The Azimuth Project
Carbon dioxide fertilization (Rev #2, changes)

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Carbon dioxide fertilization is the tendency phenomenon for plants to grow faster when the plant growth is increased due to higher atmospheric concentrations of CO2 . concentration Faster is plant higher. growth One lead possible to effect the is sequestration that of plants may sequester more CO2 . (at least during the plants’ period of growth). In the third IPCC report, models predicted that by 2050, plants will be drawing down 6 gigatonnes more carbon per year than they do now! The fourth IPCC report was similar.

This a huge effect: remember that right now we emit about 8 gigatonnes pf carbon per year. Indeed, this effect could be the difference between the land being a big carbon sink and a big carbon source. Why a carbon source? For one thing, without the plants sucking up CO2, temperatures will rise faster, and the Amazon rainforest may start to die, and permafrost in the Arctic may release more greenhouse gases (especially methane) as it melts.

Stephen Pacala

In a simulation run by Stephen Pacala, where he deliberately assumed that plants fail to suck up more carbon dioxide, these effects happened and the biosphere dumped a huge amount of extra CO2 into the atmosphere: the equivalent of 26 stabilization wedges:

So, he points out plans based on the IPCC models are essentially counting on plants to save us from ourselves.

But is there any reason to think plants might not suck up CO2 at the predicted rates? Maybe. First, people have actually grown forests in doubled CO2 conditions to see how much faster plants grow then. But the classic experiment along these lines used young trees. In 2005, Körner et al did an experiment using mature trees… and they didn’t see them growing any faster!

Second, models in the third IPCC report assumed that as plants grew faster, they’d have no trouble getting all the nitrogen they need. But Hungate et al have argued otherwise. On the other hand, Alexander Barron discovered that some tropical plants were unexpectedly good at ramping up the rate at which they grab ahold of nitrogen from the atmosphere. But on the third hand, that only applies to the tropics. And on the fourth hand—a complicated problem like this requires one of those Indian gods with lots of hands—nitrogen isn’t the only limiting factor to worry about: there’s also phosphorus, for example.

Pacala goes on and discusses even more complicating factors. But his main point is simple. The details of CO2 fertilization matter a lot. It could make the difference between their original plan being roughly good enough… and being nowhere near good enough!

Further research

On the Azimuth Blog, Martin Gisser points out that some recent research on CO2 fertilization looks worrisome:

From satellite observations, Zhao & Running (2010) estimate a 0.55 Gt (ca. 1%) decline in global terrestrial NPP (net primary production) from 2000 to 2009. Between 1982 and 1999 the increase was up to 6%.

This paper suggests that from 1997 to 2006 the Normalized Difference Vegetation Index or NDVI, a measure of the amount of vegetation, has been decreasing:

  • Shilong Piao, Xuhui Wang, Philitppe Ciais, Biao Zhuz, Tao Wang, and Jiu Liu, Changes in satellite-derived vegetation growth trend in temperate and boreal Eurasia from 1982 to 2006, Global Change Biology, preview 31 March 2011.

Abstract (…) although a statistically significant positive trend of average growing season NDVI is observed (0.5×10 30.5 \times 10^{-3} per year, P=0.03P = 0.03) during the entire study period, there are two distinct periods with opposite trends in growing season NDVI. Growing season NDVI has first significantly increased from 1982 to 1997 (1.8×10 31.8 \times 10^{-3} per year, P<0.001P \lt 0.001 ), and then decreased from 1997 to 2006 (1.3×10 3-1.3 \times 10^{-3} per year, P=0.055P = 0.055). (…)

Also see this article:

reporting on this paper:

  • Scott J. Goetz, Andrew G. Bunn, Gregory J. Fiske, R. A. Houghton, Satellite-observed photosynthetic trends across boreal North America associated with climate and fire disturbance, PNAS September 20, 2005 vol. 102.

For parts of the region, growth has not changed (gray), but in interior Alaska and a wide swath of Canada, growth has declined (brown). Only in the far north, regions of tundra, has growth increased (green).

  • Jofre Carnicera, Marta Colla, Miquel Ninyerolac, Xavier Ponsd, Gerardo Sáncheze, Josep Peñuelasa, Widespread crown condition decline, food web disruption, and amplified tree mortality with increased climate change-type drought, PNAS January 25, 2011 vol. 108.

Abstract. Climate change is progressively increasing severe drought events in the Northern Hemisphere, causing regional tree die-off events and contributing to the global reduction of the carbon sink efficiency of forests. (…) Here we report a generalized increase in crown defoliation in southern European forests occurring during 1987–2007. Forest tree species have consistently and significantly altered their crown leaf structures, with increased percentages of defoliation in the drier parts of their distributions in response to increased water deficit. We assessed (…) Our results reveal a complex geographical mosaic of species-specific responses to climate change–driven drought pressures on the Iberian Peninsula, with an overwhelmingly predominant trend toward increased drought damage.

category: carbon