By Matthew
Carroll
岩石、雨水、二氧化碳透過稱為「風化作用」的過程，數千年下來可以協助調控地球的氣候，就像是一種恆溫裝置。最近由賓州州立大學的科學家主持的新研究，或許可以讓我們更加了解這種恆溫裝置隨著溫度變化會出現什麼樣的反應。
「由於生命在地球上已經存在了數十億年，因此我們確定地球溫度一直以來還算穩定，足以使液態水存在並滋養生命，」Susan
Brantley表示。她是哥廷根大學與賓州大學地質科學系的教授。「理論指出矽酸岩的風化作用便是這道恆溫裝置，但是從來沒有人可以真的確定它對於溫度有多敏感。」
研究人員表示由於風化作用有許多因素參與在內，因此很難單獨運用實驗室的結果來估計風化作用整體會如何隨著溫度變化而改變。
該團隊整合實驗室的測量值、世界各地45個土壤場址的分析結果以及許多集水區的資料，對於地球上主要岩石類型的風化作用有了更深的瞭解，並且運用這些結果來估計風化作用整體對於溫度的反應。
「當你把實驗室試驗跟土壤或河川的樣品比對會得到不同的數值，」Brantley表示。「因此我們在這項研究所進行的嘗試，便是把各種空間尺度都含括進來，並且想辦法將全世界地球化學家長年累積下來，關於風化作用的所有數據做個合理的解釋。這篇論文便是關於我們方法的模型。」
風化作用是地球大氣的二氧化碳平衡過程中的一部份。在地球歷史上火山不斷地往大氣噴出許多二氧化碳，但是這些溫室氣體會被風化作用緩緩移除，地球才沒有因此而變成一座暖爐。
科學家表示雨水可以帶走大氣中的二氧化碳而形成一種弱酸，其落到地表後能侵蝕矽酸鹽，隨之產生的副產物由河川溪流帶到海洋，最後碳會在此被封存在沉積岩當中。
「科學家長久以來推測從數百萬年來看，二氧化碳從火山進到大氣裡的量會和風化作用從大氣中移除的量達到平衡，使得地球的溫度處在相對穩定的狀態，」Brantley表示。「其中的關鍵是當有較多二氧化碳進入大氣而讓地球變暖的時候，風化作用也會跟著加速而抽出更多二氧化碳。反之地球較冷的時候，風化的速度也會跟著減緩下來。」
但是風化作用對於溫度變化究竟有多敏感還是有很多未知之處，部分是因為風化作用牽扯到的時間與空間尺度都很廣大。
「當你看著土壤剖面，就像是看著相機對著土壤曝光大約一百萬年拍出來的相片——這是百萬年來許多作用累積起來的結果，而我們試著跟它比較的對象，卻是在燒杯中進行只有兩年的實驗，」Brantley表示。
Brantley表示「關鍵區」科學有助於科學家更加瞭解影響風化的複雜交互作用。該領域探討的空間上達最高的植被，下達最深的地下水。
比方說，岩石必須有裂縫雨水才能滲進來，然後開始分解其組成。如果岩石要產生破裂，跟外界接觸的表面積就得夠大才行，這在土壤深厚的地區就很難發生。
「在開始綜觀不同的時間與空間尺度之後，才能看出真正重要的因素是什麼，」Brantley表示。「表面積就真的很重要。要解答風化速率你需要的任何常數都可以在實驗室測量出來，但除非你能告訴我岩石在外面的自然系統中的表面積是多少，不然絕對無法對真實的系統進行預測。」
他們發表在期刊《自然》(Science)的研究結果指出從實驗室測出的溫度敏感度比土壤和河川得出的估計值還低。利用實驗室和野外場址的觀察結果，他們將他們的發現外推，估計從全球尺度來看風化作用跟溫度之間有什麼樣的關係。
他們的模型或許能幫助我們瞭解風化作用在未來隨著氣候變遷會有什麼樣的反應，也能幫我們評估藉由人為增加風化速率，從大氣中吸收更多二氧化碳的方法，比方說碳封存。
「有個想法是把許多岩石挖出來磨碎之後，運送到荒地棄置，如此便能讓風化作用發生而加速風化進行，」Brantley表示。「這是可行的——事實上也正在發生。問題是這道過程實在太慢了。」
科學家表示雖然暖化可以加速風化，但是要把人類加到大氣中的二氧化碳全部吸收掉，可能要花上數千年甚至是數百萬年。
參與這項研究的其他賓州州立大學科學家包括地質科學系的博士候選人Andrew
Shaughnessy、地球與環境系統研究所的資深科學家Marina
Lebedeva與Victor
Balashov。
Study reveals new clues about how
'Earth's thermostat' controls climate
Rocks, rain and carbon dioxide help
control Earth’s climate over thousands of years — like a thermostat — through a
process called weathering. A new study led by Penn State scientists may improve
our understanding of how this thermostat responds as temperatures change.
“Life has been on this planet for billions of years,
so we know Earth’s temperature has remained consistent enough for there to be
liquid water and to support life,” said Susan Brantley, Evan Pugh University
Professor and Barnes Professor of Geosciences at Penn State. “The idea is that
silicate rock weathering is this thermostat, but no one has ever really agreed
on its temperature sensitivity.”
Because many factors go into weathering, it has been
challenging to use results of laboratory experiments alone to create global
estimates of how weathering responds to temperature changes, the scientists
said.
The team combined laboratory measurements and soil
analysis from 45 soil sites around the world and many watersheds to better
understand weathering of the major rock types on Earth and used those findings
to create a global estimate for how weathering responds to temperature.
“When you do experiments in the laboratory versus
taking samples from soil or a river, you get different values,” Brantley said.
“So what we tried to do in this research is look across those different spatial
scales and figure out how we can make sense of all this data geochemists around
the world been accumulating about weathering on the planet. And this study is a
model for how we can do that.”
Weathering represents part of a balancing act of
carbon dioxide in Earth’s atmosphere. Volcanoes have emitted large amounts of
carbon dioxide through Earth’s history, but instead of turning the planet into
a hot house, the greenhouse gas is slowly removed via weathering.
Rain takes the carbon dioxide from the atmosphere and
creates a weak acid that falls to Earth and wears away silicate rocks the
surface. The byproducts are carried by streams and rivers to the ocean where
the carbon is eventually locked away in sedimentary rocks, the scientists said.
“It has long been hypothesized that the balance
between carbon dioxide entering the atmosphere from volcanoes and being pulled
out by weathering over millions of years holds the temperature of the planet
relatively constant,” Brantley said. “The key is when there is more carbon
dioxide in the atmosphere and the planet gets hotter, weathering goes faster
and pulls more carbon dioxide out. And when the planet is cooler, weathering
slows down.”
But much remains unknown about how sensitive
weathering is to changing temperatures, partly because of the long spatial and
time scales involved.
“In a soil profile, you are seeing a picture of soil
where the camera shutter was open for sometimes a million years — there are
integrated processes happening for a million years and you’re trying to compare
that with a two-year flask experiment,” Brantley said.
Brantley said the field of critical zone science —
which examines landscapes from the tallest vegetation to the deepest groundwater
— has helped scientists better understand the complex interactions that
influence weathering.
For example, rocks must fracture for water to get in
cracks and start breaking down the materials. For that to happen, the rock must
have large, exposed surface areas, and that is less likely to happen in regions
where soil is deeper.
“It’s only when you start crossing spatial and time
scales that you start seeing what’s really important,” Brantley said. “Surface
area is really important. You can measure all the rate constants you want for
that solution in the lab, but until you can tell me how does surface area form
out there in the natural system, you are never going to be able to predict the
real system.”
The scientists reported in the journal Science that temperature sensitivity
measurements in the laboratory were lower than estimates from soils and rivers
in their study. Using observations from the lab and field sites, they upscaled
their findings to estimate the global temperature dependance of weathering.
Their model may be helpful for understanding how
weathering will respond to future climate change, and in evaluating man-made
attempts to increase weathering to draw more carbon dioxide from the atmosphere
— like carbon sequestration.
“One idea has been to enhance weathering by digging
up a lot of rock, grinding it, transporting it and putting it out in the fields
to let weathering happen,” Brantley said. “And that will work — it’s already
working. The problem is, it’s a very slow process.”
Though warming may speed up weathering, pulling all
the carbon dioxide out of the atmosphere that humans have added could take
thousands or hundreds of thousands of years, the scientists said.
Other Penn State researchers who participated on the
study were Andrew Shaughnessy, doctoral candidate in the Department of
Geosciences and Marina Lebedeva and Victor Balashov, senior scientists in the
Earth and Environmental Systems Institute.
The National Science Foundation and the Hubert L.
Barnes and Mary Barnes Professorship supported this work.
原始論文：S. L.
Brantley, Andrew Shaughnessy, Marina I. Lebedeva, Victor N. Balashov. How
temperature-dependent silicate weathering acts as Earth’s geological thermostat. Science,
2023; 379 (6630): 382 DOI: 10.1126/science.add2922
引用自：Penn State. "Study reveals new clues about
how 'Earth's thermostat' controls climate."
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