Extreme Weather And Crop Yields

Michael Roberts reports on his research into heat, humidity and crop yields:

There are two interesting things about the VPD (ed. note: vapor pressure deficit) measure we construct. First, average VPD for July and August is closely associated with our best-fitting extreme heat measure, at least in Illinois.  Second, adding VPD for the season and VPD for July and August to our standard regression greatly improves prediction.  Using just five variables, these two plus growing degree days (degree days between 10C and 29C), extreme heat degree days (degree days above 29C) and precipitation, we can explain over 70 percent of the variance of Illinois yields, excluding the upward trend.  That's better than USDA's August and September forecasts, which are based on field-level samples and farmer interviews.  The model can explain almost half the difference between the August forecast and the final yield for Illinois.
When we simulate these variables for future climates the outlook isn't much different from our earlier predictions: really bad.  But there is somewhat greater uncertainty around projected impacts, drawing mainly from interaction effects with precipitation after we account for VPD.  And since VPD, like extreme heat, is sensitive to the distribution of temperatures, things may not turn out as bad if warming occurs mostly in cooler months, or if lows increase more than maximums do.  Of course, if maximums increase more than minimums do, things could be a lot worse.   My sense from the climate scientists is that there is a lot more uncertainty about these more subtle features of climate change.
Also, these statistical models cannot realistically account CO2 effects, because CO2 has increased slowly over time and so cannot be separated from technological change.  At this point, CO2 effects for corn (a so-called C4 crop) are expected to be modest, but may aid heat tolerance.

I think that's a fancy way of saying that he's not sure what global warming will bring specifically, but if things are hotter and more humid in the corn belt, it will seriously impact yields.  It's only been the last two years that I've noticed it, but it sure seems like it hasn't cooled down at night like it used to.  I don't think that's good for the corn.