The earth's complex of biomes and subsystems is very complex, and the study of the interrelationships of myriads of factors can be incomprehensible non-scientists. Many people tend to think that simple, single factor elements have linear effects on particular biosphere functions, such as plant growth. This tendency toward gross oversimplification is common in wingnuts on both the right and the left, and particularly with conservative politicians and global climate change and peak oil deniers. The quotes below only touch on the subject of atmospheric CO2 levels and it's effects on plant growth.
From edf.org: .
Even though higher levels of CO2 can act as a plant fertilizer under some conditions, scientists now think that the "CO2 fertilization" effect on crops has been overstated; in natural ecosystems, the fertilization effect can diminish after a few years as plants acclimate. Furthermore, increased CO2 may benefit undesirable, weedy species more than desirable species.
From the New Scientist:
Predicting the world's overall changes in food production in response to elevated CO2 is virtually impossible. Global production is expected to rise until the increase in local average temperatures exceeds 3°C, but then start to fall. In tropical and dry regions increases of just 1 to 2°C are expected to lead to falls in production. In marginal lands where water is the greatest constraint, which includes much of the developing world but also regions such as the western US, the losses may greatly exceed the gains.
An example of current research on the complexities of global climate change and plant growth from Standord University's Jasper Ridge research center:
"Most studies have looked at the effects of carbon dioxide on plants in pots or on very simple ecosystems and concluded that plants are going to grow faster in the future," said Field, co-author of the Science study. "We got exactly the same results when we applied carbon dioxide alone, but when we factored in realistic treatments -- warming, changes in nitrogen deposition, changes in precipitation -- growth was actually suppressed."
To mimic future climate conditions, Field, Mooney and their colleagues mapped out 36 circular plots of land, each about 6 feet in diameter. Four plots are virtually untouched, receiving no additional water, nitrogen, carbon dioxide or heat. Each of the remaining 32 circles is divided into four equal quadrants separated by underground partitions to prevent roots in one section from invading neighboring tracts. In these smaller quadrants, researchers study all 16 possible combinations of elevated and normal carbon dioxide, heat, water and nitrogen.
The plots thicken
The biggest surprise from the study was the discovery that elevated carbon dioxide only stimulated plant growth when nitrogen, water and temperature were kept at normal levels.
"Based on earlier single-treatment studies with elevated carbon dioxide, we initially hypothesized that, with the combination of all four treatments together, the response would be additional growth," said W. Rebecca Shaw, a researcher with the Nature Conservancy of California and lead author of the Science study.
But results from the third year of the experiment revealed a more complex scenario. While treatments involving increased temperature, nitrogen deposition or precipitation -- alone or in combination -- promoted plant growth, the addition of elevated carbon dioxide consistently dampened those increases.
"The three-factor combination of increased temperature, precipitation and nitrogen deposition produced the largest stimulation [an 84 percent increase], but adding carbon dioxide reduced this to 40 percent," Shaw and her colleagues wrote.
The mean net plant growth for all treatment combinations with elevated carbon dioxide was about 4.9 tons per acre -- compared to roughly 5.5 tons per acre for all treatment combinations in which carbon dioxide levels were kept normal. However, when higher amounts of carbon dioxide gas were added to plots with normal temperature, moisture and nitrogen levels, aboveground plant growth increased by nearly a third.
Why would elevated carbon dioxide in combination with other factors have a suppressive effect on plant growth? The researchers aren't sure, but one possibility is that excess carbon in the soil is allowing microbes to outcompete plants for one or more limiting nutrients.
If you're really interested in the complexities of this issue, buy a copy of the graduate level textbook
"Plant Growth and Climate Change," edited by James I. L. Morison and Michael D. Morecroft for about $200.
Dave