由冰與火交織而成的完美風暴造就了雪球地球

原文網址：www.sciencedaily.com/releases/2017/03/170313160813.htm

由冰與火交織而成的完美風暴造就了雪球地球

對數十億年一次的事件提出解釋

是什麼造成了地球史上稱做「雪球地球」(snowball Earth)的最劇烈冰河期事件？地質學家和氣候學家數年來不斷尋求解答，但此事件的根源仍然尚未明瞭。

哈佛大學的研究人員現在提出了一則新假說，來解釋是什麼造成了使地球兩極之間皆被冰雪所覆蓋的超級冰河期。

此研究刊登於《地球物理研究通訊》。

研究人員早已精確定出Sturtian雪球地球事件的起始時間約為7.17億年前(誤差約為數百萬年)。大約是同一時間，一起巨型火山爆發事件摧毀了從現今的阿拉斯加至格陵蘭的廣大地區。這會是件巧合嗎？

哈佛大學的教授Francis Macdonald和Robin Wordsworth並不這麼認為。

「我們知道火山活動可以對環境造成重大影響，因此關鍵問題是：這兩個事件之間有何關聯？」哈佛自然科學的John L. Loeb副教授Macdonald表示。

起初，Macdonald的團隊認為是會分解成鎂和鈣的玄武岩，跟空氣中的二氧化碳反應而導致了冷化。然而，若此理論為真，則冷化會在數百萬年之間逐漸發生。但從加拿大北極地區的火山岩得到的同位素定年結果，卻指出火山爆發跟冷化事件發生的時間更為接近。

Macdonald轉而尋求模擬地球以外行星的氣候的專家Wordsworth協助，並提出有沒有可能是由這些火山噴出的氣膠(aerosol)快速冷卻了地球？

答案是：在適當的情況下，確實如此。

「形成大型火山岩區域(large volcanic province)的噴發事件一點都不特別。」哈佛大學約翰·保爾森工程與應用科學院的環境科學與工程學助理教授Wordsworth表示。「這類型的火山噴發在整部地質史上一直反覆發生，但它們並非總是跟冷化事件有關。因此問題是：什麼造成了這個事件有所不同？」

對稱作Franklin大火成岩省進行的地質以及化學研究顯示，此區域噴發火山岩時，岩漿穿透了富含硫的沉積物，這些硫會在噴發過程中以二氧化硫的形式進入大氣層。當二氧化硫進入大氣上層時，可以對太陽輻射起到相當好的遮蔽作用。1991年菲律賓的Pinatubo火山將大約1000萬公噸的硫噴發至大氣當中，使得接下來一年全球氣溫下降了華氏1度左右。

對流層頂(tropopause)是分隔對流層和平流層的邊界，如果二氧化硫通過此處便能對太陽輻射起到最佳的屏蔽作用。若二氧化硫到達此等高度，它們就不太會經由降水作用或是跟其他粒子混和而回到地面，這會使它們待在大氣的時間延長數個禮拜到將近一年。對流層頂這道屏障的高度為何，全取決於當時地球的氣候背景值(background climate)。地球越冷，對流層頂的所在高度就越低。

「在地球歷史相當溫暖的時期，受到溫暖且位於高處的對流層頂保護，火山冷化不會有多大影響。」Wordsworth說，「在氣候較冷的情況下，地球會變得特別容易受到這類火山作用影響，使得氣候發生擾動。」

「我們的模型顯示事件本身的內容及發生背景都至關重要。」Macdonald表示。

另一個重點是二氧化硫氣柱到達對流層頂的位置。由於大陸漂移的關係，在7.17億年前發生噴發事件的Franklin大火成岩省當時是位於赤道附近，讓地球保持溫暖的太陽輻射大部份是在此處進入地球。

結論便是可以有效反射陽光的氣體於正確的地點到達大氣適當的高度，使得冷化效應發生。但還需要另一個要素才能孕育出這道「完美風暴」。畢竟，Pinatubo火山噴發也具有類似的特質，但其冷卻效應只不過持續了一年之久。

7.17億年前將硫拋射至空氣中的噴發事件可不是像Pinatubo這類由單一火山導致的單次爆發事件。研究中的火山群從加拿大綿延至格陵蘭，長度將近2000英哩。這些火山的噴發方式並非為單獨一次的爆裂式噴發，而是持續不斷的噴發，就像今日發生於夏威夷和冰島的形式。研究人員的論述中認為此種類型的火山噴發持續了十年左右，而將足以迅速顛覆氣候的大量氣膠傾倒至大氣層當中。

「由氣膠導致的冷化作用不需要將整個地球冰封起來，它只需要把冰層推進到某個緯度，冰層就會接手完成剩下的工作。」Macdonald表示。

冰層越廣，就越會將越多的陽光反射回太空，造成地球變得更冷。一旦冰層到達現今加州所在的緯度，正回饋效應造成的迴圈就會接手整個過程，超級雪球效應也變得勢不可擋。

「人們很容易將氣候想像成難以撼動的巨大系統，就很多方面來看也是事實。但在過往確實曾經發生十分劇烈的改變，同樣地，這樣的巨變也很可能在未來發生。」Wordsworth表示。

理解這些劇烈的氣候變遷事件如何發生，有助於研究人員更加了解大滅絕事件的發生原因，以及之前提出的地球工程方法可能對氣候造成的衝擊，還有其他星球的氣候變遷會是何種情況。

「這項研究提醒我們需要摒棄把地外行星看得過於單純的慣例，只考慮他們處於穩定平衡狀態和是否處於適居帶。」Wordsworth說。「我們知道地球是顆生氣勃勃，卻擁有激烈變化的行星。有充分理由可以相信像這樣的快速氣候變遷對行星來說是種常態，而非難得一見的例外。」

A perfect storm of fire and ice may have led to snowball Earth

Explaining a 'once-in-a-billion-year event'

What caused the largest glaciation event in Earth's history, known as 'snowball Earth'? Geologists and climate scientists have been searching for the answer for years but the root cause of the phenomenon remains elusive.

Now, Harvard University researchers have a new hypothesis about what caused the runaway glaciation that covered Earth pole-to-pole in ice.

The research is published in Geophysical Research Letters.

Researchers have pinpointed the start of what's known as the Sturtian snowball Earth event to about 717 million years ago -- give or take a few 100,000 years. At around that time, a huge volcanic event devastated an area from present-day Alaska to Greenland. Coincidence?

Harvard professors Francis Macdonald and Robin Wordsworth thought not.

"We know that volcanic activity can have a major effect on the environment, so the big question was, how are these two events related," said Macdonald, the John L. Loeb Associate Professor of the Natural Sciences.

At first, Macdonald's team thought basaltic rock -- which breaks down into magnesium and calcium -- interacted with CO₂ in the atmosphere and caused cooling. However, if that were the case, cooling would have happened over millions of years and radio-isotopic dating from volcanic rocks in Arctic Canada suggest a far more precise coincidence with cooling.

Macdonald turned to Wordsworth, who models climates of non-Earth planets, and asked: could aerosols emitted from these volcanos have rapidly cooled Earth?

The answer: yes, under the right conditions.

"It is not unique to have large volcanic provinces erupting," said Wordsworth, assistant professor of Environmental Science and Engineering at the Harvard John A. Paulson School of Engineering and Applied Science. "These types of eruptions have happened over and over again throughout geological time but they're not always associated with cooling events. So, the question is, what made this event different?"

Geological and chemical studies of this region, known as the Franklin large igneous province, showed that volcanic rocks erupted through sulfur-rich sediments, which would have been pushed into the atmosphere during eruption as sulfur dioxide. When sulfur dioxide gets into the upper layers of the atmosphere, it's very good at blocking solar radiation. The 1991 eruption of Mount Pinatubo in the Philippines, which shot about 10 million metric tons of sulfur into the air, reduced global temperatures about 1 degree Fahrenheit for a year.

Sulfur dioxide is most effective at blocking solar radiation if it gets past the tropopause, the boundary separating the troposphere and stratosphere. If it reaches this height, it's less likely to be brought back down to earth in precipitation or mixed with other particles, extending its presence in the atmosphere from about a week to about a year. The height of the tropopause barrier all depends on the background climate of the planet -- the cooler the planet, the lower the tropopause.

"In periods of Earth's history when it was very warm, volcanic cooling would not have been very important because Earth would have been shielded by this warm, high tropopause," said Wordsworth. "In cooler conditions, Earth becomes uniquely vulnerable to having these kinds of volcanic perturbations to climate."

"What our models have shown is that context and background really matters," said Macdonald.

Another important aspect is where the sulfur dioxide plumes reach the stratosphere. Due to continental drift, 717 million years ago, the Franklin large igneous province where these eruptions took place was situated near the equator, the entry point for most of the solar radiation that keeps Earth warm.

So, an effective light-reflecting gas entered the atmosphere at just the right location and height to cause cooling. But another element was needed to form the perfect storm scenario. After all, the Pinatubo eruption had similar qualities but its cooling effect only lasted about a year.

The eruptions throwing sulfur into the air 717 million years ago weren't one-off explosions of single volcanoes like Pinatubo. The volcanoes in question spanned almost 2,000 miles across Canada and Greenland. Instead of singularly explosive eruptions, these volcanoes can erupt more continuously like those in Hawaii and Iceland today. The researchers demonstrated that a decade or so of continual eruptions from this type of volcanoes could have poured enough aerosols into the atmosphere to rapidly destabilize the climate.

"Cooling from aerosols doesn't have to freeze the whole planet; it just has to drive the ice to a critical latitude. Then the ice does the rest," said Macdonald.

The more ice, the more sunlight is reflected and the cooler the planet becomes. Once the ice reaches latitudes around present-day California, the positive feedback loop takes over and the runaway snowball effect is pretty much unstoppable.

"It's easy to think of climate as this immense system that is very difficult to change and in many ways that's true. But there have been very dramatic changes in the past and there's every possibility that as sudden of a change could happen in the future as well," said Wordsworth.

Understanding how these dramatic changes occur could help researchers better understand how extinctions occurred, how proposed geoengineering approaches may impact climate and how climates change on other planets.

"This research shows that we need to get away from a simple paradigm of exoplanets, just thinking about stable equilibrium conditions and habitable zones," said Wordsworth. "We know that Earth is a dynamic and active place that has had sharp transitions. There is every reason to believe that rapid climate transitions of this type are the norm on planets, rather than the exception."

原始論文：F. A. Macdonald, R. Wordsworth. Initiation of Snowball Earth with volcanic sulfur aerosol emissions. Geophysical Research Letters, 2017; DOI: 10.1002/2016GL072335

引用自：Harvard John A. Paulson School of Engineering and Applied Sciences. "A perfect storm of fire and ice may have led to snowball Earth: Explaining a 'once-in-a-billion-year event'." ScienceDaily. ScienceDaily, 13 March 2017.