Sunday, November 22, 2009

USA: Climate Change likely to severely damage U.S. crop yields

Higher temperatures caused by the changing climate are likely to have a devastating effect on crop yields. Corn, soybeans and cotton are the largest three crops by production value in the US which will be affected by extreme heat. Above a certain threhold - 29 degrees - damaging effects have been described as 'damaging large' by a report by Agricultural Economists published in August in the Proceedings of the National Academy of Sciences.

Related Blog: Greed, Green and Grains Blog


In 'Nonlinear temperature effects indicate severe damages to U.S. crop yields under climate change' authors Wolfram Schlenkera and Michael J. Roberts found that

    "yields increase with temperature up to 29° C for corn, 30° C for soybeans, and 32° C for cotton but that temperatures above these thresholds are very harmful. The slope of the decline above the optimum is significantly steeper than the incline below it. The same nonlinear and asymmetric relationship is found when we isolate either time-series or cross-sectional variations in temperatures and yields. This suggests limited historical adaptation of seed varieties or management practices to warmer temperatures because the cross-section includes farmers' adaptations to warmer climates and the time-series does not. Holding current growing regions fixed, area-weighted average yields are predicted to decrease by 30-46% before the end of the century under the slowest (B1) warming scenario and decrease by 63-82% under the most rapid warming scenario (A1FI) under the Hadley III model."


Examining the implications on his blog entry August 27, 2009 Michael Roberts says that "we can't find any evidence that farmers in the south are any better at dealing with extreme temperatures than farmers in the north, even though they experience the extremes more often. This is not to say farmers or seed companies won't discover new varieties or techniques to deal with extreme heat later this century, but it casts some doubt that such change can be easily achieved."

There may be some offset effect by CO2 fertilization, seed companies may develop more heat tolerant varieties and farmers may be able to crop shift to some degree explained Roberts. Although there are issues with some of these offsets: nutritional value may decrease (Jablonski et al. 2002), little historical evidence for crop adaptation to higher temperatures. Monsanto claims to have developed a more heat tolerant corn, but it has not been able to be independently assessed. It is possible that heat tolerance may tradeoff against yield potential in both very hot and more temperate years claimed Roberts.

Roberts wrote: "To my mind, what this study makes very clear is that the worldwide face of agriculture is going to change dramatically. Even in the best-case scenario, in which losses in areas like the US are made up with gains elsewhere, we will see different crops cultivated all around the world."

A September 2009 report from the International Food Policy Research Institute (IFPRI) - Climate Change: Impact on Agriculture and Costs of Adaptation projects that in 2050, there will be 25 million additional malnourished children, a 90% increase in wheat prices, and a 15% decline in irrigated rice yields in developing countries, all compared to a scenario without climate change.

Corn and soybeans, along with wheat and rice, are important highly nutritional staple global food crops. The United States produces 41% of the world's corn and 38% of the world's soybeans. A significant reduction in US production of these crops will drive up global prices for these crops which will impact developing countries that import this food. These crops grown in developing countries for domestic consumption will change to being an export crop to the first world driving famine and further entrenching poverty.

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