美國國家大氣研究中心(NCAR)在一項研究中發現，美國以及其他許多人口稠密的國家，將因為全球暖化，而在30年內遭遇嚴重且持續的旱災。
華裔大氣科學家戴愛國(Dr. Aiguo Dai)與NCAR的博德分部共同作出結論，指出與氣候變遷相關的溫度上升，在接下來的30年間，很可能會造成全球大部分地區越來越乾燥，影響所及甚至讓許多過去沒有被注意到的地區，在本世紀末也受到影響。
戴愛國的研究顯示，美國西部2/3大部分的地區，在2030年時會明顯比現在更乾燥。在本世紀，美國大部分地區面臨到極度乾旱的風險也會逐漸增加。
集合了22台電腦所做的氣候模型、全面性的乾旱指標、以及先前所發表的研究結果，戴愛國發現西半球的大多數地區，從歐亞大陸大部分地區，非洲以及澳洲一帶，在本世紀內都可能會有極端乾旱的風險。
相反的，從阿拉斯加到北歐的高緯度地區，則可能變得更加濕潤。
戴愛國警告說，他的發現是根據目前對溫室氣體排放最佳的預測。接下來幾十年所會發生的事情取決於幾項因素，包括未來實際的溫室氣體排放量以及自然氣候如聖嬰現象的週期。
哥倫比亞大學Lamont Doherty地球天文台的氣候變遷專家西格(Richard Seager)說，「戴愛國強調，亞熱帶以及中緯度大陸大部分的地區，將面臨著更乾燥的土壤以及地表水的減少，這都是因為氣溫變暖而減少的降雨以及蒸發量增加。」他本人並沒有參與這項研究。
未來全球變遷複雜化 「全球暖化」用詞不精確
西格警告，「但是『全球暖化』這個名詞，對全球將來幾十年所面對的氣候變遷現象並不夠周詳。我們可能面臨的，有些最嚴重的狀況牽涉的不只是溫度的變化，還有水的問題。」
雖然區域性的氣候預測無法預測全球的狀況，戴愛國的研究顯示，拉丁美洲大部分地區，包括墨西哥與巴西多數區域將受到影響。地中海沿岸地區則可能會變得特別乾燥。
戴愛國預測，亞洲西南部大部分地區，橫跨東南亞、中國部分地區以及週邊國家都會發生乾旱。最後，乾旱會影響非洲與澳洲大部分區域，非洲部分特定地區可能會非常嚴重。
這項研究還發現，雖然地球上的陸地，總的來說會變得更乾燥，但是大部分的北歐、俄羅斯、加拿大、阿拉斯加以及南半球部分區域，在本世紀的乾旱風險則會下降。
先前的研究已指出，全球暖化現象會因為亞熱帶區域擴大而改變全球的降水模式。
2007年政府間氣候變遷委員會(IPCC)的評估結論是，降水在亞熱帶地區有可能下降，而高緯度地區則會逐漸增加。
戴愛國與同事在2004年發表的一份被大量引用的文獻中指出，21世紀初期全球陸域面積遭受嚴重乾旱的比例，比1970年代高了一倍以上。
乾旱的成因十分複雜，與降水減少、未能維持作物生長的乾燥土地、水庫以及其他供飲用水體的蓄水下降有關。
DPSI乾旱指標 未來恐怕已不適用
帕爾默乾旱指數(Palmer Drought Severity Index, PDSI)是分析乾旱狀態常見的方法，透過追蹤特定地點降水以及蒸發量隨時間的變化並加以比較，可以知道其乾旱狀態。
在這項研究中，戴愛國利用IPCC在2007年報告中所使用的22個電腦模型，以現代對溫室氣體排放的預測，收集有關溫度、降水、溼度、風速等與地球的輻射平衡數據。
他將這些資訊導入帕爾默模型，來計算乾旱嚴重的程度。指數值介於+0.5到-0.5之間，表示正常狀態，而等於-4或更低時，表示極度的乾旱。
結果顯示，在2030年之前，美國中部與西部大部分的地區，會面臨嚴重的乾旱，PDSI乾旱指數十年間的平均值將落在-4到-6之間。地中海的部分地區的PDSI指數將低到-8或更低。
到本世紀末，許多人口密集的地區，包括美國部分地區的指數，可能低到-8到-10，而地中海大部分地區則會低到-15到-20。這樣的乾旱指數是前所未有的。
戴愛國警告說，全球氣候模型對降水以及其他大氣因子的掌握仍不一致，特別是在限定區域的尺度之下。然而，IPCC在2007年所提出的模型中，對低緯度高海拔地區的結論，與先前的報告相比，是有相當共識的。
對未來氣候產生的各種狀況而言，帕默爾指數能掌握的程度仍有不確定性。戴愛國說，在極端的情況之下，該指數可能會高估乾旱強度。另一方面，該指數可能低估了短暫高強度降雨及降雪導致的土壤濕度流失。戴愛國說，這樣的降水趨勢近年來在美國已經發生。「事實上，目前的乾旱評估指數可能不適於21世紀的氣候狀況，本身就是件令人不安的事。」
Global warming will bring on severe and prolonged drought across the United States and many other heavily populated countries within 30 years, finds a new study by the National Center for Atmospheric Research.
Atmospheric scientist Dr. Aiguo Dai, with NCAR's Climate and Global Dynamics Division in Boulder, concludes that warming temperatures associated with climate change will likely create increasingly dry conditions across much of the globe in the next 30 years, possibly reaching a scale in some regions by the end of the century that has rarely, if ever, been observed in modern times.
Most of the western two-thirds of the United States will be significantly drier by the 2030s, Dai's study shows. Large parts of the nation may face an increasing risk of extreme drought during the century.
Using an ensemble of 22 computer climate models and a comprehensive index of drought conditions, as well as analyses of previously published studies, Dai found that most of the Western Hemisphere, along with large parts of Eurasia, Africa, and Australia, will be at risk of extreme drought this century.
By contrast, higher-latitude regions from Alaska to Scandinavia are likely to become more moist.
Dai cautioned that his findings are based on the best current projections of greenhouse gas emissions.
What actually happens in coming decades will depend on many factors, including actual future emissions of greenhouse gases as well as natural climate cycles such as El Niño.
Richard Seager of Columbia University's Lamont Doherty Earth Observatory, a climate change expert not associated with the study, said, "As Dai emphasizes here, vast swaths of the subtropics and the midlatitude continents face a future with drier soils and less surface water as a result of reducing rainfall and increasing evaporation driven by a warming atmosphere."
"The term 'global warming' does not do justice to the climatic changes the world will experience in coming decades," warned Seager. "Some of the worst disruptions we face will involve water, not just temperature."
While regional climate projections are less certain than those for the globe as a whole, Dai's study indicates much of Latin America, including large sections of Mexico and Brazil will be affected.
Regions bordering the Mediterranean Sea could become especially dry.
Dai predicts drought across large parts of Southwest Asia and also across Southeast Asia, including parts of China and neighboring countries.
Finally, drought will affect most of Africa and Australia, with particularly dry conditions in some regions of Africa, Dai's analysis shows.
While the planet's land areas should be drier overall, the study also finds that drought risk can be expected to decrease this century across much of Northern Europe, Russia, Canada, and Alaska, as well as some areas in the Southern Hemisphere.
Previous climate studies have indicated that global warming will probably alter precipitation patterns as the subtropics expand.
The 2007 assessment by the Intergovernmental Panel on Climate Change, IPCC, concluded that subtropical areas will likely have precipitation declines, with high-latitude areas getting more precipitation.
In a much-cited 2004 study, Dr. Dai and colleagues found that the percentage of Earth's land area stricken by serious drought more than doubled from the 1970s to the early 2000s.
Droughts are complex events that can be associated with reduced precipitation, dry soils that fail to sustain crops, and reduced levels in reservoirs and other bodies of water that can imperil drinking water supplies.
A common measure called the Palmer Drought Severity Index classifies the strength of a drought by tracking precipitation and evaporation over time and comparing them to the usual variability one would expect at a given location.
For this study, Dai utilized results from the 22 computer models used by the IPCC in its 2007 report to gather projections about temperature, precipitation, humidity, wind speed, and Earth's radiative balance, based on current projections of greenhouse gas emissions.
He fed the information into the Palmer model to calculate the drought severity index number. A reading of +0.5 to -0.5 on the index indicates normal conditions, while a reading at or below -4 indicates extreme drought.
By the 2030s, the results show that much of the central and western United States could experience particularly severe conditions, with average readings over the course of a decade potentially dropping to -4 to -6 on the PDSI scale.
Parts of the Mediterranean region hit -8 or lower on the PDSI scale.
By the end of the century, many populated areas, including parts of the United States, could face readings in the range of -8 to -10, and much of the Mediterranean could fall to -15 to -20. Such drought readings would be unprecedented.
Dai cautions that global climate models remain inconsistent in capturing precipitation changes and other atmospheric factors, especially at the regional scale. However, the 2007 IPCC models were in stronger agreement on high-altitude and low-latitude precipitation than those used in previous reports, he points out.
There are also uncertainties in how well the Palmer index captures the range of conditions that future climate may produce. The index could be overestimating drought intensity in the more extreme cases, says Dai.
On the other hand, he said, the index may be underestimating the loss of soil moisture should rain and snow fall in shorter, heavier bursts and run off more quickly. Dai says such precipitation trends have already been identified in the United States in recent years.
Dai says, "The fact that the current drought index may not work for the 21st century climate is itself a troubling sign."
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