Idea

Carbon dioxide fertilization is the phenomenon when plant growth is increased due to higher atmospheric concentrations of CO 2 . Faster plant growth lead to the sequestration of more CO 2 (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 CO 2 , 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 CO 2 into the atmosphere: the equivalent of 26 stabilization wedges:

Stephen Pacala, Equitable climate solutions, talk at the Energy Seminar, Woods Institute, Stanford University, 5 November 2008.

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 CO 2 at the predicted rates? Maybe. First, people have actually grown forests in doubled CO 2 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 CO 2 fertilization matter a lot. It could make the difference between their original plan being roughly good enough… and being nowhere near good enough!

The biology of soil

Plants interact with and are dependent on soil. Soils are very variable and complex ecosystems. In the long run the CO 2 fertilization effect depends on how the soil food web adapts to the increased input of C from plant litter and roots. This can lead to positive or negative feedbacks on plant growth: When soil microbes are C limited, their increased biomass leads to an increase in N mineralization, making more N available to plants - a positive feedback on plant growth. When soil microbes are N limited they claim more N for their biomass, leaving less for plant growth.

While these are relatively fast feedbacks (with a chance for modelling them), there are slower ones complicating the picture. E.g. plant community composition and diversity may change, with unpredictable influence on litter decomposition rates and soil fauna. Legumes (plants in symbiosis with atmospheric N fixating bacteria) have great influence on available N.

For more detail see the chapter 6, Soil biological properties and global change, in

Richard Berdgett, The Biology of Soil - A community and ecosystem approach, Oxford University Press, 2005

Further research

Current observations

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 − 3 0.5 \times 10^{-3} per year, P = 0.03 P = 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 − 3 1.8 \times 10^{-3} per year, P < 0.001 P \lt 0.001 ), and then decreased from 1997 to 2006 ( − 1.3 × 10 − 3 -1.3 \times 10^{-3} per year, P = 0.055 P = 0.055 ). (…)

Also see this article:

NASA Earth Observatory, April 22, 2006: Northern Forest Affected by Global Warming.

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.

Hint from paleoclimatology

Gabriel J. Bowen, James C. Zachos. Rapid carbon sequestration at the termination of the Palaeocene–Eocene Thermal Maximum. Nature Geoscience 3 (2010), 866–869

From an interview with Bowen in Science Daily:

At the beginning of the event we see a shift indicating that a lot of organic-derived carbon dioxide had been added to the atmosphere, and at the end of the event we see a shift indicating that a lot of carbon dioxide was taken up as organic carbon and thus removed from the atmosphere. (…)

Expansion of the biosphere is one plausible mechanism for the rapid recovery, but in order to take up this much carbon in forests and soils there must have first been a massive depletion of these carbon stocks. (…) We don’t currently know where all the carbon that caused this event came from, and our results suggest the troubling possibility that widespread decay or burning of large parts of the continental biosphere may have been involved.

(Note that “rapid” refers to a timescale of tens of 1000y, longer than the whole holocene.)

Possible influence on leaf structure

Lawler et al “The effects of elevated CO2 atmospheres on the nutritional quality of Eucalyptus foliage and its interaction with soil nutrient and light availability” Oecologia 109 (2007)