Six millions tons of wood pellets manufactured from trees harvested in the U.S. were shipped to Europe last year to help utilities there meet renewable energy goals, but a growing number of environmentalists say that the program does more harm than good.

In an article posted at Science 2.0, David Styles, a lecturer at Bangor University in Wales, said that shipments in 2015 were nearly double the amount of pellets shipped in 2013. Half of them went to a single plant: Britain’s Drax power station, which is switching from coal to biomass in order to reduce carbon emissions and grab what are called “renewable obligation certificates.”

The pellets were manufactured from wood harvested in five U.S. states: Louisiana, Georgia, Florida, Alabama, and Virginia.

Converting power plants from coal to wood may look good on paper, but a number of environmental groups are arguing just the opposite. In a paper published last fall, the groups argued that burning pellets to produce “bioenergy” has a number of ugly side effects, including soil and water pollution, the growth of industrial scale logging, and a loss of biodiversity. The groups argue that biomass should be excluded from the European Union’s next Renewable Energy Directive.

“A U.K. government study found that electricity generated from regenerated forests could have a carbon intensity five times higher than coal,” Styles says. “Burning wood also releases nitrogen oxides and carcinogenic compounds.”

Adding up the carbon impact

Converting a plant from coal to wood is fairly simple — “a quick and comparatively cheap way to shift towards renewables,” Styles writes — but the equation gets a lot more complicated when details of “carbon accounting” are considered.

Ironically, trucking wood pellets to a port and then shipping them across the Atlantic isn’t what tips the scales against wood, in part because ships can carry huge amounts of cargo efficiently. Even the processing of pellets, including grinding and drying the fiber, isn’t a fatal flaw.

Instead, it’s the source of the wood that’s turned into pellets that counts.

Coal releases carbon that has been stored for millions of years, while wood releases carbon that has only recently been captured through photosynthesis, and newly planted trees begin capturing carbon again in a relatively short amount of time. This balance of storing and releasing carbon, however, isn’t uniform.

Styles writes, “The government study notes that wood from intensively managed plantations could mean more carbon taken up by growing trees than emitted by the transport and processing of the pellets, leading to a net reduction in emissions even before avoided coal emissions are accounted for. Conversely … the study found that if wood pellets are sourced from regenerated natural forests, carbon emissions could be five times higher than from burning coal.”

Drax doesn’t agree. “Provided the rate at which carbon is absorbed by the forest as a whole equals or exceeds the rate at which it is being removed there is no net release of carbon,” the utility says. “Healthy demand for wood stimulates supply and ensures forests remain as forests. That is why forest cover in the U.S. is growing year on year and has been growing for each of the last 50 years.”

Drax insists that its sustainability policy requires that the sourcing of pellets not lead to a net increase in carbon levels, which is the case “at a landscape level, at state level, across the southeast U.S. and across the U.S. as a whole.”

Environmental concerns are not new

A wide scale use of wood pellets to generate electricity also has been criticized by the Natural Resources Defense Council, which said three years ago that the practice could double logging rates and significantly increase carbon emissions.

The environmental impact of the U.S. wood pellet trade may be disputed, but there’s not much doubt that it’s been a boon for the forest industry in some states. Forbes reported last year that the number of wood pellet plants has jumped significantly — two new mills planned In North Carolina alone will mean investments of $214 million and 160 permanent jobs by next year.