美國太空總署（NASA）和國家大氣研究中心（NCAR）的研究顯示，熱帶雨林吸收二氧化碳的能力超乎科學家想像，甚至比溫帶森林更高。
用二氧化碳「灌溉」 熱帶雨林效果最佳
隨著人類燃燒化石燃料、碳排放越來越高，全球森林利用這些二氧化碳快速生長，減少大氣中的二氧化碳，這個過程叫做「二氧化碳肥料效應」（carbon dioxide fertilization）。
「溫帶森林和熱帶森林誰能吸收較多二氧化碳？這個問題的答案能幫助我們瞭解全球陸地生態系統是否能持續抵消人類二氧化碳排放，還是會開始惡化氣候變遷。」研究共同作者、NCAR科學家Britton Stephens說。
目前，森林和其他陸地植被的光合作用吸收了30%的人為排放二氧化碳。研究作者指出，如果森林吸收速度減緩，全球暖化便會加速。
據作者估計，全球森林吸收25億公噸二氧化碳，其中熱帶森林就佔了14億公噸。
更新全球氣候模型 新研究顛覆過去認知
過去十年間的大氣模型有長足的進步，但由於全球氣流模型的差異，學界對於二氧化碳的大氣逆模型仍有些不同看法。
25年來，大部分大氣逆模型顯示，北半球中緯度森林吸收的二氧化碳比熱帶森林要多。研究作者認為，這是因為過去對全球氣流的認識已經過時，並根據有限的資料假設森林砍伐造成熱帶森林釋放二氧化碳多於吸收，才會有這樣的結果。
新研究首次比較相同規模的二氧化碳吸收數據，並囊括多種規模，包括生態系統過程的電腦模型、大氣逆模型、衛星影像和實驗林等資料。研究人員整合這些分析，加上蒐集自加權平均大氣生態系統模型以及地面資料的熱帶碳吸收數據。
「在我們之前，沒有人成功完成全球性的二氧化碳效應資訊整合，納入大氣、森林和各種模擬的二氧化碳效應。這些看似不同的獨立數據，似乎開始導向相同的答案。」NASA噴氣推進實驗室學者David Schimel說。
二氧化碳肥料效應 首份完整研究資料問世
2000年代中期，Stephens以飛行器測量的二氧化碳數據顯示，許多大氣逆模型並沒有精確呈現二氧化碳在大氣中的流動。
與飛行器測量結果較吻合的模型，更能精確顯示熱帶雨林的二氧化碳吸收量。不過之前缺乏全球性的資料集，難以驗證熱帶雨林二氧化碳吸收量較大的主張。
Schimel說，他們的新研究利用繼Stephens之後眾多其他科學家的研究結果，將全國性和區域性的數據整合成有力的全球資料集。科學家也必須將野火納入考量。氣候變遷減少部分地區的水源，並讓地球變暖，導致旱災頻繁和更大規模的野火。在熱帶地區，人類砍伐森林焚燒木材都會讓問題更嚴重。野火燃燒導致樹木減少，進而減少碳吸收，同時釋放大量的碳進入大氣。
「這份研究可說是完整的二氧化碳肥料效應理論，以微觀現象到看似矛盾的宏觀現象為基礎，包含光合作用的機制到全球性的觀察。」
Tropical forests have a bigger appetite for the greenhouse gas carbon dioxide than many scientists have believed – even greater than that of temperate forests, finds a new study led by NASA and the National Center for Atmospheric Research, NCAR.
As human-caused emissions from burning fossil fuels add more carbon dioxide to the atmosphere, forests worldwide are using that CO2 to grow faster, reducing the amount of the greenhouse gas that stays airborne. This effect is called carbon dioxide fertilization.
Forests and other land vegetation now remove up to 30 percent of human carbon dioxide emissions from the atmosphere by absorbing CO2 during photosynthesis. If the rate of absorption were to slow down, the rate of global warming would speed up, the authors say.
Tropical forests absorb 1.4 billion metric tons out of a total global absorption of 2.5 billion metric tons, the authors estimate.
The question of whether temperate or tropical forests absorb the most CO2 “is not just an accounting curiosity,” said NCAR scientist Britton Stephens, a co-author on the paper. “It has big implications for our understanding of whether global terrestrial ecosystems might continue to offset our carbon dioxide emissions or might begin to exacerbate climate change.”
For about 25 years, most atmospheric inverse computer models have shown that mid-latitude forests in the Northern Hemisphere absorb more CO2 than tropical forests.
Now the scientists on this study say that result was based on an outdated understanding of global air flows and on limited data suggesting that deforestation was causing tropical forests to release more carbon dioxide than they were absorbing.
The new study is the first to make apples-to-apples comparisons of CO2 uptake estimates from many sources at different scales, such as computer models of ecosystem processes, atmospheric inverse models, satellite images and data from experimental forest plots, among other sources.
The researchers reconciled these analyses and obtained their new estimate of the tropical carbon absorption from the weighted average of atmospheric, ecosystem model, and ground-based data.
“Until our analysis, no one had successfully completed a global reconciliation of information about carbon dioxide effects from the atmospheric, forestry, and modeling communities,” said co-author Joshua Fisher of the Jet Propulsion Laboratory. “It is incredible that all these different types of independent data sources start to converge on an answer.”
In the mid-2000s, Stephens used measurements of CO2 made from aircraft to show that many atmospheric inverse models were not correctly representing flows of carbon dioxide in the air above ground level.
Models that matched the aircraft measurements better showed more carbon dioxide absorption in the tropical forests. But there were still not enough global data sets to validate the idea of large tropical-forest absorption.
Schimel said their new study takes advantage of a great deal of work other scientists have done since Stephens’ paper to pull together national and regional data of various kinds into robust, global data sets.
“What we’ve had up till this paper was a theory of carbon dioxide fertilization based on phenomena at the microscopic scale and observations at the global scale that appeared to contradict those phenomena,” said David Schimel of NASA’s Jet Propulsion Laboratory in Pasadena, California.
“Here, at least, is a hypothesis that provides of a consistent explanation that includes both how we know photosynthesis works and what’s happening at the planetary scale.”
Atmospheric models have improved over the past decade, but there is still disagreement among atmospheric inverse estimates of the distribution of carbon uptake, owing to remaining differences in modeling global air flows.
The scientists must also take wildfires into account. Climate change decreases water availability in some regions and makes Earth warmer, leading to more frequent droughts and larger wildfires. In the tropics, humans compound the problem by burning wood during deforestation. Fires stop carbon absorption by killing trees and also emit huge amounts of carbon into the atmosphere as the wood burns.
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