The Washington PostDemocracy Dies in Darkness

Covid-19 sparked a run on outdoor heaters and fire pits. Which is better for the planet?

(Michael Parkin for The Washington Post)

I’m looking for a way to stay warm during covid-safe outdoor gatherings this winter. In terms of greenhouse gas emissions, is it better to use a fire pit or a propane-fueled patio heater?

-- Lyndsey Layton, Climate Solutions editor

Nelson Bryner has set a lot of things on fire in his career. Buses. Trash cans. Life-sized mannequins dressed in firefighting gear. A five-piece wooden dining set.

As chief of the fire research division at the National Institute of Standards and Technology, Bryner spends many of his working days inside the division’s 20,000-square-foot laboratory, analyzing how much heat is generated and what byproducts are produced when various items are set ablaze.

With coronavirus cases spiking and the mercury dropping, sparking a run on backyard heating devices, I knew Bryner could tell me what will happen when the fuel for those heaters is burned.

The chemical reactions that happen in a fire pit and a propane burner are more or less the same, the NIST engineer explained. Molecules in the fuel break apart, creating heat; the resulting carbon atoms combine with oxygen in the air, producing carbon dioxide.

But an inefficient fuel, or a fuel that is burning under poorly ventilated conditions, will also create other byproducts, such as carbon monoxide and methane. And these reactions don’t generate as much heat, giving you less warming bang for your carbon emissions buck.

Wood tends to fall into the “inefficient” category, according to Bryner. Wood stoves have been found to emit black carbon, a potent greenhouse gas, as well as small, inhalable particles known as PM2.5 that are a serious health threat.

When propane burns, Bryner said, it converts almost all of its carbon into carbon dioxide. “So you’re going to need less carbon, because you can get every carbon atom to work."

Even so, the more efficient gas-powered heater can still generate a lot of greenhouse gases. The average patio heater generates about 40,000 BTU (the standard unit of heat) per hour. Burning propane emits roughly 135 pounds of carbon dioxide per million BTU, according to the EPA. If you ran your patio heater for 5 hours a week over the course of three months, you’d generate about as much carbon dioxide as driving a car 450 miles.

But burning efficiency is not the only way to judge a heating device.

Anna Karion, a colleague of Bryner’s who works in NIST’s greenhouse gas measurements program, noted that wood — unlike propane — is a renewable fuel. If another tree grows in the spot where your wood was harvested, it could potentially suck enough carbon out of the atmosphere to counteract the emissions of your campfire.

That too comes with caveats. Because older, bigger trees sequester more carbon than young ones, forests must be allowed to grow to maturity to deliver the fullest carbon benefits. According to one study in the journal Environmental Research Letters, that could take as long as 100 years. But scientists say humanity needs to drastically cut greenhouse gas emissions now, or the world may soon cross the threshold of catastrophic warming.

A comprehensive comparison of the carbon footprint of different heaters also requires looking at the full life cycle of each product, Karion said. A propane-powered heater needs to be manufactured and transported, generating greenhouse gases, whereas you can easily make a fire pit with some rocks in your backyard. Recycling the heater or disposing it in a landfill will also lead to some emissions.

“Unfortunately, these decisions are not straightforward,” Karion said. “Your best option might be to get that blanket out.”

Still, it’s understandable that you want to find a way to be warm, safe and sociable this winter, and depending on where you live, a blanket might not cut it. If you decide to spring for a heater of some kind, maybe think about other ways to do something good for the planet. Measures like weatherizing your home and turning down the thermostat a few degrees can save ten times as much carbon as your patio heater emits, and they’ll save you money, too.

You can also advocate for climate action in your community and your country. While individual actions can be meaningful, experts say systemic changes are most needed to avert catastrophic warming: transitioning to renewable energy sources, converting vast swaths of land back into carbon-sequestering wilderness, swapping gasoline-powered cars for public transit and electric vehicles. United Nations scientists have called for the world to almost halve emissions by 2030.

If we don’t meet that target, your heater might become obsolete sooner than you’d like — no matter which one you choose.