More CO2 a mixed blessing for farming
Published 15th November 2009 - 28 comments - 4864 views -
While sometimes easily disregarded as wishful thinking, it is often optimistically postulated that future farmers' yields will be more bountiful due to 'carbon fertilization' via increased atmospheric carbon dioxide (CO2) levels. But things are not that simple, unfortunately.
It is correct that CO2 is one of the main ingredients of photosynthesis and that more ingredients allows for more products. But there are also other ingredients to the formula plus a labyrinth of biochemical processes from field to food to consider. More serious speculation on increased plant growth from increased CO2 levels quickly gets complicated by pro et contra whole ecosystems considerations .
German spring wheat
- 11.8% more biomass of above ground biomass (stem and ears, not leaves)
- 10.4% higher yield
But it also gave some results on the negative side:
- Smaller grains (of lower market value)
- 7.4% grain protein concentration decrease
- Decreases in amino acid concentrations
- Changes in mineral compositions (ie potassium and lead increase; iron and silicon decrease)
- Fructose content increase
The study did many other findings but most not as significant as the above mentioned. There were changes in the dough characteristics too. Overall, it seems clear that rising carbon dioxide levels will change the nutritional value of our food.
What this study did was grow 13 strains of spring wheat under conditions mimicing actual farming except adding CO2. Then analysing not just the grain, but the whole plant and the flour and bread produced. The growing plants were irrigated and fertilized - thus, under conditions where plant nutrients and water never limited growth. All future crops will not be that lucky: already drought is a major problem and is presumably only to increasingly cause wilting fields under global warming. Only (conventional) farmers in well-off countries are reasonably sure to be able to fertilize and irrigate their crops in any foreseeable future while elevated levels of CO2 will not do any good to a 3rd world farmer with soils cracked and dry.
Wheat is one of the world's major food crops so even a seemingly microscopic change in it's biology could lead to enormous world population health impacts further down the cause-effect chain. The mixed (and regarding some parameters not mentioned here, unreliable) results, the single crop type and the geographical limitation of the study to Stuttgart, Germany further studies should be undertaken.
Some crops (ie maize) already has evolved a mechanism for concentrating CO2 in their leaves, meaning raised atmospheric levels will help them little. Plants in general will not be able to evolve such biochemical mechanisms to accustom themselves to the changed climate in the scope of time we humans alter nature and project food production.
The German study is interesting because it's not from a biochemical model or a test chamber, but from an actual field. However, it only looked at rising carbon dioxide. There are many other factors to look into.
One study saw a 1ºC temperature rise reduce tree growth by 50%, ozone levels are expected to rise also which will to some degree negate the effects of more CO2, plus the CO2 could have negative effects in itself (ie via ocean acidification and an expected drop in plant biodiversity). 
And another Free-Air study found nitrogen and phosphorous levels to quickly limit growth if deficient (or rather, not in abundance). And considering agricultural crops for carbon sequestering purposes is not really serious thinking since their combined biomass is puny compared to forest biomass as well as fossil carbon emissions. 
|Perhaps crops from elevated carbon dioxide futures are a bit like bodybuilders - bigger, but not necessarily better.
Högy, P., Wieser, H., Köhler, P., Schwadorf, K., Breuer, J., Franzaring, J., Muntifering, R., & Fangmeier, A. (2009). Effects of elevated CO on grain yield and quality of wheat: results from a 3-year free-air CO enrichment experiment Plant Biology, 11, 60-69 DOI: 10.1111/j.1438-8677.2009.00230.x
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