Warming Holes

My colleague, Michael Roberts of the University of Hawaii, recently started a new Substack blog called “Grains to Gigawatts.” His first post is an important one for grain markets and I highly recommend it to anyone with a stake in these markets (which is just about everyone). The post, “Revisiting Crop Sensitivity to Extreme Heat,” discusses one of the most under-reported puzzles in the climate change and food literature. Specifically, while most of the rest of the globe has been warming, the U.S. Corn Belt has been humming along with unusually moderate summer weather for some time. Of course, this global “warming hole” has a disproportionate impact on global food supplies. Mike presents this map by Dressler and Hausfather to illustrate the point quite dramatically.

I do wonder what the figure would look like if one picked the 1940s or 1950s as the base for the map. In other words, is the “warming hole” more a function of super-hot temps in the Corn Belt during the Dust Bowl of the 1930s than relative cooling after the 1930s? Mike presents some interesting evidence in his post that there has indeed been a cooling trend in the Corn Belt in the post-WWII period. I also took a look at this question in a farmdoc daily article last May. Here is my chart for the trend in monthly average July temperatures for the Corn Belt over 1895-2023.

I find there is definitely a downtrend in Corn Belt temperatures during July, which is the most critical month of the growing season. I also looked at the shorter 1980-2023 period in order to exclude the very hot 1930s, and still found evidence of a small downtrend. The article also contains trend results for all months during the Corn Belt growing season.

If one accepts the “warming hole” as reality, the obvious question is what explains it. Mike discusses several hypotheses in his post. One is that corn plants are responsible for the cooling summer temps through evapotranspiration. Corn sweat? Maybe. I get the argument at some level (e.g., sweating green plants are cooler than bare soil), but I am skeptical that this can explain a cooling trend for the entire atmosphere over the Corn Belt. Mike explores some other hypotheses as well. One he does not explore is that global climate change is very complicated and may impact small geographic areas quite differently. As I noted in my farmdoc daily article on temperature trends, the U.S. Corn Belt is a tiny sliver of a sliver of global surface area—it represents only 0.3 percent of the total surface area of the world. 

Mike is worried about this happy state of affairs for the Corn Belt ending. While I share his worry, I also have more faith in the ability of farmers to adapt to such a negative change in climate. To me, the key is not temperature but precipitation. If there is a five-month period where precipitation averages 4 inches per month and temperatures are moderate, Corn Belt farmers will grow crops in that period. For example, if July and August temps get too hot, just move the growing season back two months and plant in February and March. We already have evidence that farmers in the U.S. can do this if properly incentivized. There has been a quiet revolution in soybean production in the southern U.S. in the last 50 years as producers have planted soybeans earlier and earlier to avoid hot late summer temps. This is part of what is known as the “Early Soybean Production System (ESPS).” This is just one example of the ingenuity of farmers in adapting management practices to optimize production. Personally, I would not bet against them in the future.

 

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