Emissions From Soil Organic Carbon Not as Bad as Previously Thought

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As I've argued many times in the past, climate models may not be foolproof but, with the right data and assumptions, they can serve an invaluable function in helping scientists and policymakers devise effective mitigation strategies. I'd be lying if I didn't say that I've seen my fair share of problematic models in the past but, as with most things, they have improved by leaps and bounds in only a few years' time. Now the findings of a new study conducted by researchers at Cornell University should help scientists do a much better job of incorporating soil organic carbon content into their models.

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Because warming is expected to accelerate soil decomposition, and thus produce more CO2 emissions (resulting in a nasty positive feedback), climate scientists have long sought to accurately estimate the amount of soil organic carbon present in various parts of the world.

Higher black C content = less CO2 emissions
The results themselves, which were published in the latest issue of Nature Geoscience (sub. required), reflect poorly on the ability of current models to accurately estimate the amount of carbon dioxide released from soil on a global scale. Johannes Lehmann, an associate professor of soil biogeochemistry, and his colleagues studied 452 sites in two Australian savannah regions -- Queensland and Northern Territory; they discovered that the levels of black carbon present in the soil were much higher than expected.

This is important because the higher content of black carbon there is in the soil, the less CO2 is released; unlike regular soil organic carbon (produced by the decomposition of vegetation), which begins releasing CO2 to the atmosphere in only a few years' time, black carbon (produced by burning organic matter), takes much longer -- roughly 1 - 2 millennia.

As Lehmann told CNN's Matthew Knight, this is a crucial distinction because most models estimate the amount of CO2 emissions from soil to be 10 times larger than the amount created by consuming fossil fuels. While that amount is still much higher, Lehman cautions against adopting a "black box" perspective to modeling -- recognizing that not all soils are the same and that the amount of CO2 released can see larger variations.

Lehmann's calculations did not take any potential feedbacks -- resulting from a higher incidence of wildfires or changing soil water contents, for instance (both of which could increase or decrease the amount of black carbon) -- into account so it's possible that his findings could over/underestimate the magnitude of the problem. (Not that this omission significantly affects the study's overall findings, though.)

Findings don't invalidate all models but suggest more research is needed
Lehmann et al. showed that a realistic estimate of the soils' black carbon content reduced projected emissions release (in response to a 3°C temperature increase over 100 years) by 18.3 and 24.4 percent in the two savannah regions. Though these results don't necessarily mean that all black carbon estimates are wildly off-mark, what they do suggest, as Lehmann explains, is that this large variation is "reason enough to have a closer look at what the same assessment would tell us for the other continents."

Or, as he concludes in his Nature article:

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. However, vegetation fires as the source of black C are a global-scale phenomenon and fire frequency and size may regionally even increase in the future. Therefore, investigation of black C in soils on a global scale is required to capture what are probably significant regional differences depending on black C production and disappearance as evident in the studied Australian soils.