Des Moines Register: Farmers are being warned they could pay a stiff price for their contributions to global warming. That could happen if the Environmental Protection Agency goes forward with regulating greenhouse gas emissions under the federal Clean Air Act, according to the U.S. Agriculture Department. Under the law, livestock operations of all sizes and farms with as few as 500 acres of corn could exceed emissions thresholds and would be required to pay for permits, USDA says. These …