A large part of Central Asia is characterized by steppes and deserts – and they are also changing in the course of climate change, as researchers have determined. Accordingly, the northern boundary of the desert climate has shifted north by around 100 kilometers since the 1980s, and the adjacent steppe has spread north by about the same amount. At the same time, precipitation is falling more frequently than rain in the higher elevations of the mountains due to warming. As a result, the snow cover in the high mountains is disappearing and the lower-lying regions hardly get any meltwater – this is also exacerbating the increasing water shortage in Central Asia.
More than 70 million people live in Central Asia – a vast, mostly steppe region in the heart of Eurasia. This region used to be home to nomadic peoples, today it is divided among the countries of Kazakhstan, Kyrgyzstan, Tajikistan, Turkmenistan and Uzbekistan, as well as China. But steppe and temperate-cool deserts still dominate 60 percent of this region. This makes Central Asia, which is already dry, particularly vulnerable to fluctuations in precipitation or periods of drought. “Even small deviations from the mean or expected rainfall during the growing season can be devastating for the agricultural and social stability of this region,” explains lead author Qi Hu from the University of Nebraska-Lincoln. “This place is therefore very vulnerable to climate change.”
Steppes and deserts shifted 100 kilometers to the north
In order to determine whether and how the climate in Central Asia has already changed, Hu and his team evaluated weather data from the 1960s onwards on temperatures and precipitation in Central Asia. Her focus was on the location and extent of the climatic zones as defined in the Köppen and Geiger system. “Because the warm season precipitation changed significantly in the 1980s, we split the data time series into two sections and focus on the differences between the period 1960-1980 and 1990-2020,” the scientists explain. The comparisons initially confirmed that the annual mean temperatures in all eleven climate types occurring in Central Asia have increased – by an average of 0.6 to 1.7 degrees. At the same time, precipitation decreased in most regions.
“The most prominent change since the 1980s is the northward expansion of the desert climate,” the researchers report. “These changes can be seen along the temperate latitudes of northern Uzbekistan and Kyrgyzstan, southern Kazakhstan, and also along the northern and eastern edges of the Junggar Basin in western China.” the team determined. This spread of the desert climate was accompanied by a substantial increase in temperatures and a decrease in precipitation, especially in summer. At the same time, the zone of the steppe climate adjoining to the north has advanced by about the same amount to the north into the zone of the cool-temperate continental climate. Among other things, the north of the Chinese region of Xinjiang has become steppe or desert in many places.
More rain in the mountains
However, it has not become drier everywhere in Central Asia: the team found that there was even a slight increase in precipitation at higher altitudes in the mountains. As a result, the climatic zone of the summer-cool continental climate has expanded a little to the south. However, this is also less positive than it sounds at first. Because this leads to a gradient in precipitation and temperature compressed into a smaller space – the change from a dry, warm steppe climate to a cool, humid climate occurs over a relatively short distance. This in turn influences the air pressure distribution and strengthens the westerly winds in this transitional region. This leads to an increased inflow of dry air and fewer rain clouds. “A possible consequence could be that the steeper gradient causes a positive feedback that even accelerates the northward spread of the dry climate,” explain Hu and his colleagues.
Another problem: Precipitation in the higher elevations of the mountains is now falling more and more frequently as rain rather than snow due to warming. As a result, the snow cover in the mountains decreases and, as a result, the spring meltwater, which is often an important water resource for the areas lying at the foot of the mountains and on the plains, also decreases. “If this continues for another 20 or 30 years, then the mountain glaciers and snowpack could disappear entirely,” says Hu. “Then all that’s left are the sparse summer rains, which will not be enough to keep lake water levels and soil moisture high enough during the agricultural growing season.” For the inhabitants of the vast steppe regions of Central Asia, these climate changes mean a reduction in their water resources two-fold Considerations: It rains less, and at the same time the rivers fed by meltwater carry less water in spring – exactly when the plants need it most.
Source: Qi Hu (University of Nebraska-Lincoln) et al., Geophysical Research Letters, doi: 10.1029/2022GL098895