Answers to why dust storms are on the rise in East Asia
.jpg "Dust storm that hit south of Beijing in 2008 (Source: NASA)")
OVER the past two decades there has been an increase in the outbreak of dust storms over East Asia. Large quantities of dust particles suspended in the air not only block the amount of sunlight reaching the earth, they also cause respiratory and visual complications.
Dust storms are attributed to two factors. First erodibility—susceptibility of soil to wind erosion and second erosivity—ability of wind to cause erosion. While erosivity depends on wind speed, erodibility depends on a number of factors like distribution of soil particles, water content, precipitation, vegetation, snow cover and land use apart from wind speed. But little data is available on these factors, making it difficult to identify the reasons behind the increase in the dust storms.
Scientists at the Arid Land Research Center and Meteorological Research Institute in Japan examined the relative contribution of these two factors in causing dust outbreaks over the East Asian region. They analysed data collected in the last two decades by the World Meteorological Organization.
The East Asian region was divided according to its topography. Southern Mongolia, western Inner Mongolia, Hexi Corridor and the Taklimakan Desert were categorised as desert, while northeastern China, north China plain were named cultivated land. Northern Mongolia and eastern Inner Mongolia were classified as grasslands.
They found that in the desert region erosivity was the cause of dust storms. In cultivated land and grasslands, wind speed was found to be relatively constant but the minimum wind speed required to create a dust storm decreased. So in these places erodibility was the culprit. They found that the quantity of dead grass or residual grass also played a part in determining the erodibilty of soil. Vegetation cover is maximum during the summer season. If there is less vegetation due to less rainfall in summer then during the spring there will be less residual grass to hold the top layer of soil and more erodibility. “Dead leaves reduce the erodibility by covering soil surface, extracting momentum from the air, and trapping soil particles in transport,” the researchers note. Hence they postulated a dead leaf hypothesis. They add these observations about plant growth and precipitation during the summer could provide a platform to forecast the frequency of dust storms the following year. “But we also need to study the growing and decaying processes of grass. It depends on species and climate,” says Yasunori Kurosaki, lead author. The study was published in June 10 issue of American Geophysical Union.
Andrew Goudie, emeritus professor at the St Cross College, Oxford, and expert in the field, though notes the study is interesting as previous studies had detected a downward trend in East Asian dust outbreaks from 1970 onwards. “This one reveals a recent change in trend. But it is more due to changes in precipitation and wind conditions than to human activities,” he opines.
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