Heat and ag yield, cont.

Michael Roberts' great paper with Wolfram Schlenker on the effect of detrimental increased heat on crop yields (which I first excitedly blogged here) is getting a lot of (well-deserved) attention.

First, Ezra Klein cites the work in the Washington Post, and, despite a few minor inaccuracies by Klein, Michael is deservedly proud of having cracked into the mainstream media.

Second, the paper was noticed as far away as Brazil, whence came a letter to the editor challenging that Brazil (being warmer and high-yielding) refutes the Schlenker/Roberts conclusion, or at least casts into doubt its generalizability. But their quick-and-dirty analysis seems to corroborate their original findings:
First, they cherry picked the state and the year from Brazil. Mato Grasso is the highest yielding state in Brazil and 2008 was a remarkably good year for them, due to unusually good weather. Other states in Brazil have average yields that are about half those of Mato Grasso.

Second, Mato Grasso yields were higher than average yields in the U.S. as a whole, but not higher than the best yielding states in the U.S.

Third, if we narrow our comparison by looking a particular state—Illinois, the number two yielding soybean state in the U.S.—and also look more closely at the data, Mato Grasso doesn’t look much warmer. In fact, the southern half of Illinois, which has average yields comparable to those in Mato Grasso, also has comparable exposure to extreme heat. (Northern Illinois is cooler and higher yielding than both Mato Grasso and Illinois). This can be seen from careful inspection of the maps below (click the figure for more detail). It turns out that there aren’t any soybeans grown in the hottest part of Mato Grasso. It’s not clear whether the commenters took into account the locations in Mato Grasso where soybeans are actually grown.
Look for more back-and-forth discussion on this interesting area, and the Schlenker/Roberts contribution in particular.

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