Extreme Heat and Corn Yields--a 2010 Update

So we finally have an update of our weather data, following from the hard work of an NCSU graduate student, Jon Eyer.

Here's a preview of what that data shows (click for a larger version):

The left panel shows degree days above 29C, measured continuously over time and space and averaged over growing areas.  The right panel shows corn yields, in bushels per acre.  The inverse relationship between extreme heat and yields is fairly clear.  The one big exception is 1993 when a flood damaged yields severely even though it wasn't very hot.

Three things to note:

First, 2010 was hot, but not nearly as hot as it has been.  Given how bad things were relative to expectations, I was expecting a much higher extreme heat measure.  Our basic regression model pretty much hit the 2010 yield on the nose, so markets (and the USDA) shouldn't have been surprised conditional on the heat.

Second, projections under most climate change scenarios are a lot worse in the coming years.  Overall, weather has been strangely good in the U.S. in recent years.  If it stays as cool as 2010, we'll be lucky.

Third, last summer I inadvertently provoked a sharp response from Ted Crosbie of Monsanto by suggesting corn tolerance to extreme heat has been declining over the last few decades.  Despite Dr. Crosbie's feelings on the issue, I think there is some tell-tale evidence of just this phenomenon in the graph.  Kansas, which is significantly hotter than the other big corn states, had yields similar to the other states 30 years ago.  But today Kansas yields there are much lower.  I think this is interesting because in earlier work we identified the phenomenon of declining heat tolerance by looking exclusively within states, not at differential trends across states. Also, this pattern doesn't require fancy non-parametric statistics--it's easy to see with the naked eye.


  1. Michael,

    I wonder if the “reverse” is also true. Did states in the northern Corn Belt, such as Minnesota and Wisconsin that maybe did not experience such extreme heat, see increases in yield?

    My own wild guess is that it is prolonged elevated temperatures during the R1 or silking stage. For example, the prolonged day and night time elevated temperatures we saw last year in the eastern Corn Belt. The largest yield reductions occur with stress at silking, especially with moisture stress. Those silks have a hard time pollinating without moisture. This may be why you are encountering some skepticism with the extreme heat only idea. I am sure there is data that indicates acceptable yields even with extreme heat. But, again I am guessing, those years had some cool periods during R1 with enough moisture to get good pollination. I would also guess that this is why seed companies are working on drought tolerance and not heat tolerance.

    Anyway…saw that you obtained tenure. Congratulations!


  2. James,

    Yes, northern, cooler areas are seeing faster yield growth.

    Also, I think extreme heat appears damaging after the silking stage. One story is that heat accelerates maturation thereby limiting the lifetime of the plant thus total exposure to radiation, thereby limiting the amount of seed formation. Cassman has written about this. Or at least that's where I learned about it.

    Whatever the channel, we see hot August temperatures negatively affecting yields a lot. That's what we saw last summer. It was relatively cool during the R1 stage but wasn't during the R2 stage, and yields still got hit.

  3. Michael

    Should have figured your thinking is a light year and time warp ahead of mine...great work. The link during R2 is interesting. And moisture stress is accounted for as well?

    Also, just FYI from my organization...the American Society of Agronomy, Crop Science Society and Soil Science Society will release a Climate Change position statement within the coming week. That's 10,000 scientists affirming the issues/challenges of climate change on ecosystem services.

    A Crop Adaptation to CC paper from the Crop Science Society should also be out soon.



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