These sandals replace petroleum with sugarcane. How smug can I be about wearing them?

Fossil fuels aren’t sustainable, but alternatives aren't perfect, either.

Allbirds, the shoe brand known for stylish sneakers with a merino wool upper, have announced a line of flip-flops that swaps traditionally-manufactured soles for a material made with Brazilian sugarcane. A common component in footwear is a foam called EVA—or ethylene-vinyl acetate. Allbirds’ new Sugar Zeffer sandals replace part of that material, which usually comes from petroleum, with sugarcane.

Here’s how it works, from field to flip-flop: Allbirds is using sugarcane grown in São Paulo state, in Brazil’s southeast. At a mill, “it’s kind of squished,” says Joey Zwillinger, one of Allbirds’ founders. One of the products of that sugarcane squishing is a liquid called molasses; added yeast feasts on that sugary substance and produces ethanol. The process continues from there, and eventually, the ethanol is made into ethylene—or the “E” in EVA foam. Allbirds adds other substances into the mix to produce what they’re calling SweetFoam.

The ethylene from the sugarcane takes the place of ethylene from petroleum. “We’ve replaced the barrel of oil with a field of sugarcane,” Zwillinger says, although he concedes that there are still “minor components” in the new flip-flops that are petroleum-derived. He says this new sugarcane foam will eventually appear in other Allbirds kicks.

While moving away from fossil fuels is good in theory—they’re a finite resource, for one, and produce greenhouse gases that contribute to climate change—industrial agriculture is not without its own consequences. Sugarcane cultivation is on the rise in Brazil, says Solange Filoso, an associate research professor at the University of Maryland Center for Environmental Science. Originally from Brazil, Filoso has researched the environmental impacts of sugarcane production in South America’s largest country. She says that sugarcane can have a negative impact on the environment, but that those problems can also be mitigated if growers follow best practices.

Erosion is one such problem with Brazilian sugarcane production. “Sugarcane plantations have a lot of roads,” she says. “When it’s cut, there are trucks that go collect the cane to take to the mills.” The erosion from this process can impact waterways. “Water quality in the state of São Paulo is a growing problem which is definitely aggravated by intensive agriculture like sugarcane,” she adds, via email.

Another issue has to do with the use of nitrogen-containing fertilizer, although sugarcane doesn’t need much. And interestingly, sugarcane agriculture is taking over land that used to be “unmanaged pasture,” she says. As pasture land becomes in shorter supply, she says, farmers are working that remaining pasture harder. “Farmers are trying to increase productivity of pasture—like how much grass can grow in one hectare—by fertilizing it.” Fertilizing pasture is otherwise unheard of in Brazil, she says. She also notes that sugarcane agriculture is replacing orange groves, too, which are gentler on the environment than the cane.

“Just because it is a renewable energy source, people think it’s all positive,” she concludes, adding that “it’s definitely better than fossil fuel.” Ultimately, she says, the important thing is for the sugarcane ethanol industry to try to minimize the harmful aspects of cane agriculture. That sentiment is important to keep in mind when considering Allbirds’ latest efforts: consumers should be skeptical of supposedly green products—goods whose manufacturing processes can still hurt the environment—even while keeping in mind that a small step away from a reliance on fossil fuels is one in the right direction.

For its part, Allbirds notes that all the sugarcane they use is certified by an organization called Bonsucro, a network focused on making sugarcane cultivation sustainable. “In partnering with a Bonsucro certified business, we are ensuring, through the best third-party verification body in the world for sugarcane production that we are following the most stringently protective practices,” Allbirds said in a statement.

Rob Verger

Rob Vergeris an associate editor at PopSci, where he covers aviation, the military, transportation, outdoor gear and gadgets, and other tech topics. A graduate of Columbia Journalism School, he's also written for The Boston Globe, Newsweek, The Daily Beast, CJR, VICE News, and other publications. Contact the author here.