In this Aug. 2012 file photo, Jerry Johnson of Ashland uses his antique 57 Ford tractor to mow vegetation around his drying pond in Ashland, Neb. Delegates from nearly 200 countries are meeting in the Qatari capital of Doha to discuss ways slowing climate change, including by cutting emissions of greenhouse gases that scientists say are warming the planet, melting ice caps, raising sea levels, and changing rainfall patterns with impacts on floods and droughts. (AP Photo/Nati Harnik, File)

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In this July 2012 file photo, dead fish float in a drying pond near Rock Port, Mo. Delegates from nearly 200 countries are meeting in the Qatari capital of Doha to discuss ways slowing climate change, including by cutting emissions of greenhouse gases that scientists say are warming the planet, melting ice caps, raising sea levels, and changing rainfall patterns with impacts on floods and droughts.(AP Photo/Nati Harnik, File)

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In this Sept. 2012 file photo, corn plants weakened by the drought lie on the ground after being knocked over by rain in Bennington, Neb. Delegates from nearly 200 countries are meeting in the Qatari capital of Doha to discuss ways slowing climate change, including by cutting emissions of greenhouse gases that scientists say are warming the planet, melting ice caps, raising sea levels, and changing rainfall patterns with impacts on floods and droughts. (AP Photo/Nati Harnik, File)

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In this Sept. 2012 file photo, a combine harvests corn near Bennington, Neb. Delegates from nearly 200 countries are meeting in the Qatari capital of Doha to discuss ways slowing climate change, including by cutting emissions of greenhouse gases that scientists say are warming the planet, melting ice caps, raising sea levels, and changing rainfall patterns with impacts on floods and droughts. (AP Photo/Nati Harnik, File)

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