Climate Denial Crock of the Week

Physics Today:

As temperatures around the world rise, the threat of wildfires is becoming increasingly common. In a populated place like California, for example, blazes in 2018 and 2020 killed dozens of people while burning hundreds of thousands of acres and causing billions of dollars in damages. Unfortunately, the situation is only getting worse (see “Fire season in the western US is intensifying,” Physics Today online, 21 June 2021).

Even though wildfires in low-populated areas may be less of an immediate danger to people, they’re still releasing vast amounts of carbon dioxide into the atmosphere and exacerbating climate change. That’s especially true in the Arctic. Because rising temperatures melt snow and ice and the liquid water reflects less sunlight than snow, the area warms further in a positive ice–albedo feedback loop. The mechanism is amplifying temperatures in northern latitudes and causing the region to warm about twice as fast as the global average. Early data on the 2019–21 fire seasons has suggested that summer blazes in the Siberian Arctic were particularly widespread.

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. Both found that several recent fire seasons had exceptional total areas burned compared with the average over the past four decades. Although the work is preliminary, it suggests that snow will continue to melt earlier each season, and a changing Arctic atmospheric circulation will accelerate fire activity.

Adrià Descals, of the Centre for Ecological Research and Forestry Applications in Spain, and his colleagues found that of the 92 000 km² of burned area in the Siberian Arctic over 1982–2020, some 44% of that total burned in 2019 and 2020 alone. Worsening fire-risk factors are to blame: climate variables, such as air temperature and atmospheric drought, and vegetation conditions, such as the length of the growing season.

The researchers used regression models to analyze how the observed burned area was affected by each variable. Most of the variables had some effect, but the large Arctic water deficit—that is, the difference in the amount of water that plants could use if it were available—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 rather mild for polar regions in the winter.

The other team, led by Rebecca Scholten of Free University in Amsterdam, reported similar findings and included data from 2021. Snow in northeastern Siberia melted an average of eight days earlier in 2020, relative to the data set’s 20-year climatological average.

In addition to the burn maps, Scholten and colleagues also looked at atmospheric-circulation data, which showed an increase in the frequency of an anomalous summer-circulation pattern in the Arctic. When that anomaly was added to their fire-activity model with the unusual snowmelt timing, the probability of a wildfire in a particular summer week increased to 44%, compared with just 2% for conditions with snowmelt occurring later in the season. The unusual atmospheric circulation also increases convection, which leads to the development of more thunderstorms and lightning strikes that spark wildfires.

The findings by both research teams may challenge conventional ideas about whether the Arctic can still be considered a carbon sink. As rising temperatures thaw more permafrost and the carbon-rich peat land underneath, the region may contribute even more carbon dioxide to the atmosphere than models have previously estimated. (A. Descals et al., Science 378, 532, 2022; R. C. Scholten et al., Science, 2022, doi: 10.1126/science.abn4419.)