Britain’s Drax power plant is one of Europe’s biggest coal plants, and it is in the process of converting its boilers to become one of the world’s biggest sources of biomass power.

These are facts. But the company also makes rather large claims about the carbon savings it is achieving from converting to biomass from coal, which in fact depend on very particular assumptions. A new report by the U.S. NGO, the Natural Resources Defense Council (NRDC), lays those assumptions bare.

Burning wood and other organic matter, called biomass, certainly produces carbon emissions, just as burning any carbon, whether coal, gas or newspapers. The question is whether these emissions are subsequently offset by planting new, replacement trees, which absorb carbon dioxide from the atmosphere as they grow.

The new report from the NRDC shows that trees will eventually offset the burning of biomass, but this will only happen over the course of many decades. That is because trees absorb most CO2 later in their life cycle, as they near maturity.

Why does this matter? First, it matters for the climate, which is warming in response to carbon emissions now, regardless of whether these emissions will be offset towards the end of the century. Second, it matters for the economics of biomass-fired power plants, which depend on supportive climate policies, which until now have assumed that biomass is zero carbon.

The NRDC report published detailed modelling of the carbon emissions from burning biomass. It found that initially, biomass may be even more carbon-emitting than coal, because of its lower energy density. Exactly how carbon-emitting is biomass depends on how wood pellets are manufactured, and in particular whether these are produced from whole trees, or waste trimmings and sawdust.

Comparing the NRDC modelling with published biomass sourcing data from Drax makes for some interesting conclusions.

Drax shows that about a quarter of its biomass, at 977,873 tonnes, in 2014 was sourced from “thinnings”, meaning whole trees (“roundwood”). The NRDC report analysed a similar situation, where a fifth of biomass (less than Drax) was sourced from whole trees. Under such a scenario, biomass would be more carbon-emitting than natural gas, and a little less than coal, until around 2065. Only after 2080 would the growth of replacement trees make the biomass cumulatively burned by then carbon neutral (see NRDC Figure 3 below).

By contrast, Drax reports that its biomass carbon emissions (34 grams per megajoule) are a fraction of those both from coal (280 grams) and gas (193 grams). This calculation simply reflects the fact that Drax assumes that biomass is entirely carbon neutral, an idea which the NRDC report debunks. Drax is only including the carbon emissions from making and transporting the wood pellets, and ignoring the far greater emissions from actually burning the wood.

Drax describes itself as “the biggest carbon reduction project in Europe”, and says it “delivers low carbon energy at scale”. The company should add the caveat – “These claims only become true after 2080”.

The NRDC findings put regulatory support for biomass at risk. There is an obvious comparison with conventional liquid biofuels, as used in motor transport. Evidence that these may compete with food crops, and that some biofuels are more carbon-emitting than others, has increased investment risk in the biofuel sector. Investments become stranded when assets are abandoned, whether because of deteriorating economics, or policy change.

In the European Union, biomass power gets support both under the EU emissions trading scheme (ETS), and under support for renewable energy. Emissions trading law assumes that biomass power is zero carbon. As a result biomass power plants do not have to buy emissions permits. That gives them a competitive advantage compared with coal and gas. In addition, in Britain, the government supports power plants which burn biomass, under renewable energy support which also assumes that biomass is environmentally friendly.

The NRDC report focused on the U.S. wood pellet supplier, Enviva, which NRDC described as the primary biomass supplier to Drax. The report presented evidence that Enviva was producing pellets from whole trees, but noted that the company did not publicly disclose the make-up of its fuel.

Drax shows that U.S. sourcing of its biomass is growing, to 58 percent of all its biomass in 2014, from 39 percent in 2013.

The NRDC report shows that the notion that burning biomass is zero carbon is a myth, and that it is time for the industry, and British and European policymakers, to take a more robust approach to carbon accounting.