岩石風化難以快速調節全球溫度

原文網址：http://www.geologypage.com/2017/05/weathering-rocks-poor-regulator-global-temperatures.html

岩石風化難以快速調節全球溫度

岩石化學風化係指岩石遭到溶解之後，沖入河川，最終沉積在海洋的周而復始過程。一則由華盛頓大學進行的新研究顯示，教科書中對全球化學風化的理解，實際上並不像地質學家過往認為的跟地球溫度那麼相關。

碳循環是碳原子在大氣、海洋和岩石中流動的過程。5月22日刊登於開放取用期刊《自然通訊》(Nature Communications)的研究，探討了碳循環中的關鍵一部份。他們的結果質疑了在長時間尺度下，調控地球氣溫的作用中岩石所扮演的腳色。

通訊作者，華盛頓大學地球和太空科學的博士生Joshua Krissansen-Totton表示：「了解地球如何從恐龍時代的溫室氣候轉變成今日樣貌，有助於我們了解未來的氣候變遷長期下來會導致何種結果。」

目前我們對於地球氣候的理解是從數百萬年為區間來看，它會受控於一種跟岩石風化相關的自然恆溫機制。火山會排放二氧化碳到大氣當中，這種氣體接下來可能會溶進雨水並跟富含矽的陸地岩石反應，導致岩石的化學風化。溶解在水中的碳之後會隨著河流進入海洋，最終被封在海床上的含碳石灰岩裡面。

大氣中的二氧化碳作為一種強力溫室氣體，會把來自太陽的熱量困在地球大氣當中。當地球變暖，雨量增加以及岩石跟水之間的化學反應速率加快，會使得化學風化速率增加。長期下來，此作用會使得空氣的二氧化碳含量降低並冷卻地球，最終會讓地球氣候回到較適宜的溫度――這大概是教科書上描繪的整體圖像。

共同作者，華盛頓大學地球和太空科學的教授David Catling表示：「一般認為如果有更多的二氧化碳釋放到大氣中，風化速率就會增加，而讓二氧化碳含量和地球溫度回到適中範圍。這種長期控溫作用可以保護地球不會變得太冷或太熱。」

這篇新研究始於研究人員想要得知約莫35億至40億年前，當最早的生命出現在地球時，它們所處的環境條件。他們先以一段科學家暸解得較為透徹的時期來驗證他們的想法。這段時期為過去1億年到現在，期間的溫度、二氧化碳含量以及其他環境變數可以從岩石和化石紀錄中得知。

地球1億年前的氣候跟現在有很大差別。在白堊紀中期，極區的氣溫比現在高了20到40℃；空氣的二氧化碳含量是今日濃度的兩倍；海平面高了100公尺；而恐龍則漫步在無冰覆蓋的兩極地區。

研究人員建立一種電腦模型來模擬若要符合所有地質紀錄則碳原子應該如何流動，才能重現從白堊紀中期的溫室氣候演變成今日的劇烈變化。

Krissansen-Totton表示：「我們發現若要解釋一切的數據――從溫度、二氧化碳、海洋化學到所有事物――化學風化跟溫度的關聯必須要比一般認為得低上許多。另外，也需要其他跟溫度無關的因子來解釋風化速率的改變。」

地質學家之前估計溫度每上升7℃，化學風化速率就會變成兩倍。但新研究結果指出溫度增加的幅度必須要變成3倍以上，也就是24℃，才能讓岩石溶解的速率變成兩倍。

Krissansen-Totton說：「這並不是一個很有效率的恆溫系統。」

作者提出其他控制風化速率的機制可能是有多少陸地暴露在海平面以上，以及陸地的坡度。當5000萬年前左右青藏高原開始形成的時候，它崎嶇的表面可能增加了全球化學風化速率，因而吸收大量二氧化碳並讓氣候降溫至今日的宜人溫度。

「回顧以前的數據，我們的結果顯得合理許多。」Catling表示，「岩石告訴我們在地質歷史之中，地球溫度的擺盪幅度相當大，因此地球的自然恆溫系統的反應速度並不是很快。」

他們的計算結果也指出大氣二氧化碳和溫度之間有很強的關係，也就是氣候敏感度很高。大氣二氧化碳濃度變成兩倍最終會讓全球溫度提高5至6℃，相較於人類排放造成大氣二氧化碳變成兩倍，一般預估在一個世紀之後的溫度上升量，這個數值是其兩倍左右。

研究人員表示雖然還未成定論，但這數字對今日發生的全球變遷來說是個壞消息。

Catling表示：「這一切意謂著如果二氧化碳含量和溫度持續上升，可以預料從長期來看，我們許久之後的後代子孫會遭受到相當長期的暖化現象影響。」

研究人員將會把他們的計算結果應用到地球過往的其他時期。

Krissansen-Totton表示：「這項結果會對地球歷史其他時期以及未來的碳循環有更多啟發，甚至有可能應用到太陽系以外的石質行星。」

Weathering of rocks a poor regulator of global temperatures

A new University of Washington study shows that the textbook understanding of global chemical weathering—in which rocks are dissolved, washed down rivers and eventually end up on the ocean floor to begin the process again—does not depend on Earth’s temperature in the way that geologists had believed.

The study, published May 22 in the open-access journal Nature Communications, looks at a key aspect of carbon cycling, the process by which carbon atoms move between the air, rocks and the oceans. The results call into question the role of rocks in setting our planet’s temperature over long timescales.

“Understanding how the Earth transitioned from a hothouse climate in the age of the dinosaurs to today could help us better understand long-term consequences of future climate change,” said corresponding author Joshua Krissansen-Totton, a UW doctoral student in Earth and space sciences.

The current understanding is that Earth’s climate is controlled over periods of millions of years by a natural thermostat related to the weathering of rocks. Carbon dioxide is released into the air by volcanoes, and this gas may then dissolve into rainwater and react with silicon-rich continental rocks, causing chemical weathering of the rocks. This dissolved carbon then flows down rivers into the ocean, where it ultimately gets locked up in carbon-containing limestone on the seafloor.

As a potent greenhouse gas, atmospheric carbon dioxide also traps heat from the sun. And a warmer Earth increases the rate of chemical weathering both by causing more rainfall and by speeding up the chemical reactions between rainwater and rock. Over time, reducing the amount of carbon dioxide in the air by this method cools the planet, eventually returning the climate to more moderate temperatures—or so goes the textbook picture.

“The general idea has been that if more carbon dioxide is released, the rate of weathering increases, and carbon dioxide levels and temperature are moderated,” co-author David Catling, a UW professor of Earth and space sciences. “It’s a sort of long-term thermostat that protects the Earth from getting too warm or too cold.”

The new study began when researchers set out to determine conditions during the earliest life on Earth, some 3.5 billion to 4 billion years ago. They first tested their ideas on what they believed to be a fairly well-understood time period: the past 100 million years, when rock and fossil records of temperatures, carbon dioxide levels and other environmental variables exist.

Earth’s climate 100 million years ago was very different from today. During the mid-Cretaceous, the poles were 20 to 40 degrees Celsius warmer than the present. Carbon dioxide in the air was more than double today’s concentrations. Seas were 100 meters (330 feet) higher, and dinosaurs roamed near the ice-free poles.

The researchers created a computer simulation of the flows of carbon required to match all the geologic records, thus reproducing the dramatic transition from the warm mid-Cretaceous times to today.

“We found that to be able to explain all the data—temperature, CO₂, ocean chemistry, everything—the dependence of chemical weathering on temperature has to be a lot weaker than was commonly assumed,” Krissansen-Totton said. “You also need to have something else changing weathering rates that has nothing to do with temperature.”

Geologists had previously estimated that a temperature increase of 7 C would double the rate of chemical weathering. But the new results show that more than three times that temperature jump, or 24 C, is required to double the rate at which rock is washed away.

“It’s just a much less efficient thermostat,” Krissansen-Totton said.

The authors suggest that another mechanism controlling the rate of weathering may be how much land is exposed above sea level and the steepness of Earth’s surface. When the Tibetan Plateau was formed some 50 million year ago, the steeper surfaces may have increased the global rate of chemical weathering, drawing down more CO₂ and bringing the climate down to today’s more moderate temperatures.

“In retrospect, our results make a lot of sense,” Catling said. “Rocks tell us that Earth has had large swings in temperature over geological history, so Earth’s natural thermostat can’t be a very tight one.”

Their calculations also indicate a stronger relationship between atmospheric CO₂ and temperature, known as climate sensitivity. Doubling CO₂ in the atmosphere eventually triggered an increase of 5 or 6 degrees Celsius in global temperatures, which is about twice the typical projections for temperature change over centuries for a similar doubling of CO₂ due to human emissions.

Though not the final word, researchers said, these numbers are bad news for today’s climate shifts.

“What all this means is that in the very long term, our distant descendants can expect more warming for far longer if carbon dioxide levels and temperatures continue to rise,” Catling said.

The researchers will now apply their calculations to other periods of the geologic past.

“This is going to have implications for the carbon cycles for other times in Earth’s history and into its future, and potentially for other rocky planets beyond the solar system,” Krissansen-Totton said.

原始論文：Joshua Krissansen-Totton et al, Constraining climate sensitivity and continental versus seafloor weathering using an inverse geological carbon cycle model, Nature Communications (₂017). DOI: 10.1038/ncomms15423

引用自：University of Washington. “Weathering of rocks a poor regulator of global temperatures.” Geology Page. May 22, 2017.