Land ecosystems currently play a key role in mitigating climate change. The more carbon dioxide (CO2) plants and trees absorb during photosynthesis, the process they use to make food, the less CO2 remains trapped in the atmosphere where it can cause temperatures to rise.
目前，在減緩氣候變遷上，土地生態體系扮演一種關鍵角色。在植物及樹木行光合作用(它們用來製造食物的過程)期間，吸收愈多二氧化碳(CO2)，愈少CO2依然被陷於，會導致溫度上升的大氣中。
But scientists have identified an unsettling trend –as levels of CO2 in the atmosphere increase, 86 percent of land ecosystems globally are becoming progressively less efficient at absorbing it.
不過，科學家們已經確認一種，令人不安的趨勢。隨著大氣中CO2含量增加，全球86％的陸地生態體系，在吸收CO2的效率上，正逐漸降低中。
Because CO2 is a main ‘ingredient’ that plants need to grow, elevated concentrations of it cause an increase in photosynthesis, and consequently, plant growth – a phenomenon aptly referred to as the CO2 fertilization effect, or CFE.
因為，CO2是植物生長所需的主要‘成分’。升高的CO2濃度會導致光合作用增加，從而導致植物生長。這是一種，適切地被稱為CO2肥沃化效應，也就是CFE的事象。
CFE is considered a key factor in the response of vegetation to rising atmospheric CO2 as well as an important mechanism for removing this potent greenhouse gas from our atmosphere – but that may be changing.
除了是從咱們大氣中，移走此強力溫室氣體的一種重要機制之外，CFE也被認為是植物，對上升之大氣CO2作出反應的一種關鍵因素。不過，那可能正改變中。
For a new study published Dec. 10 in Science, researchers analyzed multiple fields, satellite-derived and model-based datasets to better understand what effect increasing levels of CO2 may be having on CFE. Their findings have important implications for the role plants can be expected to play in offsetting climate change in the years to come.
為了一項發表於2020年12月10日版《科學》期刊的新研究，研究人員們分析了多個野地、源自衛星及以模型為基礎的數據集，來更深入瞭解升高之CO2水平，對CFE可能產生中的影響。就未來幾年，在抵消氣候變遷上，植物能被預期扮演的角色而言，他們的研究發現具有諸多重要的意涵。
“In this study, by analyzing the best available long-term data from remote sensing and state-of-the-art land-surface models, we have found that since 1982, the global average CFE has decreased steadily from 21 percent to 12 percent per 100 ppm of CO2 in the atmosphere,” said Ben Poulter, study co-author and scientist at NASA’s Goddard Space Flight Center. “In other words, terrestrial ecosystems are becoming less reliable as a temporary climate change mitigator.”
該項研究合撰人，美國航太總署所屬戈達德太空飛行中心科學家，Ben Poulter宣稱：「在該項研究中，藉由分析來自遙感及最新型地表模型，可資使用的最佳長期數據。他們已經發現，自1982年以來，大氣中CO2的全球平均CFE，已經從每100 ppm的21％穩步下降至12％。換句話說，作為暫時性的氣候變遷減緩物，陸地生態體系的可靠性正下降中。」
Without this feedback between photosynthesis and elevated atmospheric CO2, Poulter said we would have seen climate change occurring at a much more rapid rate. But scientists have been concerned about how long the CO2 Fertilization Effect could be sustained before other limitations on plant growth kick in.
Poulter表示，沒有光合作用與升高之大氣CO2間的反饋，咱們將會看到氣候變遷，以更快很多的速度發生。不過，科學家們一直憂心，在針對植物生長開啟其他限制之前，CO2肥沃化效應能持續多久。
For instance, while an abundance of CO2 won’t limit growth, a lack of water, nutrients, or sunlight – the other necessary components of photosynthesis -- will. To determine why the CFE has been decreasing, the study team took the availability of these other elements into account.
譬如，儘管豐富的CO2不會限制生長。不過，缺乏水、養分或陽光(光合作用需要的其他成分)則會。為了斷言，為何CFE一直下降中，該研究團隊將，上述其他要素的可用性納入考量。
“According to our data, what appears to be happening is that there’s both a moisture limitation as well as a nutrient limitation coming into play,” Poulter said. “In the tropics, there’s often just not enough nitrogen or phosphorus, to sustain photosynthesis, and in the high-latitude temperate and boreal regions, soil moisture is now more limiting than air temperature because of recent warming.”
Poulter宣稱：「根據他們的數據，顯然正在發生的是，發揮作用有濕度限制及養分限制兩者。在熱帶地區，氮或磷往往不够維持光合作用，而在高緯度的溫帶及北方地區，由於最近的暖化，目前土壤濕度是比空氣溫度更為限制的。」
In effect, climate change is weakening plants’ ability to mitigate further climate change over large areas of the planet.
事實上，氣候變遷正在削弱植物，於地球諸多大區域上，減緩氣候變遷的能耐。
The international science team found that when remote-sensing observations were taken into account – including vegetation index data from NASA's Advanced Very High Resolution Radiometer (AVHRR) and the Moderate Resolution Imaging Spectroradiometer (MODIS) instruments – the decline in CFE is more substantial than current land-surface models have shown.
該國際科學團隊發現，當包括來自美國航太總署所屬，先進超高解析度輻射計(AVHRR)及中解析度成像光譜輻射計(MODIS)等，儀器之植物指標數據的遙感觀測被納入考量時，CFE的下降比目前陸地表面模型，已經顯示的更為可觀。
Poulter says this is because modelers have struggled to account for nutrient feedbacks and soil moisture limitations – due, in part, to a lack of global observations of them.
Poulter表示，這是因為模型製作者一直極力於，說明養分反饋及土壤濕度限制。部分，由於缺乏有關它們的全球觀察。
“By combining decades of remote sensing data like we have done here, we’re able to see these limitations on plant growth. As such, the study shows a clear way forward for model development, especially with new remote sensing observations of vegetation traits expected in coming years,” he said. “These observations will help advance models to incorporate ecosystem processes, climate and CO2 feedbacks more realistically.”
他宣稱：「藉由結合，如同他們在這裡已經進行之數十年的遙感數據。他們能發現，此些對植物生長的限制。就此，該項研究為模型開發，闡明了一種明確的方法。特別是，使用預期於未來幾年、有關植被特徵的新遙感觀測。這些觀察將有助於提升模型，來更實際地納入生態體系過程、氣候及CO2反饋。」
The results of the study also highlight the importance of the role of ecosystems in the global carbon cycle. According to Poulter, going forward, the decreasing carbon-uptake efficiency of land ecosystems means we may see the amount of CO2 remaining in the atmosphere after fossil fuel burning and deforestation start to increase, shrinking the remaining carbon budget.
此些研究結果也強調了，生態體系的角色，在全球碳循環中的重要性。根據Poulter的說法，往後，陸地生態體系下降的碳吸收效率，意味著在燃燒化石燃料及砍伐森林之後，我們會發現大氣中，滯留的CO2數量開始增加，而縮減了剩餘的碳堆積。
“What this means is that to avoid 1.5 or 2°C warming and the associated climate impacts, we need to adjust the remaining carbon budget to account for the weakening of the plant CO2 Fertilization Effect,” he said. “And because of this weakening, land ecosystems will not be as reliable for climate mitigation in the coming decades.”
他宣稱：「這意味的是，為了避免暖化1.5或2°C及相關的氣候衝擊，我們必需調整剩餘的碳堆積，以解決植物CO2肥沃化效應的減弱。因為這種減弱，在減緩氣候變遷上，土地生態體系於未來幾十年中，將不會那麼可靠。」
原文網址：https://www.nasa.gov/feature/land-ecosystems-are-becoming-less-efficient-at-absorbing-co2
翻譯：許東榮
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