Global ChangeIntersection of Nature and Culture

Global carbon cycle sees increase in soil carbon loss

Thursday, March 25th, 2010

A lot of us are on the lookout for increased releases of soil carbon in northern ecosystems, which could signal the initiation of a positive feedback to warming.   Remember that more warming has the possibility of increasing decomposition of soil carbon, which causes the release of more CO₂ to the atmosphere, causing further warming (the feedback mechanism).

I was delighted to open today’s issue of Nature and see an article by one of my colleagues—Ben Bond-Lamberty (together with colleague Allison Thomson)—who completed a global assessment of soil carbon release.¹

To give you a bit of a primer, a couple of decades ago, we used to think that soils worldwide released about 60 gigatons of carbon each year.   That’s about six times the total current release of anthropogenic carbon from fossil fuels and deforestation combined.  So no small potatoes.   You can therefore think of humans as “decomposers” who release about 1/6 the amount of carbon that soil microbes do each year.

Based on a meta-analysis of 439 studies of soil carbon respiration, Ben and Allison discovered that this number has risen to about 98 gigatons (+/- 12 Gt) per year in 2008 and that it’s growing by about 0.1 gigaton per year.  Part of this is due to better estimates of global soil respiration, but as their data suggest, part of it is also a genuine upward trend.

The trend of increasing soil CO₂ release over time correlates with the late-20th-century rise in air temperatures.

Case closed on the positive feedback?  Not so fast they wisely warn us.  At face value, this change is consistent with temperature-driven increases in soil respiration.  However, Ben and Allison are right to point out that we need to be a bit careful and ask where that extra carbon is coming from.  For simplicity, we can think of two sources:

• As the positive feedback hypothesis suggests, it might be old soil carbon that is decomposing now that microbes are warmer and more active.
• However, a world with more CO₂ will also fertilize tree growth and lead to increased production of things like leaves, branches, and tree stems that fall to the ground each year.  If climate change is causing more of this “litterfall” to happen, then the increased decomposition Ben and Allison detected might simply reflect that fact there is more dead fresh litter falling onto soils around the globe.

Either possibility is interesting, but it’s really the first one that’s worrisome in terms of accelerated climate warming.  Why?  Because a jazzed-up carbon cycle (bullet #2) doesn’t really add any new carbon to the atmosphere.  If trees grow more and then that growth is released as greater soil decomposition, all we have done is made the plant uptake arrow bigger and the soil release arrow bigger.  No net change in atmospheric CO₂.

It’s hard to tell at this point if one or the other (or both) mechanisms are at play.  If it is the beginning of a climate warming positive feedback, it’s all the more imperative that we  get carbon emissions and temperatures under control quickly.

¹Bond-Lamberty, B., & Thomson, A. (2010). Temperature-associated increases in the global soil respiration record Nature, 464 (7288), 579-582 DOI: 10.1038/nature08930

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Photo Credit: One of my photos of a low-Arctic Sphagnum peat bog in Manitoba, Canada, which you can view at my Flickr site.