追蹤過去800年來熱帶往返移動的過程

原文網址：https://uanews.arizona.edu/story/tracking-movement-tropics-800-years-past

追蹤過去800年來熱帶往返移動的過程

亞利桑那大學的研究團隊探討熱帶最北界的移動，發現熱帶範圍擴張的時候也發生了嚴重的乾旱。

Mari N. Jensen

亞利桑那大學領導的國際團隊指出，這是科學家首度回推過去800年來熱帶最北界的南北向移動過程。

科學家藉著研究樹木的年輪，比方土耳其安那托利亞這座山坡上的歐洲黑松，他們可以解出過去800年來熱帶的最北界如何來回移動。圖片來源：Ünal Akkemik

熱帶邊界的移動會影響北半球沙漠的分布，包括索諾拉、莫哈韋以及撒哈拉沙漠。這些沙漠的位置緊鄰於熱帶(包括副熱帶)的北方。

在此之前，科學家對於熱帶位置的資訊只能追溯至西元1930年左右，當時開始有儀器持續進行確實的記錄。

在標準地圖上，熱帶的範圍大約是從北緯30度到南緯30度之間。

不過新研究顯示熱帶的最北界從西元1203年到2003年，平行著北緯30度南北變動的幅度多達4度。

Raquel Alfaro Sánchez表示：「熱帶的邊界移動會伴隨著降雨區域的變遷。」他在亞利桑那大學樹輪研究實驗室進行博士後研究時領導了此研究團隊。

團隊發現西元1568年至1634年熱帶往北方擴張。Alfaro Sánchez現為西班牙巴塞隆納生態與林業應用研究中心的博士後研究員，他說這段時期人類社會也經歷了嚴重的乾旱以及其他劇變，像是鄂圖曼土耳其帝國衰敗、中國明朝覆亡、美國維吉尼亞的詹姆斯鎮幾近廢棄。

共同作者Valerie Trouet 表示：「我們的結果指出氣候變遷是導致這些社會瓦解的成因之一。」

為了追蹤地球熱帶的最北界從西元1203年至2003年的變化，團隊利用了北半球各處五個不同地方的樹木年輪。由於樹木每年的生長輪反映了當年氣候，因此研究人員可以逐年查出過去的雨量。

亞利桑那大學樹輪年代學的副教授Trouet表示，這份800年的歷史紀錄讓研究人員可以看出大型火山爆發這類的罕見事件，和隨後發生的氣候變化之間的關聯。

大型火山爆發會把大量細微的塵埃和氣膠排放到大氣中使得地球冷卻下來。團隊寫道西元1815年坦博拉火山爆發(位處當今印尼境內)造成的冷化效應遍及全球，使得西元1816年在歐洲被稱作「無夏之年」。

Trouet說：「我們可以看到在類似坦博拉的火山爆發事件之後，熱帶的範圍有縮減的情形。」

Trouet表示瞭解氣膠對氣候的影響相當重要，因為某些研究人員提倡解決全球暖化的地球工程方案，就是把氣膠粒子送到大氣當中。

團隊的研究論文「過去800年來氣候與火山活動造成熱帶北界的移動」(Climatic and volcanic forcing of tropical belt northern boundary over the past 800 years.)預計10月15日發表在《自然―地質科學》(Nature Geoscience)的線上版。

Alfaro Sánchez表示其他研究人員證實從1970年代開始熱帶就一直往北方擴張。

Trouet表示電腦運用現今與未來的氣候模型得出的模擬結果雖然也顯示熱帶會往北方擴張，但卻比實際的幅度還小，這讓研究人員想要得到更為長久的歷史顯示熱帶區域如何移動。

研究人員利用樹輪重建過去數百年來世界各地許多地方的氣候與變遷過程。為了追蹤熱帶過去如何移動，Alfaro Sánchez和她的同事利用下列五個地方已經做出的樹輪年表：美國阿肯色州、美國西部、西藏高原、土耳其和巴基斯坦北部。

為了辨識樹輪紀錄如何反映熱帶的變化，團隊把目光放在西元1930年至2003年的樹輪，將這段期間樹木寫下來的自然文獻跟儀器在熱帶紀錄到的變化兩相比較。

團隊特別著重於紀錄中哈德里環流圈(Hadley cells)的變化，這個巨型大氣對流胞環繞了全球熱帶地區。Trouet說哈德里環流圈是驅動大氣環流的重要動力之一。

瞭解哈德里環流圈的變化跟樹輪變化的關係之後，研究團隊接著利用多個樹輪年表來觀察過去800年來熱帶的版圖如何擴張縮減。

「這是首次重建出工業革命以前的紀錄。」Trouet表示。「為了瞭解自然狀況下的氣候變化，我們需要把紀錄回推至超過150年前。」

Alfaro Sánchez和她的同事發現在工業革命之前，熱帶的範圍本身就會擴張縮減。

Trouet說地球氣候系統的內在變化影響了熱帶的移動。

其他研究指出目前紀錄顯示熱帶自1970年代開始擴張的原因，部分是大氣中的溫室氣體增加。

Alfaro Sánchez表示正在擴張的熱帶可能會對人類社會產生重大影響，因為團隊發現過去嚴重的乾旱發生時也是熱帶持續擴張的時候。

Tracking the movement of the tropics 800 years into the past

Studying shifts of the northern-most edge of the tropics revealed that periods of tropical expansion coincided with severe droughts, a UA-led research team found

For the first time, scientists have traced the north-south shifts of the northern-most edge of the tropics back 800 years, reports a University of Arizona-led international team.

The movement of the tropical boundary affects the locations of Northern Hemisphere deserts including the Sonoran, Mohave and Saharan. Those deserts sit just north of the tropical belt, which includes the subtropics.

Before now, scientists had information about the location of the tropical belt going back to around 1930, when reliable instrumental record-keeping began.

On a standard map, the tropical belt spans roughly 30 degrees north latitude to 30 degrees south latitude.

However, the new research reveals that from the year 1203 to the year 2003, the northern edge of the tropics fluctuated up to 4 degrees north and south of the northern 30th parallel.

"Movement of the limit of the tropics is associated with changes in precipitation regimes," said Raquel Alfaro Sánchez, who led the research team while a postdoctoral researcher at the UA Laboratory of Tree-Ring Research.

From 1568 to 1634, the tropics expanded to the north, the team found. That time period coincides with severe droughts and other disruptions of human societies, including the collapse of the Ottoman empire in Turkey, the end of the Ming Dynasty in China and near abandonment of the Jamestown Colony in Virginia, said Alfaro Sánchez, currently a postdoctoral researcher at the Centre de Recerca Ecològica i Aplicacions Forestals in Barcelona, Spain.

Co-author Valerie Trouet said, "Our results suggest that climate change was one of the contributing factors to those societal disruptions."

To track the northern boundary of the Earth’s tropical belt from 1203 to 2003, the team used the annual rings of trees from five different locations throughout the Northern Hemisphere. Researchers can figure out annual precipitation years into the past because each annual growth ring of a tree reflects the climate that year.

Having an 800-year history also allowed the researchers to connect rare events such as huge volcanic eruptions with subsequent changes in climate, said Trouet, a UA associate professor of dendrochronology.

Massive volcanic eruptions cool the Earth because of all the fine particles and aerosols thrown into the atmosphere. The 1815 Tambora eruption in present-day Indonesia caused such cooling worldwide that 1816 was known in Europe as "the year without summer," the team writes.

"We can see the contraction of the tropics after volcanic eruptions such as Tambora," Trouet said.

Trouet said learning how aerosols affect climate is important because some researchers have proposed sending such particles into the atmosphere as a geoengineering solution to global warming.

The team’s research paper, "Climatic and volcanic forcing of tropical belt northern boundary over the past 800 years," is scheduled for online publication in Nature Geoscience on October 15.

Other researchers have documented that the tropics have been expanding northward since the 1970s, Alfaro Sánchez said.

Because computer models of current and future climate models also show expansion of the tropical belt, but not as much as is actually occurring, the researchers wanted to develop a longer history of the movement of the tropical zone, Trouet said.

Researchers use tree rings to reconstruct past climate and climate changes for many locations around the globe. Those climate reconstructions extend hundreds of years into the past. To track past tropical belt movements, Alfaro Sánchez and her colleagues used existing tree-ring chronologies from five locations: Arkansas, the American West, the Tibetan Plateau, Turkey and northern Pakistan.

To discern how the tree-ring records reflect changes in the tropical belt, the team looked at tree rings from 1930 to 2003 and compared the trees' natural archive of climate to instrumental records of changes in the tropical belt.

The researchers focused on recorded changes in Hadley cells, the huge atmospheric convective cells that circumnavigate the globe in the tropics. Trouet said Hadley cells are an important driver of atmospheric circulation.

Knowing how changes in Hadley cells correlated with changes in tree rings, the team then used multiple tree-ring chronologies to see how the tropics expanded and contracted as much as 800 years ago.

"This is the first reconstruction that went back to pre-industrial times," Trouet said. "To know what the natural climate variability is, we need to go farther back in time than the last 150 years."

Alfaro Sánchez and her colleagues found the tropical belt has expanded and contracted on its own long before industrial times.

Internal variability in the Earth’s climate system affects the movement of the tropics, Trouet said.

The current recorded expansion of the tropical belt since the 1970s is in part due to the increase in greenhouse gases in the atmosphere, other researchers report.

The current expansion of the tropics may have important societal impacts, because the team found that past severe droughts were associated with persistent periods of tropical expansion, Alfaro Sánchez said.

原始論文：R. Alfaro-Sánchez, H. Nguyen, S. Klesse, A. Hudson, S. Belmecheri, N. Köse, H. F. Diaz, R. K. Monson, R. Villalba, V. Trouet. Climatic and volcanic forcing of tropical belt northern boundary over the past 800 years. Nature Geoscience, 2018; DOI: 10.1038/s41561-018-0242-1

引用自：University of Arizona. "Tracking the movement of the tropics 800 years into the past."