Even if you live far from the boreal forests in Canada and Siberia, you’ve likely noticed an increase in smoke from their forest fires. During major blazes in 2023, the smoke oranged the New York sky and drifted as far south as New Orleans. These blazes have surged in the last decade due to the effects of climate change — warmer summers, less snow cover in the spring, and the loss of sea ice. Experts expect that trend to continue.

Yet recent climate change projection models have not accounted for the increase. For instance, the widely used sixth Coupled Model Intercomparison Project, or CMIP6, released in the late 2010s, kept these fires constant at a relatively low severity.

A new University of Washington-led study projects that in the next 35 years these increasing boreal fires will actually slow warming by 12% globally and 38% in the Arctic. The study is the first to identify the divergence between the observed boreal fire increase and the constant fires used in climate models. Because the aerosols in smoke brighten clouds and reflect sunlight, summer temperatures during fire season drop in northern regions, leading to reduced sea ice loss and cooler winter temperatures. This effect is despite the warming effects of the fires themselves from factors such as soot that falls on the ice.

Researchers published their findings June 3 in Proceedings of the National Academy of Sciences.

“This study helps us begin to better project the impacts of climate change. The dramatic increase in these fires in the last years is itself a symptom of that,” said lead author Edward Blanchard-Wrigglesworth, a UW research associate professor of atmospheric and climate science. “It’s important to remember that these increasing fires still have a lot of negative impacts for human health and for forest biodiversity. And if the fires continue to increase, eventually they could burn through the forests and the trend could reverse. So I wouldn’t say this is good news. But it helps us better understand nature and these trends.”

Every six or seven years, climate modeling centers around the world collaborate to update their projections, using numbers going back to the 19th century and projected numbers through 2100. These data comprise things like wildfires and human-caused carbon emissions. For CMIP6, which was modeled before boreal fires became a clear anomaly, the wildfires were kept constant from 2015 to 2100.

“If you look at the time series of the fires, it starts increasing around 2015, but it really spikes in 2019 and 2021, just as this modeling was being completed,” Blanchard-Wrigglesworth said. “Those are the big years of Siberian fires. And then 2023 was the even bigger Canadian fire season.”

Because climate scientists don’t expect the causes of this increase in fires to abate anytime soon, the team reran one of the CMIP6 models with a new boreal fire projection based on the recent observed trends, resulting in a four-fold increase from 2015 to 2060. This adjusted the modeling for the smoke aerosols. It also accounted for factors like the fires’ soot, which settles on Arctic ice and darkens it, causing it to absorb more heat from sunlight (the same way sun heats asphalt). But the increased reflection of sunlight from aerosols overwhelmed this warming.

This chart compares the black carbon emissions from boreal wildfires. The red line shows actual recorded emissions. The solid blue line is the estimate from the CMIP6 model, while the dotted blue line is the estimate adjusted based on the recent increase in fires.Blanchard-Wrigglesworth et al./PNAS

While the fires occur only in the summers, researchers actually found a greater cooling effect in the winters, because the fires block some of the summer sun, resulting in thicker Arctic ice that lasts into the following winter.

The study found impacts far from boreal forests. The smoke cools temperatures across all seasons from the Arctic down to the latitude of Northern California at 40 degrees north. The fires also push tropical rains further south because tropical precipitation depends in part on the temperature difference between hemispheres.

The authors say future work should adjust other climate models to account for increasing boreal fires and investigate possible effects of changes in the land after fires.

“I hope our work raises awareness of this issue for further study and of the potential effects of any future human management of these remote fires,” Blanchard-Wrigglesworth said. “If the increase in boreal fires continues unabated over the next decade or two, society may decide we want to manage boreal fires more. But before we put a lot of resources toward that, we need to try to understand the possible consequences.”

Patricia DeRepentigny, of Université Catholique de Louvain, and Dargan Frierson, a UW associate professor of atmospheric and climate science, are co-authors on this paper. This research was funded by the National Science Foundation and the European Union.

For more information, contact Blanchard-Wrigglesworth at edwardbw@uw.edu.

  canada, glolbal, Russia, warming, wildfires