon cycle feedbacks (don’t ask me why—I’m just copying and pasting to prove that the consensus supposedly being attacked by Knorr is a straw man):

It is commonly assumed that no change in airborne fraction implies no change in the carbon cycle. This is not true. It is also commonly assumed that if AF is rising, this implies we’ve detected a climate feedback on the carbon cycle. This is also not true. In reality, the AF depends not only on this years emissions and natural fluxes, but the time history of the carbon cycle, which in itself depends on the time history of the emissions. A different rate of emissions will imply different response of AF. This is true regardless of any feedbacks from climate.

The bottom line is that while Knorr (2009) may cast doubt on the conclusion that carbon cycle feedbacks have already begun, that conclusion is hardly pertinent to the consensus on global warming. The most far-reaching conclusion a skeptic could logically draw is that global warming is as bad as we thought (and not worse than we thought).

In other words, Anthony Watts isn’t a scientist, but he plays one in real life.

NOTE: Obviously I am not a scientist, and have probably gotten some points wrong (and probably left some out as well). If you ARE a scientist with expertise in the area, please leave a comment, and I can send you a link to a Google Document that you can edit.

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