FILE - In this Sept. 19, 2012 file photo, corn plants weakened 

FILE - In this Sept. 19, 2012 file photo, corn plants weakened by the drought lie on the ground after being knocked over by rain in Bennington, Neb. Slowing the buildup of greenhouse gases responsible for warming the planet is one of the biggest challenges the U.S. _ and President Barack Obama _ faces. The impacts of rising global temperatures are widespread and costly: more severe storms, rising seas, species extinctions, and changes in weather patterns that will alter food production and the spread of disease. (AP Photo/Nati Harnik, File)
Associated Press
FILE - In this Sept. 19, 2012 file photo, corn plants weakened by the drought lie on the ground after being knocked over by rain in Bennington, Neb. Slowing the buildup of greenhouse gases responsible for warming the planet is one of the biggest challenges the U.S. _ and President Barack Obama _ faces. The impacts of rising global temperatures are widespread and costly: more severe storms, rising seas, species extinctions, and changes in weather patterns that will alter food production and the spread of disease. (AP Photo/Nati Harnik, File)
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