根據哥倫比亞大學地球研究所所屬單位，拉蒙特-多爾蒂地球觀測站（Lamont-Doherty Earth Observatory）發表於5日的最新研究顯示，本世紀初開始，人類造成的氣候變遷有可能引發美國整個西南區長期的極度乾旱。
研究者表示與過去乾旱比較，未來的缺水情形不與任何特定海洋表面溫度變化模式有關。反似是「日漸增加的溫室氣體驅使海洋表面暖化」的結果。
觀測站資深科學家也是該研究主導者之一西格（Richard Seager）表示：「北美西南的不毛地區勢必會因為人類引發的氣候變遷而更乾燥，正巧人口成長也增加用水需求。水主要還是供應農業使用。」
該研究已記錄，匯集赤道附近上升氣流和亞熱帶區下降氣流的哈得萊環流（Hadley Cell)，隨著地球暖化，如何往地球兩極擴張。
亞熱帶下降氣流使大氣下層空氣乾燥，因此抑制降雨，造成亞熱帶乾燥區的擴張。同時與該現象相關，位於中緯度含雨的風暴路徑也往兩極移動。以上牽動大氣循環變化的兩因素，仍未完全被了解，卻造成亞熱帶近兩極的邊陲區域乾燥。其他預計會被亞熱帶旱化影響的地區包括南歐、北非、中東及南美部份區域。
Human-caused climate change is likely to lead to long periods of extreme drought throughout the American Southwest starting early this century, finds a new study released today by the Lamont-Doherty Earth Observatory, a member of The Earth Institute at Columbia University.
In contrast to past droughts, future drying is not linked to any particular pattern of change in sea surface temperature but seems to be the result of "an overall surface warming driven by rising greenhouse gases," researchers said.
"The arid lands of southwestern North America will imminently become even more arid as a result of human-induced climate change just at the time that population growth is increasing demand for water, most of which is still used by agriculture," said Richard Seager, senior research scientist at the Lamont-Doherty Earth Observatory and one of the lead authors of the study.
As the planet warms, the study documented how the Hadley Cell, which links together rising air near the Equator and descending air in the subtropics, expands toward the poles.
Descending air suppresses precipitation by drying the lower atmosphere, so this process expands the subtropical dry zones. At the same time, and related to this, the rain-bearing mid-latitude storm tracks also shift poleward.
Both changes in atmospheric circulation, which are not fully understood, cause the poleward flanks of the subtropics to dry. Other land regions expected to be affected by subtropical drying include southern Europe, North Africa and the Middle East as well as parts of South America.
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