Washington, D.C. - Farmers are being warned that they could pay stiff prices 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 little as 500 acres of corn could exceed emissions thresholds and would be required to pay for permits, the Agriculture Department said.

These fees could amount to as much as $20 for every hog and $175 per dairy cow, said Rick Krause, who follows climate policy for the American Farm Bureau Federation.

Ron Sparks, a Democrat who is Alabama's agriculture commissioner and president of the National Association of State Departments of Agriculture, said the fees would drive livestock farmers out of business.

"At a time when we are becoming more and more reliant on other countries for our food, we should be looking for ways to help farmers, not punish them for producing the food we put on the table for our families," he said.

To farmers, it may sound outlandish that they would have to get permits for greenhouse gas emissions, or even that they are contributing to global warming, but here is the logic:

The Supreme Court ordered the Bush administration to declare whether greenhouse gas emissions harm the public, and if so to regulate them as a dangerous pollutant. The Bush administration hasn't taken action, but President-elect Barack Obama's advisers have indicated the environmental agency will move forward.

Advocates of agency regulation want to target the largest sources of emissions, such as automobiles and coal-fired power plants. But the Agriculture Department said that using the Clean Air Act to control greenhouse gases would trigger regulations on farms under rules that require businesses to obtain permits to emit more than 100 tons of a pollutant in a year. Two heat-trapping gases that contribute to global warming are methane, which comes from livestock, and nitrous oxide, which is emitted after nitrogen fertilizer is applied to corn fields.

Farmers with as few as 200 hogs or 50 beef cattle or 25 dairy cattle would hit the 100-ton threshold, according to a five-page Agriculture Department document assessing the impact using the Clean Air Act to regulate emissions.

"It is neither efficient nor practical to require permitting and reporting of (greenhouse gas) emissions from farms of this size," the Agriculture Department said.

Nor does that seem likely to happen, according to Vicki Arroyo, vice president for policy analysis and general counsel at the Pew Center on Global Climate Change.

The environmental agency has discretion under the Clean Air Act to focus on the largest sources of emissions, she said.

"They wouldn't necessarily use this to go after smaller, more diffuse sources or farms," she said.

Furthermore, it will take the agency months or years to write regulations. In the meantime, Congress will consider enacting emission limits through some kind of cap-and-trade system, which would be a more cost-effective way of controlling emissions than using the Clean Air Act, she said. Under a cap-and-trade system, businesses with excessive emissions of greenhouse gases must reduce them or buy emission credits.

Agricultural groups want Congress to exempt farmers from emissions limits set by a cap-and-trade bill. If they're not exempted, some farmers would have to reduce their emissions or else buy emission credits.

As it happens, one of the reasons for pushing the agency to regulate greenhouse gases is to prod lawmakers into acting on climate change. The point is to get the attention of Congress.

It's certainly gotten the attention of some in agriculture.