One of the tropes circulated by climate deniers has been that "Excess carbon dioxide is good for plants and especially crops". In other words, CO2 even at exceptionally high levels, is not really a pollutant, but something to help us attain better crop yields. While climate scientists and environmentalists have tried to correct this, i.e showing that excess CO2 actually propels "super weeds" which kill crops and that necessitates ever stronger weedicides like agents as: 2, 4- D, dicamba and paraquat, the effects have been nominal. (See also:The Wall Street Journal June 4, 2010, p A16, 'Superweeds Trigger New Arms Race')
The deniers, for their argument, probably invoked the (summary) chemical equation most of us learned in high school biology for photosynthesis:
6 CO2 + 6 H20 + light energy -> C6 H12 O6 + 6 O2
Thus, the carbon dioxide (6 molecules) combined with six of oxygen and adding in radiant energy (sunlight) gives glucose plus oxygen. What could be wrong? Well, a lot it there is too much carbon dioxide.
But this example, illustrates the recurring problem of exactly how any greenhouse gas affects crop yields. Now thanks to new work by Drew T. Shindell, e.g.
We are finally getting some useful answers. While it's been known that carbon dioxide is the largest driver of climate change, Shindell found that other anthropogenic emissions cause more damage to actual crop yields. Among the culprits named in the past: methane, halocarbons (used in refrigeration and air conditioning), and black carbon (from biomass and fossil fuel burning)
In his paper published in Earth's Future, Shindell sought to create a more complete picture of human-caused emissions. He did this by assembling data from his previous studies as well as the IPCC's fourth and fifth assessment reports and crop yield data from previous work. Shindell then created a model to determine the effect that individual greenhouse gases have on global temperatures, precipitation, carbon dioxide and ozone - all of which affect crop success.
Shindell's model revealed that in the short term (i.e. 1st decade after release of emissions), the greatest damage to crops per ton comes from black carbon and halocarbons. Methane emissions come next, given that the gas (CH4) increases surface ozone which causes harmful chlorosis, i.e. a yellowing of leaves.
Shindell found the effects of CO2 are more complicated, For example, as the amount in the atmosphere increases, crop yields initially do as well. But as the CO2 emissions continue to contribute to climate warming the overall impact becomes negative with any benefits originally accruing basically being outweighed by deleterious effects after only 10 years.
Overall, Shindell's model indicates that up to 93 percent of crop losses through the rest of the century will be caused from non-carbon dioxide emissions with methane the most damaging. (Let us also bear in mind the most massive methane outgassing, as from melting permafrost in the Arctic, is triggered by much higher warming from carbon dioxide)
The most intriguing aspect of Shindell's paper is in where it shows how mitigation procedures can increase crop yields. For reduced CO2 alone, for example, crop yields would go up 3 percent. If methane is suitably reduced (which also means CO2 and warming is reduced) we can expect a 16 percent increase in crop yields.
On the other hand, under a high emissions scenario for all greenhouse gases (likely under Trump) there will be nearly 25 percent crop losses by the end of the century. Given the world's population is expected to balloon to 11 billion or more, we are looking at extreme hunger, starvation. This will be in the most densely populated regions such as Africa.
Without a doubt, Shindell's work provides a solid basis for decisions by policy makers. The question now is whether they will even read his work, and then act on it.