Global corn and wheat yields have dropped over the past 30 years because of rising temperatures according to a study published May 5 in the journal Science.
Researchers David Lobell and Justin Costa-Roberts of Stanford and Wolfram Schlenker of Columbia University examined global yields of four major crops -- corn, wheat, rice, and soybeans -- for the period 1980-2008. They found that corn (maize) and wheat yields dropped 3.8% and 5.5% respectively. Rice and soy harvests were maintaining, as yield losses in some countries were offset by gains elsewhere.
This reported decline of global corn and wheat yields comes the same week the United Nations revised upwards its projections of global population. By century's end the UN says we'll likely exceed 10 billion people, which is a jump from previous projections of about nine billion at mid-century and then leveling off. (see link below)
Interestingly the crops/climate study showed that United States did not (yet) show yield losses during the time period studied, though in some countries climatic change was already offsetting yield increases that come from technology such as irrigation, improved seeds, and fertilizers. "Carbon fertilization" that results from rising levels of atmospheric carbon dioxide helped plant growth in all regions, offsetting some of the yield losses caused by rising temperatures from that same greenhouse gas.
The study has received press attention:
The Economist: Climate Change and Crops: Hindering Harvests: Changes in the climate are already having an effect on crop yields -- but not yet a very big one