北極暖化，乾旱來襲
美國懷俄明大學的科學家領導的新研究顯示，類似的變化可能即將發生。北極暖化會讓熱帶和極區的溫差減小，進而產生長期乾旱的完美條件：雨量下降、氣旋強度降低、中緯度西風帶減弱。
熱帶和極區的溫差是推動天氣系統運作的重要因素。這兩個完全迥異的地區溫差較大的時候會讓雨量增加、氣旋強度提高、風速更強。然而，北極冰層融化以及極區溫度提高，使得兩者溫差正在縮小。
「我們的分析結果顯示北極較溫暖的時候，噴射氣流以及其他環流模式的強度都會減弱。」懷俄明大學地質和地球物理系的教授Bryan
Shuman表示。「北極和熱帶的溫差變得較為平緩，使得中緯度地區的降雨減少。」
這項新研究的重點發表在論文「全新世中緯度地區的總雨量隨著北極暖化而減少」(Mid-Latitude
Net Precipitation Decreased With Arctic Warming During the Holocene)，Shuman為共同作者之一。論文3月27日刊登於每周發行的國際科學期刊《自然》(Nature)的線上版，紙本版則於4月4日發行。
美國北亞利桑那大學、弗洛倫斯•白斯科地球科學中心、康乃爾大學和比利時天主教魯汶大學的研究人員也參與了這項研究。
「這篇發表在《自然》的論文從全球角度，探討溫度變化跟歷史上極端乾旱時期之間的關係。重要的是，我們發現類似於現今的北極暖化在過去發生時，中緯度地區變得非常乾燥，特別像是包括懷俄明州等地的北美中部。」Shuman如此解釋，「氣候模型預期未來也會發生類似變化。」
目前北半球高緯度地區的暖化速率是全球平均值的兩倍。論文顯示這會讓極區和赤道之間的溫度梯度縮小，變成類似全新世早期至中期的狀況。
Shuman表示研究的主要貢獻在於他們運用地質證據，成功估計過去10000年來的乾旱狀況。他們的研究對象包括了懷俄明州的三座水體：杜波依斯北方的伍茲湖、雪山山脈的微風嶺池塘和貝爾圖斯山的彩虹湖。
「湖泊是紀錄乾溼狀況的天然紀錄。」Shuman表示。「湖面的上升下降會留下地質證據。」
研究人員從219個地點採集了236筆紀錄，藉此分析全新世的溫度記錄。Shuman表示過去10000年之間，他們研究的許多湖泊在早期的水位都比現在還低。
「曾有數千年的時間，懷俄明州的許多湖泊完全見底，現在長滿植被的地方則是一座座活躍的沙丘。」Shuman表示。「再往東岸前進，現今此處的地貌呈現出豐沛的雨量；但一萬年前，東岸就和北美大平原一樣乾燥。」
研究團隊以三種不同的時間區段來探討極區和熱帶的溫差變化，分別是100年、2000年和10000年。過去100年的變化可以用很多大氣觀測紀錄來分析，但是2000年前、乃至於10000年前，可用的紀錄就少了許多。樹輪可以把氣溫的研究範圍擴大至過去2000年，而更早之前的溫度雨量，就得研究湖泊沉積物、洞穴沉積物和冰河冰層的紀錄。
「我們可以用這些紀錄測試氣候模型。」Shuman表示，「如果你想要用電腦預測未來的氣候狀況，可以先用別的時間區段測試它能否做出準確的預測。而地質紀錄便是絕佳的測試對象。」
Arctic warming
contributes to drought
When the
Arctic warmed after the ice age 10,000 years ago, it created perfect conditions
for drought.
According to new research led by a University of
Wyoming scientist, similar changes could be in store today because a warming
Arctic weakens the temperature difference between the tropics and the poles.
This, in turn, results in less precipitation, weaker cyclones and weaker
mid-latitude westerly wind flow -- a recipe for prolonged drought.
The temperature difference between the tropics and
the poles drives a lot of weather. When those opposite temperatures are wider,
the result is more precipitation, stronger cyclones and more robust wind flow.
However, due to the Arctic ice melting and warming up the poles, those
disparate temperatures are becoming closer.
“Our analysis shows that, when the Arctic is warmer,
the jet stream and other wind patterns tend to be weaker,” says Bryan Shuman, a
UW professor in the Department of Geology and Geophysics. “The temperature
difference in the Arctic and the tropics is less steep. The change brings less
precipitation to the mid-latitudes.”
Shuman is a co-author of a new study that is
highlighted in a paper, titled “Mid-Latitude Net Precipitation Decreased With
Arctic Warming During the Holocene,” published today (March 27) online in Nature, an international weekly science
journal. The print version of the article will be published April 4.
Researchers from Northern Arizona University;
Universite Catholique de Louvain in Louvain-In-Neuve, Belgium; the Florence
Bascom Geoscience Center in Reston, Va.; and Cornell University also
contributed to the paper.
“The Nature
paper takes a global approach and relates the history of severe dry periods of
temperature changes. Importantly, when temperatures have changed in similar
ways to today (warming of the Arctic), the mid-latitudes -- particularly places
like Wyoming and other parts of central North America -- dried out,” Shuman
explains. “Climate models anticipate similar changes in the future.”
Currently, the northern high latitudes are warming at
rates that are double the global average. This will decrease the
equator-to-pole temperature gradient to values comparable with the early to
middle Holocene Period, according to the paper.
Shuman says his research contribution, using
geological evidence, was helping to estimate how dry conditions have been in
the past 10,000 years. His research included three water bodies in Wyoming:
Lake of the Woods, located above Dubois; Little Windy Hill Pond in the Snowy
Range; and Rainbow Lake in the Beartooth Mountains.
“Lakes are these natural recorders of wet and dry
conditions,” Shuman says. “When lakes rise or lower, it leaves geological
evidence behind.”
The researchers’ Holocene temperature analysis
included 236 records from 219 sites. During the past 10,000 years, many of the
lakes studied were lower earlier in history than today, Shuman says.
“Wyoming had several thousand years where a number of
lakes dried up, and sand dunes were active where they now have vegetation,”
Shuman says. “Expanding to the East Coast, it is a wet landscape today. But
10,000 years ago, the East Coast was nearly as dry as the Great Plains.”
The research group looked at the evolution of the
tropic-to-pole temperature difference from three time periods: 100 years ago,
2,000 years ago and 10,000 years ago. For the last 100 years, many atmospheric
records facilitated the analysis but, for the past 2,000 years or 10,000 years,
there were fewer records available. Tree rings can help to expand studies to
measure temperatures over the past 2,000 years, but lake deposits, cave
deposits and glacier ice were studied to record prior temperatures and precipitation.
“This information creates a test for climate models,”
Shuman says. “If you want to use a computer to make a forecast of the future,
then it’s useful to test that computer’s ability to make a forecast for some
other time period. The geological evidence provides an excellent test.”
原始論文：Cody C.
Routson, Nicholas P. McKay, Darrell S. Kaufman, Michael P. Erb, Hugues Goosse,
Bryan N. Shuman, Jessica R. Rodysill, Toby Ault. Mid-latitude net
precipitation decreased with Arctic warming during the Holocene. Nature,
2019; DOI: 10.1038/s41586-019-1060-3
引用自：University of Wyoming. "UW Researcher
Contributes to Study Showing Arctic Warming Contributes to Drought."
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