氣候變遷的原因不是太陽，而是浮游生物？
對於過往氣候候變遷的全新觀點
科學家認為冰河期之類的長期氣候波動，是因為地球軌道參數的變動引發，比方說地球自轉軸的傾角會以40000年的週期變化。德國基爾亥姆霍茲海洋研究中心(GEOMAR)領導的海洋科學家團隊，運用新的模型顯示海洋和大氣之間的生地化作用，也可以造成同樣時間尺度下的氣候波動。此研究最近發表於知名期刊《自然―地球科學》(Nature Geoscience)。
一群浮游生物。圖片來源:
A. Stuhr, GEOMAR (CC BY 4.0)
地球氣候史的特徵之一是週期出現的變化，科學家通常歸因於到達地表的太陽輻射。從地質時間來看日射量並非恆常不變，而是受控於地球軌道參數的週期性變動。其中一個重要參數是地球自轉軸的傾斜程度(傾角)，隨著時間經過大概會以40000年的週期來回擺動。在白堊紀和其他地質時代的沉積物中，化學與同位素訊號的記錄顯示了當時的氣溫與碳循環也以該週期規律變動。科學家認為在記錄氣候的地質文獻中，可以觀察到40000年週期是因為地軸傾角造成了日射量變化，進而影響地表溫度、海洋大氣循環、水文循環、生物圈，最終影響到碳循環。這個標準理論的其中一個問題是，地軸傾角造成的全球日射量變化其實相當微小，因此必須要有正回饋機制來放大此效應，才能影響到全球氣候，但我們對這些機制卻所知甚少。
來自德國基爾的科學家團隊最近根據海洋生物圈的新數值模型，提出了十分不同的觀點。該模型模擬了海洋浮游生物量的周轉(turnover)過程，並解析相關的微生物氧化還原作用，此作用控制了海洋的溶氧、硫化物、營養鹽和浮游生物的現存量(standing
stocks)。模型的實驗結果讓科學家十分訝異，他們發現利用生地化模型加上白堊紀的海洋環流模型，不需要引入地軸傾角的作用力，氣候自身就能維持40000年的循環。
結果最近發表在《自然―地球科學》，主要作者GEOMAR的教授Klaus
Wallmann博士解釋：「我們的模型顯示碳循環主要受控於生活在海洋表層的浮游生物。」浮游生物進行光合作用可以消耗大氣中的二氧化碳；微生物分解浮游生物之後則把二氧化碳釋放回去。由於二氧化碳是造成溫室效應的強力氣體，這種由生物造成的二氧化碳流動過程便會影響到地表氣溫和全球氣候。而浮游生物能否生長則受控於海中的營養鹽多寡，牽涉到一連串由微生物進行的氧化還原作用。
共同作者，GEOMAR的Sascha
Flögel博士表示：「我們把新的生地化模型跟白堊紀海洋環流的模型結合起來，光憑這樣氣候就能維持40000年的變化週期，而不需要引入地軸傾角的作用力。」他繼續解釋：「我們認為40000年循環是由一連串的正負回饋作用交織而成，源自於海水中的氮、磷、鐵、硫會隨著氧氣而變動。白堊紀海洋沉積物記錄中的化學和同位素數據，呈現出來的週期性變化也跟我們的模擬結果相符。」
若由此觀點看待氣候變遷，則其中的因果關係會和標準軌道理論大相逕庭：決定大氣中二氧化碳分壓有多少，進而設定氣候如何變化以及規模多大的因素，不是日射量，而是海洋生物圈。Wallmann教授表示：「我們的新理論有觀測數據的支持，而且也和我們對海洋生地化循環的理解一致。」
參與這項研究的基爾大學教授Wolfgang
Kuhnt博士總結：「雖然如此，地軸傾角和其他軌道參數對日射量的微小影響，還是有可能會被其他正回饋機制放大，進而影響全球氣候。因此地質記錄寫下的週期性氣候變遷，或許同時反映了生物圈的呼吸過程，以及地球系統對軌道和日射量等外部作用力的反應。」
Plankton as a climate
driver instead of the sun?
A new view on past climate
change
Fluctuations in the orbital parameters of
the Earth are considered to be the trigger for long-term climatic fluctuations
such as ice ages. This includes the variation of the inclination angle of the
Earth's axis with a cycle of about 40,000 years. Kiel-based marine scientists
lead by GEOMAR Helmholtz Centre for Ocean Research Kiel have shown by using a
new model that biogeochemical interactions between ocean and atmosphere could
also be responsible for climate fluctuations on this time scale. The study was
recently published in the renowned journal
Nature Geoscience.
The climate history of the earth is marked by
periodic changes that are usually ascribed to the solar radiation reaching the
surface of the earth. This insolation is not constant over geological time but
modulated by cyclic changes in the earth’s orbital parameters. One of the key
parameters affecting insolation is the tilt of the earth’s rotation axis (obliquity)
that changes periodically over time with a cycle length of about 40 000 years.
Chemical and isotopic signatures of sediments that were deposited during the
Cretaceous and other periods of earth’s history document regular changes in
temperature and carbon cycling on this time scale. The 40 kyr cycles observed
in the geological climate archives are believed to be the result of
obliquity-triggered insolation changes affecting the surface temperature, the
circulation of ocean and atmosphere, the hydrological cycle, the biosphere, and
ultimately the carbon cycle. One of the problems with this standard theory is
that changes in global insolation are very small and have to be amplified by
poorly understood positive feedback mechanisms to affect global climate.
A group of scientists from Kiel, Germany propose a
very different perspective that emerges from a new numerical model of the
marine biosphere. It simulates the turnover of plankton biomass in the ocean
and resolves the associated microbial oxidation and reduction reactions
controlling the standing stocks of dissolved oxygen, sulfide, nutrients and
plankton in the ocean. In their model experiments the scientists found surprisingly
a self-sustained 40 kyr climate cycle using the biogeochemical model integrated
in a circulation model of the Cretaceous Ocean without applying obliquity
forcing.
“In our model, the carbon cycle is largely controlled
by plankton living in the surface ocean”, explains Prof. Dr. Klaus Wallmann
from GEOMAR, lead author of the study which was recently published in Nature Geoscience. Plankton consumes
atmospheric CO2 via photosynthesis and by microorganisms that degraded
plankton biomass and release CO2 back into the atmosphere. Since CO2
is a potent greenhouse gas, the biological CO2 turnover affects
surface temperatures and global climate. The growth of plankton is controlled
by nutrients that take part in a range of microbial oxidation and reduction
reactions.
“We have integrated this new biogeochemical model in
a circulation model of the Cretaceous Ocean, and it creates a self-sustained 40
kyr climate cycle without applying obliquity forcing”, says Dr. Sascha Flögel,
co-author from GEOMAR. “From our perspective, the cycle is induced by a web of
positive and negative feedbacks that are rooted in the oxygen-dependent
turnover of nitrogen, phosphorus, iron and sulfur in the ocean. Chemical and
isotopic data recorded in sediments deposited in the Cretaceous Ocean show
periodic changes that are consistent with the model results”, Flögel
continues
In this new view on climate change, the relationship
between causes and effects is radically different from the standard orbital
theory. The marine biosphere rather than insolation is setting the pace and
amplitude by controlling the partial pressure of CO2 in the
atmosphere. “Our new theory is supported by observations and consistent with
our understanding of biogeochemical cycles in the ocean”, according to Prof.
Wallmann.
“However obliquity and other orbital parameters may
also affect global climate change when their delicate effects on insolation are
amplified by positive feedback mechanisms. Therefore, the periodic climate
change documented in the geological record may reflect both the breath of the
biosphere and the response of the earth system to external orbital and
insolation forcing”, summarizes Prof. Dr. Wolfgang Kuhnt from Kiel University
who participated in this study.
原始論文：Klaus
Wallmann, Sascha Flögel, Florian Scholz, Andrew W. Dale, Tronje P. Kemena,
Sebastian Steinig, Wolfgang Kuhnt. Periodic changes in the Cretaceous
ocean and climate caused by marine redox see-saw. Nature Geoscience,
2019; DOI: 10.1038/s41561-019-0359-x
引用自：Helmholtz Centre for Ocean Research Kiel
(GEOMAR). " Plankton as a climate driver
instead of the sun?"
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