As temperatures around the world rise, the threat of wildfires is becoming increasingly more frequent. For a heavily populated place like California, wildfires are particularly hazardous: Blazes in 2018 and 2020 killed dozens of people while burning hundreds of thousands of acres and causing billions of dollars in damages. Those risks are only getting worse (see “Fire season in the western US is intensifying,” Physics Today online, 21 June 2021).
Even though wildfires in low-population areas may be less of an immediate danger to people, when they do happen they release vast amounts of carbon dioxide into the atmosphere. That is especially worrisome for the Arctic, which has a large amount of carbon stored in permafrost. Moreover, the region is subject to a positive ice–albedo feedback loop: Rising temperatures melt snow and ice and the liquid water reflects less sunlight than snow, so the area warms further. The mechanism is one contributor to Arctic amplification—temperatures in northern latitudes are warming at least twice as fast as the global average (see the article by Martin Jeffries, James Overland, and Don Perovich, Physics Today, October 2013, page 35).
Historically, the area burned by fires has been less in the Arctic than that at lower latitudes.1 Early data on the 2019–21 fire seasons, however, have suggested that summer blazes in the Siberian Arctic—one of which is shown in figure 1—were widespread. That’s particularly true in eastern Siberia, where high-pressure systems often develop in the summer. They generate several conditions that are more favorable for fire activity: stable high temperatures, precipitation deficits, and more lightning ignitions because of atmospheric convection.
A pyrocumulus cloud formed from a wildfire that burned this 30-kilometer-wide area on 6 August 2020, during one of the most intense Siberian Arctic fire seasons of the past 40 years. (Satellite photo created by Adrià Descals; imagery acquired by the European Space Agency.)
A pyrocumulus cloud formed from a wildfire that burned this 30-kilometer-wide area on 6 August 2020, during one of the most intense Siberian Arctic fire seasons of the past 40 years. (Satellite photo created by Adrià Descals; imagery acquired by the European Space Agency.)
To better connect how warmer-than-average temperatures lead to exceptional fire activity, two teams analyzed satellite-derived maps of burned areas in the Siberian Arctic.2,3 Both found that compared with the past 20–40 years, several recent fire seasons were exceptional in the total area burned. The studies suggest that snow will continue to melt earlier each season, and a changing Arctic atmospheric circulation will accelerate fire activity.
Fire factors
The burn maps analyzed in both papers were made possible only by the decades of satellite data that were carefully collected during multiple missions led and funded by NASA, NOAA, and the European Space Agency. Adrià Descals, of the Centre for Ecological Research and Forestry Applications in Spain, and his colleagues found that of the 9.2 million hectares of burned area in the Siberian Arctic over 1982–2020, shown in figure 2, some 44% of that total burned in 2019 and 2020 alone. (The total area of the Siberian Arctic is some 286 million hectares.)
Widespread wildfires burned millions of hectares of forest in the Siberian Arctic in 2019 and 2020, based on six data sets made with satellite measurements. The unusual fire activity persisted into 2021 and may continue in the future because of a complex interplay of earlier snowmelt and varying atmospheric conditions brought about by rising temperatures. (Adapted from ref. 2.)
Widespread wildfires burned millions of hectares of forest in the Siberian Arctic in 2019 and 2020, based on six data sets made with satellite measurements. The unusual fire activity persisted into 2021 and may continue in the future because of a complex interplay of earlier snowmelt and varying atmospheric conditions brought about by rising temperatures. (Adapted from ref. 2.)
Other high-latitude regions in North America and western Europe experienced wildfires during those years too, but the blazes across the Siberian Arctic were noteworthy because of how widespread they were. Descals and his colleagues suspected that worsening fire-risk factors are to blame: climate factors, such as air temperature and atmospheric drought; vegetation conditions, such as the length of the growing season; and the number of ignition events over an area.
To analyze how the observed burned area was correlated with each of seven fire factors, Descals and his colleagues used regression models. He says, “We found a quite clear exponential relation between the annual burned area and several factors of fire. I expected a noisier relationship.” The results showed that a 1 °C rise in temperature was correlated with an expansion of the burned area by 150–250%. But just because atmospheric drought or another climate variable covaries with a fire factor doesn’t mean that it caused or exacerbated the blaze. The fire itself could, for example, make the local region drier.
To unpack any potential cause and effect between the environmental variables and the wildfires, Descals and his colleagues developed a structural equation model. It starts by hypothesizing that changes in an environmental variable could cause an accompanying change to a fire factor. Quantitative relationships in the literature show, for example, that a higher surface temperature over a longer period of time would stand to lengthen the growing season, and a lower total precipitation in the region would heighten the water stress on plants. To test whether those causal relationships could produce the observed wildfire conditions, a statistical goodness-of-fit test assesses how similar the model estimates are to remote and field measurements.
Most of the variables in the structural equation model had some effect on the incidence and extent of wildfires. But the large Arctic water deficit—that is, the difference between the available water and the maximum amount of water that plants could potentially use—was the biggest contributing factor to the total burned area. In addition, Descals and colleagues found that the years with the largest burned areas all had average summer air temperatures exceeding 10 °C, which is more than 2 °C higher than the Siberian Arctic’s historical 40-year average. Those high summer temperatures are projected to become the new normal by 2040, according to climate models with an intermediate emissions scenario.
Atmospheric anomaly
The factors that affect wildfires are varied, complex, and tied to more than just surface temperature. Rebecca Scholten of Free University of Amsterdam, who led another research team, says, ”It’s not enough to just look at how much the average temperature is going to rise. If we want to understand how future fire activity looks in Siberia, we have to take into account the complexity of the system.”
To obtain a more complete picture of wildfires in the Siberian Arctic, Scholten and her team analyzed the timing of snowmelt in the spring by reviewing maps made with data from the MODIS instrument aboard NASA’s Terra and Aqua satellites. They saw that snow melted an average of four to eight days earlier in 2020, relative to the data set’s average over 2001–21.
In addition to the burn maps, Scholten and her colleagues studied atmospheric-circulation data over the past 40 years. Over that time period, the data showed an increase in the frequency of a high-altitude wind pattern over the Arctic coastline. Scholten and her team suspect that the pattern may be anchoring high-pressure systems over northeastern Siberia and thus may partially be responsible for the drier conditions there.
The years 2019, 2020, and 2021 all had that wind pattern and earlier snowmelt timing. When both were added to a fire-activity analysis, the probability of wildfire activity in a summer week increased to 44%, compared with just 2% for conditions with snowmelt occurring later in the season than usual. The peculiar atmospheric circulation also increases convection and consequently generates more thunderstorms with lightning strikes that can spark wildfires.
To make Arctic wildfire predictions under changing future scenarios requires Earth system models that compute the complex interactions between the atmosphere, land, oceans, cryosphere, and biosphere. So far, however, those models lack enough detail to make robust wildfire predictions. One nagging unknown is whether wildfires are diminishing the Arctic boreal forests’ ability to act as a carbon sink. Eastern Siberia contains a large share of continuous permafrost. If rising temperatures and wildfires thaw the carbon-rich permafrost, the region would emit more carbon dioxide to the atmosphere than previously estimated.4
More observations, particularly ground-based measurements, of carbon emissions should help researchers better understand the interplay among all the variables and how that translates to changes in the incidence and extent of wildfires. Scholten began collecting field data on the carbon emissions from wildfires in Siberia in 2019. But the global shutdown in 2020 forced by the coronavirus pandemic and, more recently, the 2022 Russian war against Ukraine have prevented her and her team from collecting all the Siberian wildfire observations they wanted. “It’s a big loss for the climate science community to not be able to get these data,” says Scholten. “And I think this paper really shows that it’s a very important region where a lot of things are changing very rapidly.”
