Deep in fashion’s supply chain sits the dirtiest part of the business: the energy-intensive process of dyeing and treating fabrics.

More than half of the industry’s planet-warming emissions are linked to this step in the production process, largely because of the coal-fired boilers commonly used to generate the high temperatures needed to colour and finish textiles.

Indeed, it’s the cheap and abundant supply of coal, the world’s most polluting fossil fuel, that powers the fashion industry’s supply chain and its ability to churn out millions of low-cost garments per year.

To meet the industry’s environmental commitments that needs to change. Many of the world’s biggest brands have pledged to weed out coal from their supply chains under flagship industry climate initiative the UN Fashion Charter for Climate Action.

In their scramble to cut out coal, companies like H&M Group, Zara-owner Inditex, Gap and Nike are turning to biomass. It’s an alternative widely embraced as a lower-carbon transition fuel, but one some environmental organisations say isn’t any better.

“It’s a false solution,” said Xixi Zhang, a research specialist at environmental advocacy group Stand.earth. “We don’t think there is responsible biomass.”

What Is Biomass?

Biomass is a broad category covering renewable organic material that comes from plants and animals. That includes things like wood, agricultural waste and even dung.

It’s an appealing option for companies looking to score a quick win on climate action because it’s a relatively simple swap for coal that doesn’t require much investment to adapt existing infrastructure.

Though burning biomass still emits carbon like fossil fuels, it is often referred to as a renewable or “carbon neutral” energy source because replanting trees or growing new crops can offset the emissions released during combustion.

Several governments, from the UK to Indonesia, have embraced biomass as a climate solution, offering subsidies to help grow the sector. Exactly how much it is used in the fashion industry is difficult to pin down because it’s not something that is consistently disclosed, but in an analysis of large companies’ climate reports and manufacturing base published Monday, Stand.earth concluded that the use of biomass boilers in the sector is significant and expanding.

Why Is Biomass Controversial?

Environmental groups say the notion that biomass is always less polluting than coal is wrong.

Scientists have raised doubts about the potential for biomass to cut emissions; wood is a less efficient fuel source than coal or gas, which means it can emit more carbon per unit of energy produced. Where trees are cut down to generate energy they can take years to grow back enough to offset the carbon released.

Biomass plants have been found to release nasty air pollution and there are concerns growing demand could fuel deforestation, threaten indigenous communities and compete with food crops for land, water and other resources.

Murky supply chains are already a problem in the fashion industry; a 2021 Royal Holloway University study on Cambodia concluded that hundreds of tonnes of illegally harvested forest wood are likely burned daily by factories making clothes for international fashion brands in the country. Over the course of a year, Cambodia’s garment factories are estimated to consume enough wood to clear up to 1,400 hectares of forest, the study found.

Finally, climate advocates worry that a shift to biomass boilers could delay a push to renewable energy, especially in Asia, by locking manufacturers into infrastructure dependent on the fuel for years to come.

What Are Brands Doing About It?

Large brands say they are mindful of the risks associated with biomass and are working to ensure its use in their supply chains is responsible.

Efforts are underway to establish sourcing standards and guidelines to reduce the risk of links to deforestation. Several companies say they already recommend suppliers only use agricultural or forestry waste as feedstock and limit their support for biomass to instances where other options aren’t currently available.

Climate groups argue that such efforts are likely to rely on certifications that aren’t always robust and there simply isn’t enough consistent supply of waste biomass to meet demand.

Earlier this month some 30 environmental organisations wrote to the UN Fashion Charter, calling on the initiative to exclude the use of biomass from its recommendations for climate action. The Fashion Charter said it has not taken an official position on biomass.

Climate groups are pushing for brands to focus on alternative solutions like electrification, heat pumps and investment in renewable energy supply — steps that are likely to be costlier, more complicated and lengthier to implement.