The United States and many other heavily populated countries face a growing threat of severe and prolonged drought in coming decades, according to a new study by National Center for Atmospheric Research (NCAR) scientist Aiguo Dai.

The detailed analysis 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.

Using an ensemble of 22 computer climate models and a comprehensive index of drought conditions, as well as analyses of previously published studies, the paper finds most of the Western Hemisphere, along with large parts of Eurasia, Africa, and Australia, may be at threat of extreme drought this century.

In contrast, higher-latitude regions from Alaska to Scandinavia are likely to become more moist.

Dai cautioned that the 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 Nino.

The new findings appear this week as part of a longer review article in Wiley Interdisciplinary Reviews: Climate Change. The study was supported by the National Science Foundation, NCAR's sponsor.

"We are facing the possibility of widespread drought in the coming decades, but this has yet to be fully recognized by both the public and the climate change research community," Dai says. " If the projections in this study come even close to being realized, the consequences for society worldwide will be enormous."

While regional climate projections are less certain than those for the globe as a whole, Dai's study indicates that most of the western two-thirds of the United States will be significantly drier by the 2030s. Large parts of the nation may face an increasing risk of extreme drought during the century.

Other countries and continents that could face significant drying include: much of Latin America, including large sections of Mexico and Brazil; regions bordering the Mediterranean Sea, which could become especially dry; large parts of Southwest Asia; most of Africa and Australia, with particularly dry conditions in regions of Africa; Southeast Asia, including parts of China and neighboring countries.

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. However, the globe's land areas should be drier overall.

"The increased wetness over the northern, sparsely populated high latitudes can't match the drying over the more densely populated temperate and tropical areas," Dai says.