The Island of Doubt

Black carbon is one the complicating factors that have to be taken into account when trying to figure out what the net effect of greenhouse-gas emissions really is. It is formed artificially through the incomplete combustion of fossil fuels, but that's not the kind we're interested in here. Instead, it's the carbon found in the soil after burning organic matter, such as a forest.

That carbon eventually ends up in the atmosphere as carbon dioxide, and how fast it gets into the air depends on how warm it is. In other words, it's a consequence of and contributor to global warming. Because carbon emissions from the soil represent 10 times what humans are emitting, getting a handle on just how long it takes this feedback mechanism to work is critical for a full understanding of climate change. As Johannes Lehmann of Cornell University and his Australian and British colleagues write in the paper:

...for models that will fully incorporate soil C stocks9, failure to identify significant but highly stable fractions could render predictions of global climate change, even by simulation models with multiple soil C pools, inaccurate.

What Lehmann et al found when they closely examined Australian soils is that it take a lot longer for soil organic carbon to find its way back to the air than previously thought. For example, "If the existence of black C is not considered, the three-pool model predicts an 18% greater increase in CO2 production compared with a scenario that recognizes black C." And that kind of difference could have serious implications for the total amount of atmospheric CO2. Remember that soil organic carbon releases are an order of magnitude greater than anthropogenic sources.

But that's not the whole story. First, of all, Lehmann et al only looked at Australian dirt. And "An extrapolation of these results to the global scale is currently not possible, because very few data are available that report black C contents in soils outside Australia."

Second, and more importantly, what the Lehmann study primarily shows is that our climate models may have to be rejigged if they are to accurately simulate the global carbon cycle. It doesn't say anything about climate change observations.

Here's a relevant excerpt from Cornell's press release:

"We know from measurements that climate change today is worse than people have predicted," said Lehmann. "But this particular aspect, black carbon's stability in soil, if incorporated in climate models, would actually decrease climate predictions."

This is an important reminder that current climate models still don't yield the magnitude of warming actually being recorded. (Globally that's about 0.8° C). I asked Lehmann in an email to elaborate. He reiterated the main idea in his response:

We know that burning is part of many ecosystems worldwide, providing justification to look closely. And this provides the need to look where we underestimated emissions. If soil feedback is part of a particular prediction model, that portion may have overestimated warming feedback.

And CNN got an even better quote from him:

It means that if we overestimated the soil response to warming then we must have underestimated something else dramatically because we have a net effect of faster global warming. That is really something to think about.

So, if taking into account the burning of forests and the resulting role of black carbon will make our models produce even lower levels of warming that what we're seeing, that means we'll have to find other reasons to explain the discrepancies between the observations and models.

In other words, if the Australian situation is representative of the global story (something that by no means certain), then we know even less than we thought we knew about how we're affecting the climate. It's theoretically possible that what we've overlooked will prove easy to mitigate, which would be great. But it could just as easily turn out to be the opposite case.
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Johannes Lehmann, Jan Skjemstad, Saran Sohi, John Carter, Michele Barson, Pete Falloon, Kevin Coleman, Peter Woodbury, Evelyn Krull (2008). Australian climate-carbon cycle feedback reduced by soil black carbon Nature Geoscience DOI: 10.1038/ngeo358