Planting trees has long been viewed as a cost-effective and useful method of helping offset the ravages of climate change. After all, trees (and other plants) take planet-warming carbon dioxide out of the air and convert it to wood, roots, and other forms of carbon-containing biomass.
But what happens when climate change causes large portions of this biomass to burn up?
That’s the core question of a new study in this week’s issue of Nature, examining the impacts of Canada’s catastrophic 2023 wildfires.
These fires, which were spread across the entire country, burned 4 percent of Canada’s entire forested area, forced hundreds of thousands of people to evacuate, and sent smoke across much of eastern North America. “New York was briefly the most polluted city in the world,” says Brendan Byrne, a carbon-cycle scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California, and lead author of the new study.
But they also released enormous quantities of carbon dioxide into the air. To find out how much, Byrne’s team used a European Space Agency satellite called Sentinel-5 to measure the amount of a related chemical, carbon monoxide, also produced by combustion.
Although it’s also possible to measure carbon dioxide from space, Byrne says, carbon monoxide worked better because it is rarer, and “doesn’t have tons of sources and sinks. When you have big fires, you see giant increases in carbon monoxide, so it’s really easy to pick out.” Not to mention that Sentinel-5 measures it globally, every day. “We don’t have quite that density of observations from our carbon dioxide sensors,” he says.
From that data, he says, his team could “back calculate” how much wood must have burned, and from that, how much carbon dioxide must have been released.
The results were staggering. Overall, his team calculated, a total of approximately 647 teragrams of carbon was emitted (a teragram is one trillion grams) – more than four times more than what all of Canada produced the year before.
2023 was, of course, an unprecedented fire season, a product of the hottest and driest year in the 44 years of climate records Byrne’s team examined. But that doesn’t mean it should be dismissed as a one-off. “When we look at climate projections,” Byrne says, “by the time you get to the 2050s, climate models suggest that those kinds of temperatures are going to be pretty average over the Canadian boreal forests.”
That makes it a concerning finding for those hoping forests can play a major role in offsetting climate change. “Are we going to see these kinds of fires more frequently?” Byrne asks. “That could have a big impact on forests’ ability to store carbon.”
Other climate researchers see the study as important. “Impressive work,” says Richard Alley, a Nobel-laureate climate-change researcher at Pennsylvania State University.
“We know that over long times (100,000 years plus), the Earth system will remove the CO2 we release,” he says. In fact, “over short times (decades), that is happening now.” But at “in-between” time scales, he says, “it is possible, or even likely, that removal will be small, or even that the Earth system will become a source of CO2, amplifying what we do.” And, “of the many ways the Earth system could become a source, burning a lot of stored carbon rapidly is a worrisome possibility.”
Jon Wang, an earth systems scientist at the University of Utah, agrees. “There is a prevailing notion in carbon-cycle science that the far north is ‘greening’,” he says, meaning that as the climate warms and growing seasons expand, vegetation growth in these areas is increasing the rate at which these ecosystems pull carbon dioxide out of the atmosphere. “However, this study adds to a growing chorus of research that questions the reliability of a northern carbon sink because of recent increases in fire activity.”
His own research, using satellites to measure forest biomass, has shown that the rate of carbon accumulation in forests in western Canada “has largely been shut down by wildfires.” In fact, he says, “many of these ‘greening’ signals are the recovery of forests from prior fires.”
