The research was conducted by two economists, Dr Michael Roberts of North Carolina State University and Dr Wolfram Schlenker at Columbia University, who used a well-known climate change prediction model to assess how much yields could decline by the end of the century. They found that corn, soy and cotton yields could fall by 30 to 46 percent in this time under the slowest warming scenarios – if greenhouse gas emissions are cut to 50 percent of 1991 levels by 2050 – and by 63 to 82 percent if emissions continue at current levels.
They wrote that although yields increase with temperature up to 29C for corn and 30C for soybeans, there is a sharp decline in yield above these thresholds.
Roberts said: "While crop yields depend on a variety of factors, extreme heat is the best predictor of yields. There hasn't been much research on what happens to crop yields over certain temperature thresholds, but this study shows that temperature extremes are not good."
Soaring food prices
And decreasing yields mean that prices of staple grains are likely to skyrocket, bringing serious consequences for consumers as well as food manufacturers, many of which are still reeling from last year’s record-high input prices.
The study comes on the heels of a report released last week by the Economic Research Service of the US Department of Agriculture, warning Americans that food prices are once more on the rise. Overall it said that grocery prices are likely to rise by two to three percent this year, although that compares to a huge 6.4 percent jump in 2008, the largest increase for nearly two decades.
This latest study adds to a bleak picture, considering that experts estimate that global food supplies need to increase by at least 70 percent by 2050 in order to feed the world’s booming – and increasingly urbanized – population.
On his blog, 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.”
He sees three ‘caveats’ which could ease the problem: The so-called ‘greening theory’ in which more CO2 could offset some negative effects, which he mentions is still a topic of intense debate; the development of heat tolerant crops by companies like Monsanto, although he says that there is “little evidence of adaptation in the past”, and by farmers shifting where they grow different kinds of crops.
“But with projected damages this large for the world's biggest bread basket, no clear evidence of adaptation to warmer temperatures in the historical data, and with projections already rather dismal for much of tropics and subtropics, at present I don't know why we should be particularly optimistic,” he wrote.
Although the study focused on US crop yields alone, the authors said that their findings have global implications due to the quantity of grains exported from the US, which produces 41 percent of the world’s corn and 38 percent of its soybeans.
Roberts said: "Effects of climate change on US crop production will surely be felt around the globe, especially in developing countries.”
Source: Proceedings of the National Academy of Sciences
Published online before print August 28, 2009, doi: 10.1073/pnas.0906865106
“Nonlinear temperature effects indicate severe damages to U.S. crop yields under climate change”
Authors: M. Roberts and W. Schlenker