A recent Chatham House report claimed that burning wood for electricity is worse for the climate than using coal.
It sparked a backlash from a group of 125 academics in the field who said the research was deeply flawed.
Now supporters of the original study have hit back, saying that to avoid dangerous warming the world needs to plant more trees, not burn them.
Producing electricity from burning biomass such as trees has boomed in recent years, with the amount of energy generated doubling between 2005 and 2015.
Cash for ash?
Many energy firms have seen it as a more reliable green power source than wind or solar. The EU is the world’s biggest consumer of biomass, with some imported as wood pellets from southern US states. Bio-energy is expected to contribute more than half of the EU’s renewable energy by 2020.
It’s a big money spinner, with subsidies worth £800m paid by the UK government for biomass electricity in 2015.
But the Chatham House study said that the financial support for this type of power generation was based on some flawed assumptions.
The first was that the emissions of CO2 from the burned wood are balanced by the planting of new trees.
The author argued that there is no accounting for the soil carbon lost during the harvesting of these trees, and that older trees used for burning can sequester far more carbon that younger ones planted in their stead.
The study also said that the amount of time it would take to re-absorb the carbon produced from the burning of pellets was critically important. With global scientific concern that the world has only a few decades left to make significant cuts in CO2, opting for a policy that would also take many decades to achieve carbon payback was dubious.
“Overall, while some instances of biomass energy use may result in lower life-cycle emissions than fossil fuels, in most circumstances, comparing technologies of similar ages, the use of woody biomass for energy will release higher levels of emissions than coal and considerably higher levels than gas,” the report said.
The idea that burning wood released more CO2 than coal has enraged those who work in the biomass industry and many who carry out research in the field.
A group of 125 academics, from Europe and the US, have attacked the Chatham House study saying it “gives an inaccurate interpretation of the impact of harvesting on forest carbon stock”.
It presumes that forests would continue to grow if no biomass was used for bio-energy which is “unrealistic”.
The letter also criticises the study for assuming that roundwood is the type of material used for burning when the “on the ground reality is that in the EU, by-products and residues from silviculture [the growing and culture of trees] are the most common types of feedstock”.
“The author failed to appreciate that it is the net cumulative emissions of carbon that are principally responsible for long-term temperature change, so high emissions one year can legitimately be countered by negative emissions (or forest growth) in subsequent years,” said Prof Piers Forster, director of the Priestly International Centre for Climate at the University of Leeds, who signed the letter.
“This is not a simple accounting trick by the bio-energy industry – this is science. In fact nearly all emission pathways that keep the world below a 2 degrees C temperature rise rely on such negative emissions in the future, typically from bio-energy combined with carbon capture.”
But now another group of more than 50 scientists involved in this field have written in support of the original study.
Prof William Moomaw was a lead author for the Intergovernmental Panel on Climate Change (IPCC) report on renewable energy in 2010.
He says he was solicited by opponents of the Chatham House report to sign their critical letter but refused to do so. He says the bio-energy industry is pushing “alternative facts”.
“Until about six years ago I bought into the notion that because another tree grows it’s carbon neutral, it’s so comforting!” he told BBC News.
“But it’s like saying I had £100,000 in my bank account, and I spent it all on a Ferrari – but it’ll be ok because in my lifetime I’ll have £100,000 in my bank sometime in the future. This accounting is troubling.
“We are telling women in Africa that cutting down a tree to cook dinner is deforestation and we have policies to stop you from doing that, however if the UK government gives £500m to burn biofuels that are cut from trees in North America and shipped across the ocean, that’s zero carbon – It doesn’t compute!” he said.