Desert climate overtaking more of Central Asia

An analysis of the region's climate has revealed that what was once a zone of semi-arid climate, featuring at least some summer precipitation, has since transitioned to a drier and hotter clime offering little rainfall during the growing season. The average annual temperature of the once-temperate areas rose roughly 9 degrees Fahrenheit when comparing the 20-year stretch of 1960-1980 to the 30-year period of 1990-2020.

More than 70 million people live in Central Asia, which comprises five former Soviet republics -- Kazakhstan, Kyrgyzstan, Tajikistan, Turkmenistan and Uzbekistan -- but is sometimes considered to encompass Afghanistan, western China and fragments of other neighboring nations. Because more than 60% of its land contends with arid or semi-arid climate, the region is especially susceptible to drought and sensitive to fluctuations in precipitation, the researchers said.

"The region is dry, so small deviations from the average or anticipated amount of growing-season rainfall can be devastating to the agricultural production and social stability of the region," said Qi "Steve" Hu, professor of natural resources and of Earth and atmospheric sciences at Nebraska. "It's a place very vulnerable to climate change."

That vulnerability spurred Hu and Lanzhou University's Zihang Han to examine Central Asia's monthly air temperature and precipitation data, which stretches back to the mid-20th century. But unlike most prior studies, which analyzed variations in individual elements of climate, Hu and Han tied the region's temperature and precipitation to its vegetation and ecology by instead assessing shifts in climate types, from tundra and temperate continental to subtropical and desert.

The duo found that, even as the latitudinal zones of desert climate were expanding northward, an envelope of wetter, colder continental climate farther north was busy spreading southward. By narrowing the distance between the two climates, that pincer movement has also steepened the gradient of temperature and pressure disparities across the latitudes. The steeper gradient favors a clockwise rotation of the atmosphere that itself favors the sinking of air, Hu said, essentially acting as a "cloud killer" that could lead to even less precipitation and a more severe desert climate.

All 11 of the identified climate types in Central Asia saw substantial increases in annual temperature from 1990 to 2020. Whereas most of those climates also experienced decreases in annual precipitation, the region's high-altitude areas -- among them, the Tianshan Mountains bordering China, Kazakhstan, Kyrgyzstan and Uzbekistan -- have generally seen their precipitation increase. Combined with the simultaneous warming of those mountainous areas, a significant proportion of that extra precipitation -- about 8 inches' worth -- has taken the form of rain rather than snow.

Those increases in temperature and rain have likely driven the depletion of glaciers that previously dominated the mountainous landscapes, Hu said. In the short term, a corresponding increase in meltwater is feeding rises in groundwater and bodies of surface water, including lakes, across western China's Xinjiang Province and similarly mountainous areas. But whatever temporary gains those areas are enjoying, he said, are likely coming at the expense of the more sustainable meltwater that the region's mountains have supplied for generations.

"That's coming at the cost of your future water," Hu said, adding that the risk of flash flooding will grow in the meantime. "If this continues, after maybe 20 or 30 more years, those glaciers and that snowpack could be gone. Then you have just the summer precipitation, which is not going to be enough to keep up the water level in your lakes and your soil to sustain agricultural production in the growing season.

"So you're possibly going to be in a one-way drought that you don't come back from."

Though the situations are distinct, the projected uncertainty in Nebraska's summer precipitation and in the snowpack of the Rocky Mountains -- snowpack that feeds the Platte River -- should have Huskers thinking about the future of water in their own state, Hu said.

"This has some potential implications for the situation we're facing in the west-central United States, especially around the Rockies," he said. "That could undermine the availability of water resources in the next 50 years."