科學家終於發現了全球暖化造成全球海底深層洋流逆流的證據，這首度的發現是由美國斯克雷普斯海洋研究所（SIO）的科學家所揭露。他們所形容的海流逆轉事件發生在距今5500萬年前，而據他們所說，今日的氣候型態與當時類似，因此可能對洋流造成相同的影響。
在距今約5,500萬年前，也就是古新世與始新世交替時期，地球曾有一段溫度上升4~7度的暖化時期；而在最近一期《自然》期刊中，科學家那恩斯（Flávia Nunes）與諾瑞斯（Richard Norris）描述他們如何檢驗該時期的暖化效應。
那恩斯表示：「地球是一個快速變遷的系統。5,500萬年前，當時地球正處於全球暖化時期，海流方向改變快速，這個現象一直持續了2萬年左右，海流方向才恢復到暖化前的狀況。」
科學家表示，現代從石化燃料來源排放到地球大氣層的二氧化碳，慢慢接近預測值裡「古／始新世交替時期最大熱量」(Paleocene/Eocene Thermal Maximum，PETM)週期的程度。他們說古新世與始新世的例子暗示不只對全球氣候，甚至是對海底深層洋流，人類活動所造成的變遷都可能帶來長久的影響。
化石紀錄顯示在PETM時期的全球暖化造成的變遷範圍之大，包括棲息海底深層的海洋生物大量滅絕，以及陸棲哺乳動物因的大遷徙──因暖化開啟了過去寒冷氣候所冰封的遷移路線。此時期即是科學家在北美大陸和歐陸發現馬與靈長類遺跡的最早時期。
兩位科學家的發現，主要是根據他們對5,500萬年前海洋微生物所觀察到的化學組成。
For the first time, evidence that global warming triggered a reversal in the circulation of deep ocean patterns around the world has been uncovered by scientists affiliated with the Scripps Institution of Oceanography. While the changes they describe occurred 55 million years ago, the scientists say today's conditions are similar and could have similar drastic effects on ocean circulation.
In today's issue of the journal "Nature," scientists Flávia Nunes and Richard Norris describe how they examined a four to seven degree warming period that occurred some 55 million years ago during the closing stages of the Paleocene and the beginning of the Eocene eras.
"The Earth is a system that can change very rapidly," said Nunes. "Fifty-five million years ago, when the Earth was in a period of global warmth, ocean currents rapidly changed direction and this change did not reverse to original conditions for about 20,000 years."
Modern carbon dioxide input to the Earth's atmosphere from fossil fuel sources is approaching the same levels estimated for the Paleocene/Eocene Thermal Maximum (PETM) period, say the scientists. They say the Paleocene/Eocene example suggests that changes produced by human activities may have lasting effects not only on global climate, but on deep ocean circulation.
Fossil records show that the global warming at the time of the PETM created changes ranging from a mass extinction of deep sea bottom dwelling marine life to migrations of terrestrial mammal species, as warm conditions may have opened travel routes frozen over when climates were colder. This time period is when scientists find the earliest evidence of horses and primates in North America and Europe.
Nunes and Norris base their findings on the chemical makeup of microscopic sea creatures that lived 55 million years ago.
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