While you are driving to work, plume of carbon drifting lazily into the sky, do you ever wonder where all of your carbon will end up?  Now, work by Simon Lewis and a large team of collaborators (Lewis et al 2009) have shown that about 1/6th of the carbon emitted annually to the atmosphere by combustion of fossil fuels (total ~6 Pg = Petagrams = 10¹⁵ g) may be stored in the biomass of trees in tropical forests around the world (~1.3 Pg per year).  More specifically, they have shown that about 0.3 Pg are stored annually in ancient trees in the relatively undisturbed forest regions of Africa.  The reason?  “Carbon Fertilization”- increases in carbon dioxide concentrations have stimulated their photosynthesis, leading trees to grow faster and larger than they would at lower concentrations.

This is far more than just a curiosity.  For more than a decade, researchers have struggled and failed to balance global carbon budgets, which must balance carbon emissions to the atmosphere from fossil fuels (6.3 Pg per year; numbers here from Skee Houghton at Woods Hole Research Center) and land use change (2.2 Pg; deforestation, agriculture etc.) with carbon dioxide accumulation in the atmosphere (3.2 Pg) and the carbon sinks taking carbon out of the atmosphere, especially carbon dioxide dissolving in Ocean surface waters (2.4 Pg).  Until recently, balancing these numbers required assuming the existence of a 2.9 Pg “Missing Carbon Sink” (measurement error = +/- 1.1 Pg) .  While carbon accumulation in the atmosphere and carbon emissions from fossil fuels are well measured, most global carbon numbers are not known with a high degree of certainty, so even the size of the missing sink has long been a fairly open question.  However, the unknown carbon sink(s) must be on land- it cannot be in the Ocean.

So Lewis et al. (2009) have made a great contribution to understanding global changes in the carbon cycle that contribute to global warming.  Their solid measurements of a terrestrial carbon sink removing 0.3 Pg of carbon per year from the atmosphere in Africa’s trees, combined with those of other tropical forests, can explain more than one third of the missing carbon sink (1.3 Pg carbon per year). 

These observations also raise some very large questions.  What about forest soils in these regions? Forest soils often contain more carbon than tree biomass – though this is less true in the Tropics.  What about the long-term strength of this terrestrial carbon sink?  Trees usually become weaker carbon sinks as they age (growth slows over time)- though the ancient trees in this study did show sustained growth.  And taken globally, increases in tropical forest carbon may be at least partly explained not by carbon fertilization, but by a recovery of carbon after past disturbances such as fire (both natural and anthropogenic) and land clearing by humans even centuries earlier- a factor that will reduce sink strength over time as forests recover.

All of this also gets me thinking about the other two thirds of the missing terrestrial carbon sink that has yet to be explained (~2 Pg).  Given the forest recovery going on in many regions, including the US and most of Europe, and even China- how much of the missing sink can be explained by carbon fertilization versus regrowth of these rapidly growing forests and patches of trees?   This is much harder to measure than in undisturbed forests – these are trees in diverse small to large patches in abandoned agricultural lands intermingled with human settlements and are surely growing differently than trees in undisturbed forests or in the experimental planted and regrowing forests where carbon sink strength has been measured using precise methods.

Finally, this work brings home the fact that undisturbed trees in the tropics are cooling the planet for you while you and me are busy adding carbon to the fire.  Better hope these trees keep doing their thing!  And don’t make them work so hard!

• Link to Nature news item (requires access to Nature): http://www.nature.com/nature/journal/v457/n7232/full/457969a.html
• Reference for Lewis et al. (2009): Lewis, S. L., G. Lopez-Gonzalez, B. Sonke, K. Affum-Baffoe, T. R. Baker, L. O. Ojo, O. L. Phillips, J. M. Reitsma, L. White, J. A. Comiskey, M.-N. D. K, C. E. N. Ewango, T. R. Feldpausch, A. C. Hamilton, M. Gloor, T. Hart, A. Hladik, J. Lloyd, J. C. Lovett, J.-R. Makana, Y. Malhi, F. M. Mbago, H. J. Ndangalasi, J. Peacock, K. S. H. Peh, D. Sheil, T. Sunderland, M. D. Swaine, J. Taplin, D. Taylor, S. C. Thomas, R. Votere, and H. Woll. 2009. Increasing carbon storage in intact African tropical forests. Nature 457:1003-1006. Download the paper (requires access to Nature): http://dx.doi.org/10.1038/nature07771
• More about the “Missing” carbon sink:  http://www.whrc.org/carbon/missingc.htm