Why does East Asian dust activity decrease in the past two decades?

Dust activity strongly decreases in last two decades over East Asia. A model-based study reveals that the weakening of surface winds and increase of vegetation cover and soil moisture all contribute to this decline. The weakening of surface winds plays the dominant role.
Why does East Asian dust activity decrease in the past two decades?

Dust storm is one of the major “natural” disasters in the world. It has many environmental consequences including severe air pollution, land degradation, and damage to crop and livestock. Dust entrained into the atmosphere serves as a major aerosol type, exerting a significant effect on weather and climate system via aerosol-radiation-cloud interactions and delivering nutrients from continents to other continents and oceans. Dust storm is also regarded as an important mechanism for the formation of loess sediments. Therefore, it is important to understand the evolution of dust activity in the past and project its changes in the future.

In recent decades, East Asian dust activity has decreased greatly. In particular, there is a strong decline of dust activity after a dusty period of 2000-2002. What are the reasons for the recent decline of dust activity in East Asia? How will dust activity change in the future? It is of great concern whether this decreasing trend is going to continue.

As we know, dust emission depends on various factors including surface winds, soil conditions, vegetation cover, and human disturbances. A research team led by Associated Professor Chenglai Wu and Professor Zhaohui Lin from the Institute of Atmospheric Physics, Chinese Academy of Sciences have revealed the drivers for the recent decline of East Asian dust activity. This is obtained by using a physically-based dust emission model (DuEM) which can account for the impacts of these factors in a numerical way. They first reconstructed the dust emission flux from 2001 to 2017 by using latest meteorology reanalysis and satellite vegetation cover data to force the DuEM model. Then, they conducted several sensitive experiments with one individual factor (such as surface wind, soil moisture, and vegetation cover) varying during 2001-2017 but other factors fixed to the level of the beginning year (i.e., year 2001). By comparing the results from these experiments, they isolate the impacts of surface wind, soil moisture, and vegetation cover on the temporal variations of dust emission during 2001-2017.

They show that DuEM reproduces well the interannual variations and decreasing trends of dust activity in East Asia during 2001 to 2017. Regional accumulated dust emission over the main source regions (35-49 ºN, 94-126.5ºE) has decreased significantly from 308 million ton per year in 2001 to 202 million ton per year in 2010s (2010-2017). They also find that the weakening of surface wind and the increase of vegetation cover and soil moisture all contribute to such dust emission reduction. The relative contributions of these three factors are 46%, 30%, and 24%, respectively. Weakening of surface wind plays the dominant role and increase of vegetation cover and soil moisture also have key contributions.

These results suggest that changes in meteorological factors are the main drivers for recent decline in dust activity over East Asia. In fact, the weakening of surface wind can be ascribed to the reduced meridional temperature gradients due to the polar amplification of global warming. The weakening can also be partly explained by internal variability in the climate system such as the Pacific Decadal Oscillation. Increase of vegetation cover is closely related to the increase of CO2 concentrations and temperature as well as ecological restoration in China. Soil wetting may be mainly ascribed to the increase of precipitation in the source regions. Therefore, it is needed to take into account the evolution of all these relevant factors for projecting future dust change in this region.

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