“This is just the beginning” – Arctic warming increases fire risk

According to a study, the warming of the Arctic threatens to end in a vicious circle. Accordingly, even a relatively small amount of warming could significantly increase the spread of wildfires in Siberia in the future. That would release large amounts of carbon that are stored in the permafrost there. And this in turn would further drive global warming, reports an international research team in the journal Science.

The scientists had analyzed the wildfires in the Siberian Arctic - i.e. north of the Arctic Circle - from 1982 to 2020 by evaluating satellite images. In these almost four decades, almost half of the total area burned (44 percent) was in the last two years 2019 and 2020: Of the 9.24 million hectares affected in the total period, 4.7 million hectares (47,000 square kilometers) burned in these two years. - more than the area of ​​Switzerland with its 41,000 square kilometers.

The Arctic is warming up much faster than the rest of the planet, writes the group led by Adrià Descals from the CRAF research center in Barcelona. Experts speak of the "Arctic reinforcement". This is also due to the fact that the earth's surface absorbs more heat due to the reduction in snow and ice cover on water and on land.

"Compared to pre-industrial times, the average annual temperature in the Arctic has increased by more than 2 degrees Celsius," the team writes. “By the year 2100 it is expected to be between 3.3 and 10 degrees Celsius higher than the average for the period from 1985 to 2014.”

The consequence of warming: ever larger areas of permafrost are thawing and could release the carbon stored in them into the atmosphere. The researchers write that there is particular concern about the increase in large fires. Analysis of data from various satellites showed that fires in the Siberian Arctic released almost 150 million tons of carbon into the atmosphere in 2019 and 2020. That corresponds to a CO₂ equivalent of almost 413 million tons. In 2020 alone, 423 fires burned over an area of ​​almost 3 million hectares. According to a statement from the Spanish National Research Council (CSIC) involved in the work, the released CO₂ equivalent of 256 million tons corresponds roughly to the annual CO₂ emissions of Spain.

To determine the key drivers of fire risk, the team correlated annual burned areas with ten climate factors known to contribute to fire risk. These include precipitation, air and ground temperature, wind strength and direction, duration of the vegetation period and the number of thunderstorms.

According to the analysis, the risk of fire depended significantly on the temperature: 2019 and 2020 were the two warmest years of the study period. In June 2020, a heat record for the Arctic was measured in the Siberian city of Verkhoyansk at 38 degrees Celsius.

The team describes the diverse influences of temperature: With higher temperatures, the snow cover tends to disappear earlier in the year. This not only promotes warming of the soil, but also lengthens the vegetation period and thus increases the amount of fuel that is potentially available. In addition, at higher temperatures, the water consumption of the plants increases and the water content of the soil decreases. And last but not least, the likelihood of thunderstorms increases with the heat in summer - and thus the main factor that causes fires to occur in the first place.

"Climate change has a dual effect on fire risk," the team writes. "Warming increases the vulnerability of vegetation and moorland to fire and increases the rate of lightning-related ignitions."

In the years 2019 and 2020, the mean air temperatures in summer were 11.35 and 11.53 degrees Celsius. That was 2.65 and 2.82 degrees more than the average for the years 1982 to 2020. In general, the four years with the largest burned area - in addition to 2020 and 2019, these were 2018 and 2001 - had an average air temperature of more than 10 in summer Centigrade.

"This suggests that even small increases in mean summer temperatures above a threshold of 10 degrees Celsius tend to be associated with extensive burned areas," the authors conclude. The threshold would be a kind of tipping point, beyond which fire activity in the region increases significantly. If the mean summer temperatures rise linearly, a value of 10.2 degrees will already be reached in 2024, according to the team, the mean value for 2020 - i.e. 11.53 degrees - in 2045.

The researchers predict what that could mean for various emission scenarios. Under a business-as-usual scenario (RCP 8.5), the area burned in 2020 would become the norm over the century. Annual emissions of 135 million tons of carbon - or 382.5 million tons of CO₂ equivalent - could be normal by the end of the century.

In a Science commentary, Eric Post of the University of California and Michelle Mack of Northern Arizona University warn of the long-term consequences of this development. “The northern peatlands - in Asia, North America and Europe - currently store about 100 million tons of carbon annually. The massive release of 150 million tons of carbon in 2019 and 2020 from the Siberian fires shows how quickly ecosystems can transform from carbon sinks to carbon sources as the Arctic warms steadily.”

Increased burning of the insulating peat layer could thaw larger reservoirs of organic matter and release carbon that has been stored underground for centuries or even millennia, commentators warn.

Guido Große from the Alfred Wegener Institute (AWI) in Potsdam, who studies permafrost landscapes with satellite data, speaks of a solid study. "It's pretty convincing," he says. “We have some important pieces of the puzzle here that have not previously been included in climate models. Permafrost soils are a globally important carbon reservoir that threatens to become an additional source of greenhouse gases sooner than expected.”

The AWI researcher Moritz Langer, who researches the dynamics of permafrost, also speaks of an important study. Nevertheless, the authors' conclusion that even small increases in temperature could exponentially boost wildfires above a specific threshold value probably does not do justice to the complex relationships. "There is a lot of uncertainty in such forecasts," says Langer. As the study shows, in addition to temperature, a combination of many factors play an important role, such as the distribution of precipitation in summer and winter and the circulation of the atmosphere.

But to what extent do the amounts of greenhouse gases released during fires in the Siberian permafrost actually have a global impact? For the year 2020, the authors of the study assume 256 million tons of CO₂ equivalent. For comparison: Mankind emits around 35 billion tons of CO₂ equivalent per year, says Langer.

But the expert warns against underestimating emissions from the Arctic. “You have to reckon with these quantities in the long term.” Große also emphasizes this: “This is only the beginning, and we are already observing increased fires and permafrost thawing with a few degrees of warming. When the permafrost ecosystem goes from being a sink to a source of greenhouse gases, that's a problem.”

Overall, Langer adds, the arctic moorlands contain around 400 billion tons of carbon - about half of which is in permafrost. How exactly their thawing affects the greenhouse gas balance is still largely unknown.

A team led by Norman Rößger from the University of Hamburg and Torsten Sachs from the Geoforschungszentrum in Potsdam recently reported in the journal "Nature" that these soils can release more of the greenhouse gas methane (CH4) into the atmosphere not only when they burn, but also when they "normally" thaw Climate Change”. The researchers measured methane emissions in the Lena Delta in northern Siberia from 2002 to 2019. Accordingly, methane emissions there in June and July have risen by almost 2 percent annually since 2004.

"However modest the observed trends may be," writes the team. "But in view of the very thick and cold permafrost in the study area compared to many other places, they represent a remarkable development."