When it comes to burning wood to generate electricity, forest scientist John Gunn says we need to do the math.

Five Questions for John Gunn

Burning wood pellets to generate electricity instead of relying on fossil fuels like coal sounds like a good deal for the planet. The carbon dioxide released into the atmosphere when burning wood, after all, is reabsorbed over time by trees planted to replace timber used as fuel.

Many European countries are racing to meeting carbon reduction goals by burning wood instead of coal to generate power. That’s a tenuous strategy.

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That’s the theory anyway – and a whole industry has grown up around it. In the U.S., dozens of “woody biomass” companies are burning plant-based fuel or exporting thumb-sized pellets of compressed wood from the Southeast to be burned in Europe to meet climate-protection goals. The energy bill that died this month in the lame-duck session of Congress included a bipartisan Senate provision stating that burning wood to produce energy would be considered carbon-neutral.

But scientists who have studied woody biomass say that is a dubious proposition. When a tree is cut down and burned, they argue, it creates a “carbon debt” that may not be repaid for decades, depending on an array of factors like how fast the replanted trees grow. The U.S. Environmental Protection Agency’s science advisory board has repeatedly warned that not all biomass is carbon-neutral.

The seminal study raising these kinds of doubts was produced for the state of Massachusetts in 2010 by the Manomet Center for Conservation Studies (now called Manomet Inc.), a nonprofit group based in Plymouth. For this installment of the Undark Five, we talked with John Gunn, one of the authors of that study, along with a subsequent analysis of other research about woody biomass.

Gunn is a research and extension assistant professor of forest management at the University of New Hampshire, and he now serves as the volunteer director of the nonprofit Spatial Informatics Group – Natural Assets Laboratory, which recently published a study concluding that burning wood pellets for home heating in the Northeast reduces greenhouse gas emissions compared to fossil fuel alternatives. Another of his projects involves developing a protocol to generate carbon credits under California’s cap-and-trade system by thinning overgrown forests so that when wildfires hit, they are less severe and release less carbon dioxide.

Our conversation was edited for length and clarity.

UNDARK — From your work, it seems like the answer to the question about the utility of woody biomass in fighting climate change is: It all depends on the specific circumstances. Is that a fair way to sum up what you have learned?

JOHN GUNN — It sure does. The analytical approach we took in Massachusetts can be applied anywhere, and you get different answers depending on that context. Everything from how the forest is managed in the absence of biomass to how the forest is managed with biomass — that makes a difference.

And other key components are the feedstocks that are used to make the biomass. That varies all over North America, depending on the forest context — restoration treatments in the Western U.S., which are thinnings that tend to reduce fire severity and size, compared to here in the Northeast where whole-tree biomass and wood chips are part of a typical harvest. In some cases, chips could be coming from green trees that could be used to make pulpwood or energy. So all of those distinctions matter.

UD — If you could find a fast-growing feedstock, that could really change the whole equation, right?

JG — Yes, that’s right. The forest is renewable, sure, but are you renewing enough to recapture the carbon that’s lost through combustion? The forest has to make it up in some way.

UD — Wood is less carbon-dense than coal. It appears to take about 150 percent of the greenhouse emissions to produce a unit of energy from woody biomass compared to coal. So is it accurate for opponents to say that burning woody biomass is worse for the climate than burning coal?

JG — I guess it depends on how you define worse. There are certainly upstream impacts of coal that we don’t account for. In the analysis that we do, we are strictly talking about greenhouse gas emissions. The Massachusetts example shows that for the 30- to 50-year period, it seems likely the pathway of burning wood instead of coal would result in more carbon dioxide in the atmosphere for that period of time.

[But] suggesting that wood isn’t the best pathway in certain situations doesn’t mean that we promote coal as the answer. I know some of the press and industry spin on some of our findings has been that we’re promoting coal over wood. But nowhere have we said that coal is preferable.

UD — Should countries such as members of the European Union be able to count woody biomass as carbon-neutral?

UD — It depends on where they’re getting it from and what are the implications for the forest that it comes from. In the Southeast, some of our work found there’s been a period of greater emissions from wood than the fossil alternatives in Europe. But others have suggested that low-grade markets will cause more land to be planted to forest, which changes the equation. Again, the assumptions about the future forest matter.

Also, I think there’s a lot of confusion about what’s actually being used to make pellets. On the one hand people are talking about waste wood, and I think that can be misleading because we know that some of that waste wood is actually coming from green trees that were standing and growing.

UD — It looks like you’ve been doing some thinking generally about what kinds of bioenergy make sense for the climate and which should be viewed with more caution. Can bioenergy in some form help head off the disastrous results from climate change?

JG — Yes. I think that’s why we need to do the math on these different contexts. The work we are doing in California shows that using wood from forest-restoration thinning for energy seems like a good pathway. Other work we just finished here in the Northeast looks at wood pellets for heat — such as from small-scale pellet mills in Maine and New Hampshire that people use to heat homes or small buildings. Here in the Northeast, home heating oil is the dominant heat source. You’re basically burning diesel, so that’s not a super-efficient way to heat homes.

In this analysis, we looked at the implications of a declining pulp and paper industry in the Northeast; we assumed that harvest practices and volumes don’t change and the wood that was going to be made into pulp and paper is used to make pellets and heat homes instead. If you can use those pellets in modern wood-heat systems where there’s 80 percent efficiency, then we show that there’s a clear benefit to making that switch. To me, that points to a good pathway that can have a lot of positive benefits.

Robert McClure is executive director of InvestigateWest, a nonprofit newsroom in Seattle with a focus on the environment, public health, and government accountability. He is currently a 2016-17 Knight Science Journalism fellow at MIT.