A Warming Siberia, Wracked by Wildfires, Nears a Crucial Threshold

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Rapid warming of the Arctic has led to the extreme wildfire seasons experienced in Siberia in recent years, scientists said Thursday, and such severe fires are likely to continue.

The researchers said that the Siberian Arctic, with its vast expanses of forest, tundra, peatlands and permafrost, was approaching a threshold beyond which even small temperature increases could result in sharp increases in the extent of fires.

“Global warming is changing the fire regime above the Arctic Circle in Siberia,” said David L.A. Gaveau, one of the researchers. His company, TheTreeMap, monitors deforestation around the world.

In the Arctic, wildfires can result in the burning of decayed organic matter in peat and thawed permafrost. That releases carbon dioxide, adding to warming and making the goal of reining in climate change more difficult.

Over the past four decades, the Arctic as a whole has been warming about four times faster than the global average. Recent summers in eastern Siberia have been marked by particularly extreme temperatures — as much as 38 degrees Celsius, or 100 degrees Fahrenheit.

The warmth has been accompanied by severe and extensive wildfires. “Observations indicated that the fire seasons were exceptional,” Dr. Gaveau said. “But there were no precise quantitative assessments to justify these claims.”

He and his colleagues analyzed satellite data to map the burned area each summer from 1982 to 2020. Over that time, a total of nearly 23 million acres burned. The researchers found that together, 2019 and 2020 accounted for nearly half of the total. “The burning was much, much higher than in the last 40 years,” Dr. Gaveau said. The study was published in the journal Science.

They then looked at factors that affect wildfire risk, including the length of the growing season (which results in more vegetation available to burn) and air and surface temperatures (warm conditions dry out the vegetation, making it easier to burn) and found that these have increased over the decades.

Those and other factors “are causing what we’re seeing — an increase in areas of burning,” he said.

In 2019 and 2020, average summer temperatures in the Siberian Arctic have been above 10 degrees Celsius, or 50 degrees Fahrenheit. Dr. Gaveau said that 10 degrees could be a tipping point, or threshold, beyond which wildfire activity greatly increases with just a small increase in temperature.

“It’s worrying because predictions essentially indicate that the fires of 2019, 2020 will become the norm by the end of the century,” he said.

They estimated that the fires of 2019 and 2020, which burned large areas of peatland, resulted in the release of more than 400 million metric tons of carbon dioxide, which is greater than the total annual emissions of Australia. With more extreme fire years, Dr. Gaveau said, “there’s going to be much more carbon released into the atmosphere every year because of global warming in a region that would not normally burn as much.”

Brendan M. Rogers, who studies the effect of climate change on Arctic ecosystems at Woodwell Climate Research Center in Massachusetts and was not involved in the study, said the findings “are adding to the story that we keep seeing year by year and expect to keep happening as long as the planet is warming.”

“We’re just getting more fires in these systems and they’re emitting carbon.”

The fires are also affecting the permafrost, permanently frozen ground that underlies much of the Siberian Arctic. The organic matter in the thawed ground begins to decompose, releasing carbon dioxide and methane, but it can also dry out and eventually burn, resulting in even more emissions.

The study “adds to the urgency of reducing emissions,” Dr. Rogers said, with global climate talks to take place next week in Egypt. It also reinforces what he and other climate scientists have been saying: Emissions from thawed permafrost and Arctic wildfires currently are not fully accounted for in global carbon budgets, and need to be, because these emissions will affect how much countries need to reduce emissions from fossil-fuel burning to limit global warming.

A separate study published in Science looked at factors that drove the extreme fire season of 2021, in addition to 2019 and 2020.

Rebecca C. Scholten of Vrije Universiteit Amsterdam and colleagues found that earlier snowmelt was an important contributor. Over the past half-century, spring snowmelt in northeastern Siberia has started an average of 1.7 days earlier per decade. An earlier snowmelt leads to a longer period when soil and vegetation dry out, increasing the risk of burning.

The researchers also found that changes in the polar jet stream that circles the planet most likely contributed to greater fire activity. During many weeks when extreme fires occurred, the jet stream was temporarily split in two, with a northerly branch and a more southerly one. Referred to as an Arctic front jet, it is marked by a region of lower-level air that is stationary and allows heat to build up, increasing fire risk.

This divergent jet stream is the same phenomenon that scientists say likely contributes to increasing heat waves in Europe.

Dr. Scholten said the research showed that the two factors worked together.

“It’s a compound effect,” she said. “It’s only if we have early snowmelt, which we have more with climate warming, and then if we have an Arctic front jet, which we also have more frequently with climate warming, then we have like really extreme fire risk.”

Sumber: www.nytimes.com

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