This story was co-published with The Weather Channel as part of Collateral, a series on climate, data and science.

BRISTOL, Vermont — Wood smoke rising from a stovepipe is as about as common a sight in Vermont as roadside signs advertising maple syrup for sale.

Vermonters have been heating their homes with wood for centuries. Many still do. The Green Mountain State leads the nation in its reliance on wood, with about a quarter of households using it as their primary heating fuel.

In a state where winters are cold, forests abundant and people celebrate self-reliance, wood has also made its way into the Vermont's latest renewable energy planning, billed as a way to cut climate-warming pollution.

But is it a clean energy solution?

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"With the capabilities to burn wood that we have now, with sophisticated equipment, it's our local homegrown renewable energy source," argues Emma Hanson, the wood energy coordinator at the state Department of Forestry, Parks and Recreation, a position created in 2017 to advocate for wood burning.

Wood burning means tradeoffs, though—for air quality, public health and the climate.

Wood smoke contains a diverse mix of pollutants that can harm people's health: carbon monoxide, sulfur dioxide, nitrogen oxides, volatile organic compounds such as benzene and formaldehyde, and copious amounts of sooty fine particles called black carbon that can penetrate deep into the lungs and cross into the bloodstream. Vermonters already produce about 22 pounds of particulate matter emissions per capita from wood burning each year, by far the highest in the nation.

And then there's the climate problem. Living trees absorb planet-warming carbon dioxide and store it, but once trees are cut down and burned, that carbon is released.

Loggers cut down trees near Reading, Vermont. "It’s our local homegrown renewable energy source,” says Emma Hanson, the wood energy coordinator at the state Department of Forestry, Parks and Recreation. Credit: Jessica Rinaldi/Boston Globe via Getty Images

While advocates of wood burning argue that sustainably harvested wood is "carbon neutral," recent research suggests that many of the climate benefits of burning biomass instead of fuel oil or propane are cancelled out by other climate-warming pollutants, including black carbon and methane, both many times more potent than carbon dioxide.

Despite those concerns, when the state updated its Comprehensive Energy Plan (CEP) in 2016, laying out detailed steps for reaching the state's goal of getting 90 percent of its total energy needs from renewable energy by 2050, wood burning was part of the plan.

The plan calls for increasing the share of Vermont's total building heating demand met with wood heating from 21 percent in 2016 to 35 percent by 2030. This would effectively double the number of households relying on wood.

One reason for the state's big bet on biomass is what wood-burning would displace: Vermonters rank second nationwide in their use of heating fuel oil per capita.

According to the Energy Information Administration, about three-quarters of Vermonters rely on fossil fuels—mostly fuel oil, natural gas or propane—to heat their homes. Heating produces almost 28 percent of the state's greenhouse gas emissions, the second biggest source after transportation. In comparison, Vermont has one of the cleanest electricity mixes in the nation, the majority of it hydropower.

State agency leaders and some energy advocates see modern wood heat as a "win-win-win": good for the climate because it displaces fossil fuels, good for the local economy and good for the state's forest-related industry.

"We import 100 percent of the fossil fuels we use in the state of Vermont," said Johanna Miller, the energy and climate program director for the Vermont Natural Resources Council, the state's oldest independent environmental advocacy group. "That reliance is wreaking havoc on our planet, and 80 percent of those fossil fuel dollars leave the state. Advanced wood heating is about creating an opportunity for keeping dollars in state, putting people to work and recirculating in our economy."

But doubling down on wood heat as a climate change mitigation strategy is predicated on some assumptions, including that fuel from Vermont forests will be sustainably harvested.

Wood Burning Can Be Worse Than Fossil Fuels

No matter how it burns, a wood fire produces carbon dioxide.

From the moment a tree is felled until a mature tree grows to take its place, the carbon released from the fire represents an addition of warming pollution to the atmosphere. There is a lag time for that carbon to be absorbed again by the growth of new trees. It could take decades or even centuries, depending on the type of forest.

Whether that climate penalty is worth paying to reduce fossil fuel use depends in part on where the wood is coming from.

Researchers at the University of Vermont and the University of New Hampshire analyzed lifecycle greenhouse gas emissions from the harvesting, processing and burning of wood pellets for heating in homes in a 2017 study. Pellet mills can use the residue left over from paper manufacturing or sawmills as a feedstock.

Researchers found that if roughly half of the feedstock used to make pellets for wood-burning stoves was from sawmill residue and half from harvested pulpwood, then emissions would be roughly the same as from heating with fossil fuels. Credit: Unkel/Ullstein Bild via Getty Images

The researchers determined that whether pellet stoves made sense as a carbon emissions reduction strategy depended largely on harvest levels: If roughly half of the feedstock used to make pellets was from sawmill residue and half from harvested pulpwood, then emissions would be roughly the same as from heating with fossil fuels.

In other words, their findings suggested that a wide-scale shift to pellet heating could lead to a net reduction in climate-warming emissions if it was done right. But it could also cut the other way if increased pellet demand leads to more felling of standing trees.

What Else Is in That Wood Smoke?

The carbon accounting of wood heat is quite sensitive to assumptions about rates of forest re-growth, and expert views diverge on the subject. But debates around the "carbon neutrality" of biomass often fail to acknowledge the climate impacts of non-CO 2 emissions.

Burning biomass incompletely can generate short-lived climate pollutants like black carbon and methane, and produce high levels of volatile organic compounds (VOCs) such as benzene and ethylene. (Methane and other VOCs contribute to the formation of ground-level ozone, which is both a respiratory irritant and a short-lived greenhouse gas.)

The dark smoke curling out of a stovepipe is a telltale sign of emissions of black carbon—a climate pollutant that the state's Comprehensive Energy Plan doesn't mention.

Black carbon contributes to climate change in multiple ways.

Suspended in the atmosphere, it is extremely efficient at absorbing incident sunlight and turning it into heat. It affects cloud formation and regional rainfall patterns.

When it falls onto snow and ice, black carbon reduces the amount of light reflected back to space. The darkened surface absorbs more energy, accelerating the melting of glaciers and permanent snowfields. Because it only lasts about one or two weeks in the atmosphere before it settles out, black carbon has an outsized warming impact close to its point of origin.

A truly "carbon neutral" wood fuel must not only be harvested sustainably. It must also be completely combusted, resulting in as little black carbon and methane as possible. Credit: Robert Nickelsberg/Getty Images

All of this means that the bar for "carbon neutrality" may be even higher than some wood heating skeptics suggest. A truly "carbon neutral" wood fuel must not only be harvested sustainably. It must also be completely combusted, resulting in as little black carbon and methane as possible.

"It's imperative that regulatory agencies take black carbon into account," said Zoё Chafe, the technical lead for air quality at C40 Cities, a group that provides scientific advice for cities trying to integrate air quality into their climate planning.

Taking into account those non-CO 2 pollutants could build a stronger case for the state to pursue a wholesale shift to zero emissions strategies, such as improving buildings' insulation (Vermont has some of the oldest housing stock in the country) and incentivizing the wider adoption of heat pumps powered by renewable electricity.

What About More Efficient Stoves?

State officials envision a broad swath of Vermonters embracing advanced wood heating systems—a category that includes automated woodchip and pellet boilers and furnaces along with the newest generation of wood and pellet stoves.

These are designed to emit less particulate matter, carbon monoxide and other pollutants than the older stoves that still heat many Vermont farmhouses.

But it's very difficult, even with modern technology, to eliminate emissions of those pollutants.

"There is no minimum threshold for damage to health from exposure to fine particulate matter," said Chafe. "Any time any amount of these tiny air pollutants enters your body there is the potential for some damage."

That's why the American Lung Association recommends avoiding burning wood for heat. Exposure to the fine particles in wood smoke is linked to increased risk of cardiovascular disease, stroke and lung cancer, and is especially dangerous for people with existing health conditions such as asthma and compromised respiratory systems, children, pregnant women and the elderly.

If you're going to burn biomass, "pellet stoves are far and away the best option" to reduce health-damaging emissions, Chafe said. "Emissions can be and usually are so much lower than any other sort of wood stove setup."

Pellets are of uniform quality and are fed into stoves and boilers with controlled, automated systems, leading to more complete combustion—and less black carbon and methane. Cord wood, however, comes in all kinds of shapes and sizes and degrees of moisture content.

A recent study in Italy examined methane emissions from several different wood and pellet heaters, and found that automated pellet stoves produce much less methane than cord wood stoves. Its authors concluded that, over a 20-year time-scale, the warming caused by short-lived climate pollutants emitted from wood stoves—but not pellet stoves—exceeded the reductions in carbon dioxide from avoided fossil fuel combustion.

The Human Element

In May 2020, new, tighter EPA smoke emissions limits for wood stoves are scheduled to take effect. But just because a wood stove is EPA-certified doesn't automatically translate to less smoke. There's plenty of room for "user error."

While a seasoned stove user can keep a fire burning hot and achieve more complete combustion, not all homeowners are burning wood properly.

"It can be really hard to tell if a stove is performing as cleanly as it should at the household level," Chafe said.

And there's another challenge: getting Vermonters to change from the cord wood they can chop themselves to pellet stoves or non-wood heat.

Vermont's long love affair with wood is inextricably linked to its creed of self-reliance. During Hanson's first week on the job, she found herself touting the virtues of advanced wood heating systems to an older Vermonter. "Well, Emma," he replied, "I have 100 acres of trees on my property, but I don't have a single pellet tree."

Top photo: Shawn Patrick Ouellett/Portland Press Herald via Getty Images