A new estimate of global desert area and quantity ofdust emission

Abstract

<p indent=0mm>Deserts and their margins have long presented a challenge for human habitation, as an unstable and easily degraded environment. Deserts are the most important source of global dust emission, and the dust flux can be used as an index of land degradation and climate change, as well as a way to assess the dust’s own effects on global climate. In this study, we used the Moderate Resolution Imaging Spectroradiometer (MODIS) data, combing with Support Vector Machine (SVM) analysis, trial-and-error method and visual interpretation analyses, to extract the global and regional desert area, and analyze the accuracy of classification of the extracted desert area. We use data on topography, precipitation, evapotranspiration, and surface wind speed to characterize the environment of the world’s deserts, in order to explore the natural conditions that produce and accompany desert and produce a clear objective definition for desert environment. Our results show that global desert area is <sc>~17.5×10⁶ km²,</sc> and the overall classificatory accuracy is 92%, according to the cross-checked by the multi sources data calibrations. Globally, desert is mainly distributed in eastern hemisphere and northern hemisphere; North Africa and East Asia have the largest desert area. The classificatory accuracy of all individual regions is greater than 75%. Global desert, as characterized by our multi-faceted definition, is mainly distributed in low regions below<sc>2000 m</sc> elevation, mean annual precipitation less than <sc>200 mm,</sc> and annual wind speed is about <sc>4–6 m/s.</sc> Based on our defined distribution of global deserts, we estimated dust emissions from those areas. Because dust emission is a small-scale, random process, it is necessary to collect field data of dust emission from typical landscape types, in order to describe dust emission and quantitatively assess their impact. We used data from the Portable <italic>In-Situ</italic> Wind Erosion Laboratory (PI-SWERL), which is a newly developed portable wind erosion instrument. Its observation data are in good agreement with the wind tunnel experiment results. Because of its small size and portability, it has great advantages in measuring dust emission at specific sites. We integrated measured data about 1400 PI-SWERL observation points of dust emission (PM₁₀), combing with land cover type, soil moisture and friction wind speed data, and obtained about 14000 points quantifying dust emission through interpolation. These 14000 dust emission amounts points are distributed across different areas of the desert. Using these 14000 points to make Tyson polygons, we estimated the annual dust emission amount of each polygon area, and then added each annual dust emission amounts polygon area to obtain the annual dust emission amount of global desert. Dust emission amount shows considerable geographical differences between different desert areas. The region around the Caspian Sea and Aral Sea in central Asia, Mesopotamia in western Asia, sandy areas in northern West Asia and eastern Australia have the highest annual dust emission rates. This result is in good agreement with previous published results on global dust source regions. The annual dust emission amounts in desert area are ~1792.65 Tg/a, and North Africa has the largest annual dust emission amounts, ~786.03 Tg/a, accounting for 43.85% of the total dust emission amounts of global desert. According to the relationship between dust deposition in land and sea, we estimate that ~1344.49 Tg/a dust is deposited in land and ~448.16 Tg/a in ocean. The global desert area interpreted from remote sensing images using our new standardized protocol, and our dust emission estimates, will facilitate further understanding of the environmental changes within deserts and their impacts.