跟從花粉的訊息——聖路易斯華盛頓大學的氣候科學家進行的研究指出，過去植物留下的紀錄訴說了全球氣溫如何變化的實際情形。
根據4月15日發表在《科學前緣》(Science
Advances)的全新模擬結果，氣溫升高會帶來更多植物，這又會讓氣溫進一步升高。
Alexander Thompson是該所大學文理學院地球與行星科學系的博士後研究員，他對一個重要的氣候模型進行更新之後，模擬結果反映出過去一萬年造成全球氣溫變化的過程中，植被的改變是關鍵因素之一。
許多模型模擬了地球自從末次冰期以來的氣溫變化，令Thompson長久以來感到困擾的是，有太多模擬結果都顯示氣溫會隨著時間持續升高。
但是氣候代用指標的紀錄卻訴說了完全不同的故事：許多來源都暗示了全球氣溫在六千到九千年前有道明顯的高峰。
Thompson的直覺認為模型可能太重視大氣二氧化碳濃度或是冰層覆蓋面積的影響，卻忽略了植被變化所扮演的角色。
「花粉紀錄指出在那段期間植被的面積增加了許多，」Thompson表示。
「但是之前的模型展現出的植被增加面積卻有所限制，」他說，「因此，就算這些模擬中有一些考慮到植被會有動態變化，其變化量還是少到幾乎無法解釋花粉紀錄指出的現象。」
事實上，植被的覆蓋面積有著非常重大的變化。
在全新世(目前的地質時代)早期，非洲的撒哈拉沙漠比現在更加翠綠——實際上更像是一座草原。其他北半球的植被，像是極區的針葉林與中緯度的落葉林也相當茂盛。
Thompson利用花粉紀錄得來的證據設計一連串實驗。他在各式各樣的氣候模型當中，使用評價最好的其中之一：Community
Earth System Model，接著他進行模擬來解釋一些沒人探討過的植被變化。
「全新世的植被擴張最多可以讓地球的氣溫升高華氏1.5度，」Thompson表示。「我們新的模擬結果非常契合古氣候的代用指標。因此我們認為在解決具有爭議的全新世氣溫矛盾的時候，北半球的植被可能是成因之一，這點讓我們相當興奮。」
了解全新世期間的溫度變化量與發生時間十分重要，因為從地質來說這是最近的一段歷史。由於人類的農業與文明也是在這段期間出現，因此不同領域的許多科學家與歷史學家都很想了解全新世早期與中期的氣候跟現在有什麼差別。
Thompso還是密西根大學的研究生時開始了這項研究，在他加入華盛頓大學的氣候學家Bronwen
Konecky的實驗室後仍持續著。
「總而言之，我們的研究強調出納入植被變化是很重要的，」Thompson表示，「在預估未來氣候變遷的時候如果可以納入植被變化，就更有機會得出值得信賴的結果。」
Changes in
vegetation shaped global temperatures over last 10,000 years
Follow the pollen. Records from past
plant life tell the real story of global temperatures, according to research
from a climate scientist at Washington University in St. Louis.
Warmer temperatures brought plants -- and then came
even warmer temperatures, according to new model simulations published April 15
in Science Advances.
Alexander Thompson, a postdoctoral research associate
in earth and planetary sciences in Arts & Sciences, updated simulations
from an important climate model to reflect the role of changing vegetation as a
key driver of global temperatures over the last 10,000 years.
Thompson had long been troubled by a problem with
models of Earth's atmospheric temperatures since the last ice age. Too many of
these simulations showed temperatures warming consistently over time.
But climate proxy records tell a different story.
Many of those sources indicate a marked peak in global temperatures that
occurred between 6,000 and 9,000 years ago.
Thompson had a hunch that the models could be
overlooking the role of changes in vegetation in favor of impacts from
atmospheric carbon dioxide concentrations or ice cover.
"Pollen records suggest a large expansion of
vegetation during that time," Thompson said.
"But previous models only show a limited amount
of vegetation growth," he said. "So, even though some of these other
simulations have included dynamic vegetation, it wasn't nearly enough of a
vegetation shift to account for what the pollen records suggest."
In reality, the changes to vegetative cover were
significant.
Early in the Holocene, the current geological epoch,
the Sahara Desert in Africa grew greener than today -- it was more of a grassland.
Other Northern Hemisphere vegetation including the coniferous and deciduous
forests in the mid-latitudes and the Arctic also thrived.
Thompson took evidence from pollen records and
designed a set of experiments with a climate model known as the Community Earth
System Model (CESM), one of the best-regarded models in a wide-ranging class of
such models. He ran simulations to account for a range of changes in vegetation
that had not been previously considered.
"Expanded vegetation during the Holocene warmed
the globe by as much as 1.5 degrees Fahrenheit," Thompson said. "Our
new simulations align closely with paleoclimate proxies. So this is exciting
that we can point to Northern Hemisphere vegetation as one potential factor
that allows us to resolve the controversial Holocene temperature
conundrum."
Understanding the scale and timing of temperature
change throughout the Holocene is important because it is a period of recent
history, geologically speaking. The rise of human agriculture and civilization
occurred during this time, so many scientists and historians from different
disciplines are interested in understanding how early and mid-Holocene climate
differed from the present day.
Thompson conducted this research work as a graduate
student at the University of Michigan. He is continuing his research in the
laboratory of climate scientist Bronwen Konecky at Washington University.
"Overall, our study emphasizes that accounting
for vegetation change is critical," Thompson said. "Projections for
future climate change are more likely to produce more trustworthy predictions
if they include changes in vegetation."
原始論文：Alexander J.
Thompson, Jiang Zhu, Christopher J. Poulsen, Jessica E. Tierney, Christopher B.
Skinner. Northern Hemisphere vegetation change drives a Holocene
thermal maximum. Science Advances, 2022; 8 (15) DOI: 10.1126/sciadv.abj6535
引用自：Washington University in St. Louis.
"Changes in vegetation shaped global temperatures over last 10,000
years."
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