為什麼地球每十萬年就會歷經一次冰河期？

原文網址：www.sciencedaily.com/releases/2016/10/161026081537.htm

為什麼地球每十萬年就會歷經一次冰河期？

卡迪夫大學的專家對地球為何開始每十萬年就會進入及退出冰河期的現象提出了解釋。

這項被暱稱為「十萬年謎題」的神秘現象大約發生在過去一百萬年，並產生了曾覆蓋北美大陸、歐洲和亞洲的廣大冰層。目前為止，科學家仍未能解釋其發生的原因。

科學家可以預測地球季節如何變化，而最寒冷的夏季會以四萬年為間距發生。因此，對他們來說，過去地球冰河期以同樣地間距發生是相當合理的。然而，在一百萬年前左右，於「中更新世轉型期」(Mid-Pleistocene Transition)這個時間點，冰河期的間距從四萬年一次轉變成了十萬年一次。

今日刊登於期刊《地質》(Geology )的新研究提出海洋可能是導致此轉變的推手，精確來說跟海洋從大氣吸收二氧化碳的方式有關。

研究團隊分析海床上微小化石的化學成分，發現在以十萬年為規律的區間，冰河期時深海儲存的二氧化碳量變多了。

這代表當時有更多二氧化碳從大氣抽離並儲存至海洋，造成地球溫度更加降低而讓廣大冰層可以吞沒整個北半球。

此篇研究的第一作者，地球和海洋科學院的教授Carrie Lear說：「我們可以想像海洋會吸入並呼出二氧化碳。因此冰層擴大時，表示海洋從大氣吸入了二氧化碳，造成整個地球變冷；當冰層縮小，則代表海洋呼出二氧化碳，使大氣二氧化碳變多進而讓地球氣溫升高。」

「藉由研究海床上的微小生物化石，我們發現冰層以十萬年為周期前進後退時，於冷期海洋吸入了更多二氧化碳，代表同一時期留在大氣中的二氧化碳變得較少。」

身為光合作用生物中的主要成員，海洋藻類在移除大氣二氧化碳的作用中扮演了重要角色。

經由稱做湧升流(upwelling)的作用，深海海水上湧回到海表時二氧化碳也會跟著回到大氣。但是，當有大量海冰出現時，海冰會阻斷海洋呼出二氧化碳，使得冰層更加擴張而冰河期也會延長。

「若我們想像海洋正在吸進呼出二氧化碳，大量冰層的出現就像有顆巨大糖球將其噎住。它好比是個鍋蓋封住了海洋表層。」 Lear教授繼續說道。

地球氣候現今正處於兩次冰河期之間的溫暖間隙。上次冰河期約在11000年前結束。從那時開始，氣溫和海平面便持續升高而冰帽也退回兩極。除了這些自然循環之外，人類排放的二氧化碳也對氣候暖化造成了深遠影響。

Why does our planet experience an ice age every 100,000 years?

Experts from Cardiff University have offered up an explanation as to why our planet began to move in and out of ice ages every 100,000 years.

This mysterious phenomena, dubbed the '100,000 year problem', has been occurring for the past million years or so and leads to vast ice sheets covering North America, Europe and Asia. Up until now, scientists have been unable to explain why this happens.

Our planet's ice ages used to occur at intervals of every 40,000 years, which made sense to scientists as the Earth's seasons vary in a predictable way, with colder summers occurring at these intervals. However there was a point, about a million years ago, called the 'Mid-Pleistocene Transition', in which the ice age intervals changed from every 40,000 years to every 100,000 years.

New research published today in the journal Geology has suggested the oceans may be responsible for this change, specifically in the way that they suck carbon dioxide (CO₂) out of the atmosphere.

By studying the chemical make-up of tiny fossils on the ocean floor, the team discovered that there was more CO₂ stored in the deep ocean during the ice age periods at regular intervals every 100,000 years.

This suggests that extra carbon dioxide was being pulled from the atmosphere and into the oceans at this time, subsequently lowering the temperature on Earth and enabling vast ice sheets to engulf the Northern Hemisphere.

Lead author of the research Professor Carrie Lear, from the School of Earth and Ocean Sciences, said: "We can think of the oceans as inhaling and exhaling carbon dioxide, so when the ice sheets are larger, the oceans have inhaled carbon dioxide from the atmosphere, making the planet colder. When the ice sheets are small, the oceans have exhaled carbon dioxide, so there is more in the atmosphere which makes the planet warmer.

"By looking at the fossils of tiny creatures on the ocean floor, we showed that when ice sheets were advancing and retreating every 100,000 years the oceans were inhaling more carbon dioxide in the cold periods, suggesting that there was less left in the atmosphere."

Marine algae play a key role in removing CO₂ from the atmosphere as it is an essential ingredient of photosynthesis.

CO₂ is put back into the atmosphere when deep ocean water rises to the surface through a process called upwelling, but when a vast amount of sea ice is present this prevents the CO₂ from being exhaled, which could make the ice sheets bigger and prolong the ice age.

"If we think of the oceans inhaling and exhaling carbon dioxide, the presence of vast amounts of ice is like a giant gobstopper. It's like a lid on the surface of the ocean," Prof Lear continued.

The Earth's climate is currently in a warm spell between glacial periods. The last ice age ended about 11,000 years ago. Since then, temperatures and sea levels have risen, and ice caps have retreated back to the poles. In addition to these natural cycles, humanmade carbon emissions are also having an effect by warming the climate.

原始論文：Caroline H. Lear, Katharina Billups, Rosalind E.M. Rickaby, Liselotte Diester-Haass, Elaine M. Mawbey, Sindia M. Sosdian. Breathing more deeply: Deep ocean carbon storage during the mid-Pleistocene climate transition.Geology, 2016; G38636.1 DOI: 10.1130/G38636.1

引用自：Cardiff University. "Why does our planet experience an ice age every 100,000 years?." ScienceDaily. ScienceDaily, 26 October 2016.