I recently had an experience both rare and pleasant, a civil and informative argument about climate on FaceBook. It was started by
One of the commenters, although not prepared to defend the hysterical tone of the posted piece, was willing to argue that climate change was making the global food situation worse and threatened to make it much worse in the future. In defense of that claim, he cited "one recent study showing four major global crops declining (relative to no climate change)." The article, "Climate Trends and Global Crop Production Since 1980" (Lobell et. al. 2011), was an attempt to separate out the effects on four major crops of different environmental changes–temperature, precipitation, and CO2 concentration–occurring from 1980 to 2008.
Reading it, I noticed that what the authors defined as the effect of climate change included temperature and precipitation but not CO2; its (positive) effect was listed separately. Including it changed the conclusion from four crops down to two down, two up. The commenter who offered the article as evidence had apparently missed that fact.
I also noticed that while they found a significant warming trend over the period, the trend in precipitation was statistically insignificant – consistent with random change. Redoing the calculation using only the two effects we knew were associated with AGW, warming and increased CO2 concentration, made the percentage increase in rice equal to the decrease in maize, the increase in soybeans larger than the decrease in wheat. The figures are shown in Table 1 from the article.
The table showed no effect of increased CO2 on the yield of Maize. Maize, as the gentleman I was arguing with pointed out, is a C4 crop, the other crops C3, the difference being in the details of the mechanism for photosynthesis. The effect of CO2 fertilization on C4 crops is substantially less than on C3 crops but not zero. Looking at another article that had been linked in the discussion, this one from the EPA, I found:
The yields for some crops, like wheat and soybeans, could increase by 30% or more under a doubling of CO2 concentrations. The yields for other crops, such as corn, exhibit a much smaller response (less than 10% increase).
That suggests that the effect is less than a third as large as the effect on the C3 crops but still substantial. Including it on Table 1 makes the negative net effect on maize smaller than the positive effect on rice.
Looking at the EPA article I noticed that the increase in yield due to CO2 fertilization was presented as a fact, various things that might decrease yields as possibilities.
"if temperature exceeds a crop's optimal level or if sufficient water
and nutrients are not available, yield increases may be reduced or
"Extreme events, especially floods and droughts, can harm crops and reduce yields."
No evidence was offered that any of those things would happen or how large the effects would be if they did. It looked as though the authors wanted to give the impression that climate change would reduce agricultural yields but prudently stopped short of saying so.
I also noticed:
"Overall, climate change could make it more difficult to grow crops,
raise animals, and catch fish in the same ways and same places as we
have done in the past."
As conditions change, people change what they do in response. If temperatures rise, farmers will shift to crop varieties suited to a warmer environment. If rainfall increases or decreases, they will adjust crop varieties, irrigation, other details accordingly. What would happen if farmers ignored environmental changes in deciding how to farm tells us very little about what will happen in the real world. Whether or not we have global warming, it is quite unlikely that, a
century from now, people will grow crops, raise animals, and catch fish
in the same ways and the same places as they do now.
The same issue is relevant to the other article. The authors estimated the effect of increased temperature on yield by looking at how yields had varied with temperature, year by year, in the past. Those estimates were used to calculate the effect of the overall increase in temperature over the period and suggest possible effects of future increases.
To see the problem with that approach, consider
a farmer at planting time. He does not know how hot the year will be, how much rainfall there
will be. Decisions such as when to plant and what varieties to plant can only be
based on the expected value of those variables.
A farmer in 2100 knows what changes in climate have occurred over the previous century so can take account of those changes in how he farms. It
follows that models based on observations of year to year variation will show a more negative effect of climate
change than can be expected from gradual change over a
long period of time. The authors of the article noted that problem along with other limitations to their analysis.
Most of the time, all I learn from arguing climate with people on FaceBook is how unreasonable most people engaged in the argument, on both sides, are. This was a pleasant change.
I will have to wait to see whether my opponent has become less confident that climate change threatens the global food supply now that he knows that the article that he thought supported that claim is, if anything, mild evidence against it.