北極圏ツンドラ、ＣＯ２排出が吸収上回る ＮＯＡＡ報告

【ワシントンＡＦＰ＝時事】過去数千年にわたり大気中の二酸化炭素（ＣＯ２）を吸収・貯蔵してきた北極圏のツンドラ（凍土帯）が、温暖化に伴う永久凍土の融解と山火事の発生件数増加によってＣＯ２の排出源に急激に変わりつつあるとする報告書を米海洋大気局（ＮＯＡＡ）が１０日、発表した。（写真はロシア・ヤクーツクから南へ約７０ｋｍの集落付近で湖から氷を取る村人）
　ＮＯＡＡのリック・スピンラッド局長は、「私たちの観測によると、温暖化と山火事の増加に伴い、北極圏のツンドラでは現在、ＣＯ２の放出量が貯蔵量よりも多い。それにより、気候変動への影響も悪化するだろう」と指摘した。
　発表は、２００１～２０２０年までの観測記録の平均値に基づいている。
　温暖化は北極圏に二重の影響を及ぼす。植物の成長を促し、大気中からのＣＯ２除去が進む一方で、地表の温度を上昇させ、永久凍土の融解を引き起こす。
　永久凍土が融解すると、凍土に閉じ込められていたＣＯ２が放出され、さらに微生物の分解によって温室効果ガスのＣＯ２やメタンが放出される。
　同報告書によれば、アラスカの永久凍土では今年、観測史上２番目に高い温度が記録された。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2024/12/11-20:58）

After locking carbon dioxide in its frozen soil for millennia, the Arctic tundra is undergoing a dramatic transformation, driven by frequent wildfires that are turning it into a net source of carbon dioxide emissions, a US agency said Tuesday.
This stark shift is detailed in the National Oceanic and Atmospheric Administration's 2024 Arctic Report Card, which revealed that annual surface air temperatures in the Arctic this year were the second-warmest on record since 1900.
Our observations now show that the Arctic tundra, which is experiencing warming and increased wildfire, is now emitting more carbon than it stores, which will worsen climate change impacts, said NOAA Administrator Rick Spinrad.
What happens in the Arctic doesn't stay in the Arctic, Anna Virkkala of the Woodwell Climate Research Center, who co-authored the report, added to AFP. We should try to stop anthropogenic climate change as soon as possible, so that we can also stop the emissions from the Arctic eventually as well.
The finding is based on an average of observations recorded from 2001-2020.
Climate warming exerts dual effects on the Arctic. While it stimulates plant productivity and growth, which removes carbon dioxide from the atmosphere, it also leads to increased surface air temperatures that cause permafrost to thaw.
Thawing permafrost releases carbon previously trapped in frozen soil as carbon dioxide and methane -- two potent greenhouse gases -- through microbial decomposition.
In 2024, Alaska recorded its second-warmest permafrost temperatures on record, the report said.
Human-caused climate change is also intensifying high-latitude wildfires, which have increased in burned area, intensity, and associated carbon emissions.
Wildfires not only combust vegetation and soil organic matter, releasing carbon into the atmosphere, but they also strip away insulating soil layers, accelerating long-term permafrost thaw and its associated carbon emissions.
Since 2003, circumpolar wildfire emissions have averaged 207 million tons of carbon annually, according to NOAA. At the same time, Arctic terrestrial ecosystems have remained a consistent source of methane.
Last year, 2023, was the largest fire year on record due to Canadian wildfires, which burned more than twice any other year on record in Canada, report co-author Brendan Rogers said during a press conference.
The fires emitted nearly 400 million tons of carbon -- more than two-and-a-half times the emissions from all other sectors in Canada combined, he added.
Meanwhile, 2024 ranked as the second-highest year for wildfire emissions within the Arctic Circle.
- 'Alarming harbinger' -
Asked whether the Arctic's shift from carbon sink to source might be permanent, Rogers said it remains an open question. While boreal forests further south still serve as carbon sinks, northern regions are of greater concern.
The best as we can tell, permafrost emissions are not going to dwarf fossil fuel emissions, but they are a significant layer, and so they need to be accounted for, he told AFP, adding that aggressively limiting human-caused warming would stem the problem to some extent.
Reacting to the news, Brenda Ekwurzel, a climate scientist at the Union of Concerned Scientists said that the climate catastrophe we're seeing in the Arctic is already bringing consequences for communities around the world.
The alarming harbinger of a net carbon source being unleashed sooner rather than later doesn't bode well. Once reached, many of these thresholds of adverse impacts on ecosystems cannot be reversed.
Warmer temperatures are impacting wildlife too, with the report finding tundra caribou numbers have decreased by 65 percent over the past two to three decades -- with summer heat disrupting their movements and survival, alongside changes to winter snow and ice conditions.
Surprisingly, however, Alaska's ice seal populations remain healthy.
The report found no long-term negative impacts on body condition, age of maturity, pregnancy rates, or pup survival for the four species of ice seals -- ringed, bearded, spotted, and ribbon -- inhabiting the Bering, Chukchi, and Beaufort seas.