原文網址：https://www.bristol.ac.uk/news/2023/september/sahara-desert-greening.html
一項開創性的研究揭曉了北非過去80萬年之間出現的濕潤時期，並且解釋了撒哈拉沙漠為什麼會週期性地變成綠地。
北非濕潤與乾燥期的植被變化。植物帶的劃分依據為各類植被生長所需的最低降雨量。圖片來源：Jani Närhi / University of Helsinki
這篇發表在《自然通訊》(Nature Communications)的研究顯示撒哈拉週期性出現的濕潤時期，成因為地球繞日軌道的變化，而且在冰河期時會被壓抑下來。
氣候科學家首次模擬出歷史上撒哈拉遭到「綠化」的時段，並且提出證據顯示這些濕潤事件的發生時間與強度，也會因為位在遠方、北半球高緯度地區的大型冰層造成的效應而受到影響。
赫爾辛基大學與布里斯托大學的氣候科學家Edward
Armstrong博士是主要作者，他說：「撒哈拉沙漠的生態系會週期性地轉變成莽原和林地，這是地球上最令人驚訝的環境變遷之一。」
「在氣候模型的研究當中，我們模擬出來的非洲濕潤期(African Humid Periods)的事件規模，是頭幾個和古氣候的觀察結果差不多的。意味著我們的模型可以揭露這些事件發生的時間及原因。」
許多地方都有證據顯示撒哈拉在變成現今的沙漠之前，曾經週期性地擁有植被以及廣布的河川湖泊，使得河馬這類依水而生的動物可以居住。這些北非濕潤期(North
African Humid Periods)或許提供了一條綠色通道，讓包括早期人類在內的許多物種可以散布到非洲之外的地區，因此相當重要。
科學家認為「綠化」是由地球軌道的狀態變化所導致，具體來說是地球軌道的歲差。歲差是指地球自轉軸的搖擺，它會以大約21000年的週期來影響季節性(也就是季節之間的對比有多強)。歲差的變化決定了地球在不同季節接收到的能量多寡，進而控制了非洲季風的強度以及在這片廣大的地區當中植被如何分布。
探討北非濕潤期的過程中有一個重大的阻礙：大多數的氣候模型都無法模擬出這些濕潤期的規模，因此科學家一直沒辦法確定造成它們的詳細機制。
本研究運用了近期開發出來的氣候模型來模擬北非濕潤期，結果使我們對於造成這些事件的機制有了深入許多的理解。
研究確認了北非濕潤期每過21000年就會發生，受控於地球軌道的歲差變化。當此變化造成北半球夏季變得更熱，就會增加西非季風系統的強度並讓撒哈拉的降雨增加，造成莽原類型的植被擴散到沙漠各處。
研究也顯示北非濕潤期不會發生在冰河期，也就是廣闊的冰層覆蓋大部分高緯度地區的時候。這是因為廣闊的冰層降低了大氣的溫度，壓抑非洲季風系統往外擴張的趨勢。結果顯現這兩個距離遙遠的地區之間具有強烈的遙相關(teleconnection)，或許造成了過去80萬年以來地球處在冰河期的時候，人類在內的許多物種就難以擴散到非洲之外。
布里斯托大學的自然地理學教授Paul
Valdes是研究共同作者，他說：「這項成果確實讓我們感到十分振奮。以往的氣候模型通常難以呈現撒哈拉的『綠化』程度。經過我們改良的模型成功呈現了過去的變化，也讓我們更有信心它們可以用來了解未來的變遷。」
伯明罕大學的氣候科學家也參與了這項研究。赫爾辛基大學的通力基金會贊助此研究所屬的計畫，目標為探討在古人類如何分佈及其生態棲位的演變中，氣候有什麼樣的影響。
赫爾辛基大學的人族環境學助理教授Miikka
Tallavaara是研究共同作者，他說：「撒哈拉地區就像是道閘門，控制了北非與撒哈拉以南的非洲，以及非洲大陸內部與外部的物種能否散佈到另一邊。」
「當撒哈拉遭到綠化這道閘門便會開啟，而以沙漠為主時就會關閉。這些交替出現的乾燥與濕潤時期對於非洲物種的演化與散播來說具有重要影響。我們成功把北非濕潤期給模擬出來是項重大的成果，意味著我們現在也可以更加真實地模擬人類的散佈過程，並了解人屬在非洲是如何演化。」
New research reveals why and when the
Sahara Desert was green
A pioneering study has shed new light on
North African humid periods that have occurred over the past 800,000 years and
explains why the Sahara Desert was periodically green.
The research, published in Nature Communications, showed periodic wet phases in the Sahara
were driven by changes in Earth’s orbit around the Sun and were suppressed during
the ice ages.
For the first time, climate scientists simulated the
historic intervals of ‘greening’ of the Sahara, offering evidence for how the
timing and intensity of these humid events were also influenced remotely by the
effects of large, distant, high-latitude ice sheets in the Northern Hemisphere.
Lead author Dr Edward Armstrong, a climate scientist
at the University of Helsinki and University of Bristol, said: “The cyclic
transformation of the Sahara Desert into savannah and woodland ecosystems is
one of the most remarkable environmental changes on the planet.
“Our study is one of the first climate modelling
studies to simulate the African Humid Periods with comparable magnitude to what
the palaeoclimate observations indicate, revealing why and when these events
occurred.”
There is widespread evidence that the Sahara was
periodically vegetated in the past, with the proliferation of rivers, lakes and
water-dependent animals such as hippos, before it became what is now desert.
These North African Humid Periods may have been crucial in providing vegetated
corridors out of Africa, allowing the dispersal of various species, including early
humans, around the world.
The so-called ‘greenings’ are thought to have been
driven by changes in Earth’s orbital conditions, specifically Earth’s orbital
precession. Precession refers to how Earth wobbles on its axis, which
influences seasonality (i.e. the seasonal contrast) over an approximate
21,000-year cycle. These changes in precession determine the amount of energy
received by the Earth in different seasons, which in turn controls the strength
of the African Monsoon and the spread of vegetation across this vast region.
A major barrier to understanding these events is that
the majority of climate models have been unable to simulate the amplitude of
these humid periods, so the specific mechanisms driving them have remained
uncertain.
This study deployed a recently-developed climate
model to simulate the North African Humid periods to greatly advance
understanding of their driving mechanisms.
The results confirm the North African Humid Periods
occurred every 21,000 years and were determined by changes in Earth’s orbital
precession. This caused warmer summers in the Northern Hemisphere, which intensified
the strength of the West African Monsoon system and increased Saharan
precipitation, resulting in the spread of savannah-type vegetation across the
desert.
The findings also show the humid periods did not
occur during the ice ages, when there were large glacial ice sheets covering
much of the high latitudes. This is because these vast ice sheets cooled the
atmosphere and suppressed the tendency for the African monsoon system to
expand. This highlights a major teleconnection between these distant regions,
which may have restricted the dispersal of species, including humans, out of
Africa during the glacial periods of the last 800,000 years.
Co-author Paul Valdes, Professor of Physical
Geography at the University of Bristol, said: “We are really excited about the
results. Traditionally, climate models have struggled to represent the extent
of the ‘greening’ of the Sahara. Our revised model successfully represents past
changes and also gives us confidence in their ability to understand future change.”
The research, including climate scientists from the
University of Birmingham, is part of a Kone Foundation-funded project at the
University of Helsinki, which studies the impacts of climate on past human
distributions and evolution of their ecological niche.
Co-author Miikka Tallavaara, Assistant Professor of
Hominin Environments at the University of Helsinki, said: “The Sahara region is
kind of a gate controlling the dispersal of species between both North and
Sub-Saharan Africa, and in and out of the continent.
“The gate was open when Sahara was green and closed
when deserts prevailed. This alternation of humid and arid phases had major
consequences for the dispersal and evolution of species in Africa. Our ability
to model North African Humid periods is a major achievement and means we are
now also better able to model human distributions and understand the evolution
of our genus in Africa.”
原始論文：Edward
Armstrong, Miikka Tallavaara, Peter O. Hopcroft, Paul J. Valdes. North
African humid periods over the past 800,000 years. Nature
Communications, 2023; 14 (1) DOI: 10.1038/s41467-023-41219-4
引用自：University of Bristol. "New research
reveals why and when the Sahara Desert was green."
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