野火可以把碳封存起來數百年至數千年之久？
最近發表的一則研究顯示，野火製造出來的木炭可以把碳封存數百年，而有助於減緩氣候變遷。
大火正在肆虐北極圈。圖片來源：ITAR-TASS
News Agency/Alamy Live News
今年北極地區史無前例地爆發大規模的野火，並且排放相當大量二氧化碳的消息，登上了全世界的頭條新聞。
但是發表在《自然―地球科學》(Nature Geoscience)的新研究，計算出木炭具有一種相當重要的功能：可以抵銷野火排放到大氣中的碳。研究團隊表示這些木炭在接下來的歲月中，可以相當有效地封鎖住數量可觀的碳。
全世界的野火平均每年燒掉的面積等同於印度，排放到大氣中的二氧化碳比全球公路、鐵路、航運和空運排放量的總和還多。
焚毀的地區重新長出植被之後，可以經由光合作用重新吸收大氣中的二氧化碳。這是正常野火復原循環中的一部份，在草原地區整個循環可以在一年之內完成，而在適應火燒的森林中則可能長達數十年。
但在極區或熱帶的泥炭地這類極端例子中，可能經過數個世紀都還無法完全復原。
重新長出來的植被相當重要，因為沒有被植被吸收的碳會繼續留在大氣當中，造成氣候變遷。
尤其是砍伐森林時放的火會讓碳長期流失到大氣當中，使其成為氣候變遷相當重要的來源之一。
英國斯旺西大學和荷蘭阿姆斯特丹自由大學的研究人員最近進行的研究中，計算出野火產生的碳(稱為火成碳，pyrogenic
carbon)具有一種相當重要的功能：可以抵銷野火排放到大氣中的碳。
主要作者Matthew
Jones最近剛從斯旺西大學轉任至英國東英吉利大學的環境科學院。他說：「正常來說，野火燃燒期間排放出的二氧化碳會在植被長出之後重新封存起來，因此研究人員通常認為一旦生物量可以完全回復，那麼野火就能視為一種碳中和的事件。」
「不過，火焰燃燒時有些植被並不會被燒得精光，而是變成木炭。這些富含碳的物質可以保存在土壤或是海洋裡很長一段時間。」
「我們結合了實地研究、衛星資料、電腦模擬，更加精確地定量出以全球尺度來看，有多少碳在野火事件中保存下來。」
論文共同作者包括斯旺西大學的Cristina
Santin博士與Stefan
Doerr教授、阿姆斯特丹自由大學的Guido
van der Werf教授。結果說明了地表的野火不僅會釋放二氧化碳到大氣當中，植被含有的碳質也有很大一部分會被野火轉化成木炭和其他碳化物。
研究人員表示模擬全球野火排放出多少二氧化碳的時候，需要考慮這些火成碳。
Jones博士說：「我們的結果顯示從全球來看，野火製造的火成碳相當於其二氧化碳排放量的百分之12，因此火成碳可說是大幅減緩了野火產生的二氧化碳帶來的影響。」
「人們認為氣候暖化會讓野火在許多地方變得更加頻繁，特別是森林。這會增加野火排放到大氣的二氧化碳總量，但是以火成碳儲存下來的碳也會更多。如果未來數十年植被可以自然回復，這些排放到大氣中的二氧化碳，便能經由重新長出的植物而再次吸收，另外一部分則會以火成碳的形式留存在土壤、湖泊、海洋之中。」
「我們認為這些火成碳可以保存數百年到數千年。雖然它們最終還是會分解而回到大氣當中，但這段時間之內它們會被封存起來，無法對我們所處的氣候產生影響。」
「包括森林砍伐和某些泥沼地發生的火災在內，雖然人類活動排放出越來越多二氧化碳而對全球氣候持續造成嚴重威脅，但這項研究至少帶來了某些好消息。」
氣候變得更加溫暖且乾旱越常發生，會對未來全球的野火規模產生什麼影響，仍有某些重要的問題有待解答。
舉例來說，極區的泥炭地會更常發生跟今年夏天一樣的火災嗎？排放出來的二氧化碳中，又有多少會被之後長回來的植被重新吸收？
但是這項新研究顯示野火產生出來的火成碳應該視為一種相當重要的產物，而且也是全球碳循環中十分重要的一道環節。
論文「Global
fire emissions buffered by the production of pyrogenic carbon」發表於期刊《自然―地球科學》。
How wildfires trap carbon for
centuries to millennia
Charcoal
produced by wildfires could trap carbon for hundreds of years and help mitigate
climate change, according to new research published today.
The extensive and unprecedented outbreak of wildfires
in the arctic and the vast amounts of CO2 they are emitting have been hitting
the headlines across the world.
But a new Nature
Geoscience study quantifies the important role that charcoal plays in
helping to compensate for carbon emissions from fires. And the research team
say that this charcoal could effectively ‘lock away’ a considerable amount of
carbon for years to come.
In an average year, wildfires around the world burn
an area equivalent to the size of India and emit more carbon dioxide to the
atmosphere than global road, rail, shipping and air transport combined.
As vegetation in burned areas regrows, it draws CO2
back out of the atmosphere through photosynthesis. This is part of the normal
fire-recovery cycle, which can take less than a year in grasslands or decades
in fire-adapted forests.
In extreme cases, such as arctic or tropical
peatlands, full recovery may not occur for centuries.
This recovery of vegetation is important because
carbon that is not re-captured stays in the atmosphere and contributes to
climate change.
Deforestation fires are a particularly important
contributor to climate change as these result in a long-term loss of carbon to
the atmosphere.
Now, a new study by researchers at Swansea University
and Vrije Universiteit Amsterdam has quantified the important role that
charcoal created by fires - known as pyrogenic carbon - plays in helping to
compensate for carbon emissions.
Lead author Dr Matthew Jones, who recently joined the
UEA’s School of Environmental Sciences from Swansea Univsersity, said: “CO2
emitted during fires is normally sequestered again as vegetation regrows, and
researchers generally consider wildfires to be carbon neutral events once full
biomass recovery has occurred.
“However, in a fire some of the vegetation is not
consumed by burning, but instead transformed to charcoal. This carbon-rich
material can be stored in soils and oceans over very long time periods.
“We have combined field studies, satellite data, and
modelling to better quantify the amount of carbon that is placed into storage
by fires at the global scale.”
The paper, which was co-authored by Dr Cristina
Santin and Prof Stefan Doerr, from Swansea University, and Prof Guido van der
Werf, of Vrije Universiteit Amsterdam, explained that, as well as emitting CO2
to the atmosphere, landscape fires also transfer a significant fraction of
affected vegetation carbon to charcoal and other charred materials.
The researchers say this pyrogenic carbon needs to be
considered in global fire emission models.
Dr Jones said: “Our results show that, globally, the
production of pyrogenic carbon is equivalent to 12 per cent of CO2 emissions
from fires and can be considered a significant buffer for landscape fire
emissions.
“Climate warming is expected to increase the
prevalence of wildfires in many regions, particularly in forests. This may lead
to an overall increase in atmospheric CO2 emissions from wildfires, but also an
increase in pyrogenic carbon storage. If vegetation is allowed to recover
naturally then the emitted CO2 will be recaptured by regrowth in future
decades, leaving behind an additional stock of pyrogenic carbon in soils, lakes
and oceans.
“We expect any additional pyrogenic carbon to be
trapped for a period of centuries to millennia, and although it will eventually
return to the atmosphere as charcoal degrades, it is locked away and unable to
affect our climate in the meantime.
“This brings some good news, although rising CO2
emissions caused by human activity, including deforestation and some peatland
fires, continue to pose a serious threat to global climate.”
There are still important questions to be answered
about how a warmer, more drought-prone climate will affect the global extent of
wildfires in the future.
For example, will there be more fire in arctic
peatlands as we are experiencing this summer, and what proportion of CO2
emissions will be recaptured by future vegetation regrowth?
But this new research shows that pyrogenic carbon
production should be considered as a significant product of fires and an
important element of the global carbon cycle.
“Global fire emissions buffered by the production of
pyrogenic carbon” is published in the journal Nature Geoscience.
原始論文：Matthew W.
Jones, Cristina Santín, Guido R. van der Werf, Stefan H. Doerr. Global
fire emissions buffered by the production of pyrogenic carbon. Nature
Geoscience, 2019; DOI: 10.1038/s41561-019-0403-x
引用自：University of East Anglia. "How wildfires
trap carbon for centuries to millennia."
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