Federal legislation  promoting wood-burning power plants as a “carbon neutral” way to make energy is drawing criticism from some scientists. The provision is part of a much larger bipartisan energy bill now pending in the U.S. Senate, which could approve the bill as early as next week.

In a letter sent to Senate leaders on 24 February and released earlier this week by the Massachusetts-based Woods Hole Research Center, 65 scientists warned that “this well-intentioned legislation, which claims to address climate change, would in fact promote deforestation in the U.S. and elsewhere and make climate change much worse.”

The dispute is the latest skirmish in a fight over whether power plants fueled by wood should be promoted as climate-friendly, or discouraged for putting more carbon into the atmosphere and imperiling forests. The debate has pitted scientists against each other, and some environmental groups against the timber industry.

The latest battle comes as power producers in some European countries are embracing wood as a source of renewable energy that would also help them meet targets to reduce greenhouse gas emissions. The U.S. Environmental Protection Agency (EPA), meanwhile, isconsidering how to treat wood-burning power plants under the Clean Power Plan, the Obama Administration’s initiative to cut greenhouse gases coming from the electricity sector.

The forestry and biomass energy industries, and a number of political leaders from heavily-forested states, argue that burning wood to drive turbines can be a low-carbon way to make electricity. The reasoning is relatively straightforward: Carbon released into the atmosphere when trees are burned should eventually be reabsorbed as new trees grow in their place.

“Forests in the United States are robust and sustainably managed, and climate science has consistently and clearly documented the carbon benefits of utilizing forest biomass for energy production,” Senator Susan Collins (R–ME), said on the Senate floor on 3 February, a day after lawmakers approved her amendment adding the wood-burning provision to the energy bill.

The amendment directs the secretaries of both the Energy and Agriculture departments and the head of the EPA to create policies for forest biomass energy, and to recognize its “carbon neutrality” so long as the forests involved continue to be managed to grow trees. It was co-sponsored by a bipartisan group of seven senators from states with active forest industries —Maine, New Hampshire, Minnesota, Montana, and Idaho.

The goal, Collins said in a press release, is to “help ensure that federal policies for the use of renewable biomass are clear, simple, and reflect the importance of biomass for our energy future.”

But critics charge the carbon math for these power plants is far more complicated, and that the amendment seeks to replace scientific judgment with a political declaration. “The Senate is on the verge of mandating that it’s carbon neutral, and it isn’t,” says Philip Duffy, president and executive director of the Woods Hole center, a nonprofit climate research group in Falmouth, Massachusetts. He helped write the letter and recruited scientists with backgrounds in climate, energy, and forest ecology to sign it. “It should be the scientific community and the engineering community, and not the Senate or another legislative community, saying what’s carbon neutral.”

The letter notes that burning wood releases more carbon dioxide than coal for each unit of power produced. And there’s no assurance all that carbon will wind up back in trees and the ground, the scientists caution. Logged land could be put to other uses, new forests might be managed differently than the ones they replaced, and insect infestations or droughts could make it hard to reestablish trees.

A scientific panel advising the EPA on how to account for carbon released from biomass power plants has cautioned against treating all projects equally. “Carbon neutrality cannot be assumed for all biomass energy a priori,” the panel wrote in a 2012 report.

Today a similar panel is reviewing proposed EPA guidelines for how states or businesses can calculate the carbon footprint of power plants using biomass such as wood. That work started following the Obama Administration’s decision to regulate greenhouse gas emissions from new power plants. It could also have implications under the Clean Power Plan, which the U.S. Supreme Court has put on hold during a legal challenge.

While some scientists express reservations about a rush to biomass energy, others say it could be a valuable, low-carbon power source. That depends partly on where the fuel comes from, says Tim Volk, a forestry scientist at the State University of New York in Syracuse. He’s experimenting with fast-growing shrub willows that can be harvested on cycles as short as 3 years.

Volk says states and the biomass power industry are hindered by a lack of clear, uniform federal guidelines about which power plants will pass muster and which won’t. He was one of more than 100 scientists that signed a 2014 letter to EPA Administrator Gina McCarthy urging the agency to consider the carbon benefits of using wood for energy.

“I think that would be a mistake to declare all forest biomass as carbon neutral,” Volk says. “But let’s set up an assessment system where we can identify those that will have a low-carbon footprint and those that will have a high-carbon footprint.”

The fate of Collins’ amendment remains uncertain. Although Senate leaders this week said they would like to finish work on the bill soon, perhaps as early as next week, action has stalled amid disagreements over a number of issues, including proposed funding to address drinking water problems in Flint, Michigan. Any bill emerging from the Senate would also need to be reconciled with a different version already passed by the House, and then signed by the president.