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Does More Carbon Dioxide Mean Increased Crop Water Productivity? (arstechnica.com)
An anonymous reader points us to an Ars Technica report: For the most part, we think of rising levels of carbon dioxide as an environmental problem. But atmospheric CO2 can also boost agricultural productivity by helping plants grow. How do these potential issues balance out? In an investigation recently published in Nature Climate Change, scientists have looked into the global implications of carbon dioxide's ability to enhance agricultural productivity. Increased levels of CO2 can enhance photosynthesis and reduce leaf-level transpiration, the process by which some of the water that plants draw from the ground gets released back into the atmosphere. These changes can reduce growing seasons and water loss. The result could be an increase in what's called "crop water productivity," i.e. the amount of food produced for each unit of water expended. If elevated CO2 levels increase crop yield and reduce water consumption at large scales, this could help ensure water and food security despite the climate disruptions. By combining data from a massive network of field experiments and global crop models, the scientists claimed that depending on the crop type, global crop water productivity will increase by 10 to 27 percent by the 2080s. Arid regions exhibited large increases that were based on crop type.
It sounds good that crops will be more productive. So will other things, though. There was an experiment in which poison ivy was grown in higher CO2 conditions. It grew better and produced more urushiol (the stuff that causes you to have an allergic reaction). Crops may grow better, but so will weeds. It wouldn't surprise me if that included some invasive weeds like kudzu. If it stays warmer, pests might not die off in the cold; the mountain pine beetle is an example. Furthermore, there's not only increased temperatures and longer growing seasons; rainfall patterns will shift, too. Areas that currently grow crops might become arid and either have to grow different crops or not be productive at all. Other areas, such as the northern US, are likely to become wetter. Maybe you have longer growing seasons in some areas, but I'm not sure how much of a real gain there will be if the rainfall moves poleward with the warm temperatures.
I've got a planted freshwater aquarium. In addition to good lighting and appropriate fertilization, people who like to keep this kind of aquarium tend to inject carbon dioxide to keep the plants growing well. The difference in plant performance in the aquarium with and without CO2 injection is substantial.
-- "Oh. This guy again."
The difference this time around is that Carboniferous period fungi were unable to break down lignin. Lignin adds rigidity to plant cell walls and was what plants needed to grow into tall trees with the capacity to bind trillions of tonnes of carbon into wood. For about 50 millions years, all the tree trunks that fell over from storms, disease, old age, insects, earthquakes, dinosours knocking them over, what have you, did not rot completely. Much of their carbon was sequestered underground and compressed into coal. The Caboniferous period ended as Fungi evolved the means to digest lignin and ended the massive carbon sequestration.
http://www.mining.com/coal-sto...
So yeah, plant growth will spike, but don't expect that to mean much for reducing global C02 levels.
Free-Air Concentration Enrichment studies such as soyFACE artificially raise CO2 (among other variables) and monitor plant response. SoyFACE, as the name implies, is focused on soy, an important food crop. Imagine a crop field surrounded by CO2 sprayers and heaters to simulate elevated CO2 and its effects.
Findings from the experiment include that increased temperatures will likely reduce yields of soy, even at elevated CO2. Higher average temperatures also increased susceptibility to herbivory by the Japanese beetle.
A related meta including 228 experimental observations found that barley, rice, wheat, soybean, and potato all have lower protein content at elevated CO2.doi: 10.1111/j.1365-2486.2007.01511.x
14 years of publications can be found here: http://www.igb.illinois.edu/so...
In short: even if water use efficiency were to increase, that does not result in increased yield, or crop quality.