A pump handle for E85 fuel is shown at a gas station in Albany, N.Y. The Obama administration will decide soon how to measure biofuels’ impact on climate.

Washington ? President Barack Obama’s commitment to take on climate change and put science over politics is about to be tested as his administration faces a politically sensitive question about the widespread use of ethanol: Does it help or hurt the fight against global warming?

The Environmental Protection Agency is close to proposing ethanol standards. But two years ago, when Congress ordered a huge increase in ethanol use, lawmakers also told the agency to show that ethanol would produce less pollution linked to global warming than would gasoline.

So how will the EPA define greenhouse gas emissions from ethanol production and use? Given the political clout of farm interests, will the science conflict with the politics?

Environmentalists, citing various studies and scientific papers, say the agency must factor in more than just the direct, heat-trapping pollution from ethanol and its production. They also point to “indirect” impacts on global warming from worldwide changes in land use, including climate-threatening deforestation, as land is cleared to plant corn or other ethanol crops.

Ethanol, which in the future may come from cellulosic sources such as switchgrass and wood chips, is promoted by its advocates as a “green” substitute for gasoline that will help the U.S. reduce its reliance on fossil fuels, especially foreign oil. That transition is a priority of the Obama White House.

In 2007, Congress ordered huge increases in ethanol use, requiring refiners to blend 20 billion gallons with gasoline by 2015 and a further expansion to 36 billion gallons a year by 2022.

Congress said any fuel produced in plants built after 2007 must emit 20 percent less in greenhouse gases than gasoline if it comes from corn, and 60 percent less if from cellulosic crops.

Meeting the direct emissions would not be a problem. But if indirect emissions from expected land use changes are included, ethanol probably would fail the test.

Twelve farm-state senators, both Democrats and Republicans, wrote EPA Administrator Lisa Jackson in March, urging the agency to stick to assessing only the direct emissions.