Future Climate and the Food Supply
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 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).
"if temperature exceeds a crop's optimal level or if sufficient water and nutrients are not available, yield increases may be reduced or reversed."
"Extreme events, especially floods and droughts, can harm crops and reduce yields."
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."
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.