For a long time, conventional wisdom had it that increasing temperatures in the U.S. might lead to increased agricultural productivity.
Longer summers in Kansas and shorter and milder winters meant longer growing seasons. There's a reason it's hard to grow corn in Siberia: It's very cold. But there's also a reason it's hard to grow wheat in equatorial Brazil. It's very, very hot.
It's about to get hotter in the U.S., too, and the effects on American crop production could be disastrous, according to a study just published in the Proceedings of the National Academy of Sciences by agricultural and resource economist Michael Roberts of North Carolina State University and Wolfram Schlenker, an economist at Columbia University.
According to their calculations, corn, cotton and soybean yields—assuming that the growing area remains the same—will decrease between 30 and 46 percent by the end of the century under the slowest warming scenario that they model, between 63 and 82 percent under the most rapid warming scenario, using the Hadley III model.
The numbers are staggering enough. But then factor in what percentage of total global production the U.S. is responsible for: in the case of corn, 41 percent, and soybeans, 38 percent.
Keep in mind that by the end of the century total global population is expected to increase by close to 50 percent, and the word that should come to mind is "famine".
"These crops comprise two of the four largest sources of caloric energy produced and are thus critical for world food supply," the authors write.
The changes are far greater than those that economist William Cline reported in his overview of climate change's effects on global agriculture—including the carbon fertilization effects, he estimated U.S. gross agricultural crop yields to increase between 5 and 20 percent. When the effect is excluded, estimates on the change in yield range from -15 percent to 5 percent.
The researchers emphasize that they did not account for the carbon fertilization effect in their study. As they admit, "higher CO2 concentrations may boost yields," although they cite research suggesting that estimates of the CO2 fertilization effect are often overly high.
The world's population is now overtly dependent on American agricultural commodities, with Brazil not far behind. Along with the U.S., Argentina and Brazil are the world's major corn exporters and the world's major soy and soy-derivative exporters. In 2005, Brazil produced 57 million tons of soy, to Argentina's 41 million tons and the U.S.'s 84 million tons.
The Roberts-Schlenker study doesn't deal with the effects of global warming on Brazilian and Argentinean agriculture, but other studies have. One, carried out by Brazilian researchers, found potential reductions of 50 percent or more in the country's major grain crops under a worst-case scenario. The area that will be able to be planted with soy will decrease by 20 percent by 2020, 40 percent by 2070, even under the most optimistic model.
Such massive changes are forecast to take place because plants' reaction to climate change is non-linear—different crops exhibit modest increases to a certain point, then drop sharply. In the case of corn, that inflection point is at 29° C; for soybeans it is 30° C; and for cotton it is 32° C.
In the southern and central U.S., summer growing temperatures already are close to that band. In countries closer to the equator, in the northern or southern hemispheres, temperatures exceed those thresholds regularly.
It shouldn't shock then that if methods used by the American researchers were used to estimate crop losses in other regions of the world, the findings would be catastrophic. Indeed, many already think that African agriculture will totally breakdown if climate change proceeds unfettered.
There are a few positive things to keep in mind: Grains are partially substitutable, even though over 50 percent of global calories are consumed in the form of cereals; farmers may choose to grow more fruits and vegetables in lieu of cereal crops, as they are less vulnerable to extreme temperatures; there is the possibility of locating crop varietals that are resistant to extreme temperatures, an initiative that is rapidly gaining speed; and, finally, around 30 percent of American corn production is diverted to ethanol. There's some room for maneuver.
Schlenker and Roberts also note:
"If climate change were anticipated to induce severe yield impacts on a global scale, then anticipated increases in commodity prices would likely encourage greater investments in new seed varieties, irrigation systems and other technological changes."
But the trouble is that the very processes of crop destruction that will increase the pressure to find replacement species for currently widely-planted will destroy the native land-races that are likely to be the source of heat-resistant varieties, since they're located in yet hotter climates. Mexico, for example, is the source of corn, and climate change will devastate its agricultural systems—a Catch-22 of the climate-change conundrum.
Meanwhile, those very land-races that are resistant to extreme heat are ones that yield considerably less than the monocultures planted across the United States—another Catch-22.
Looking for solutions to the problems presented in such studies is a good idea, but a better idea is to head problems off at the source.