Decreasing Dust Over the Middle East Partly Caused by Irrigation Expansion.

The importance of the effects of anthropogenic activities on modulating the global dust cycle has been increasingly recognized. Over the Middle East, we find in observations that there has been a significant decrease in dust optical depth from 2007 to 2019 during which global irrigated areas especially in the Middle East and South Asia have rapidly expanded. Whether irrigation expansion contributes to the decrease of dust in the Middle East is investigated based on observations/reanalyses and global climate model simulations with and without irrigation. Results show that irrigation over the northeast Middle East and Pakistan supplements water to the soil. By increasing local evaporation and moisture advection, irrigation enhances precipitation over the whole Middle East and the northwest Indian subcontinent. As a result, dust wet deposition by precipitation is elevated. Owing to irrigation‐induced land surface cooling, surface wind speed decreases as the planetary boundary layer becomes stable. Along with increased soil moisture, reduced surface wind speed suppresses local dust emissions. Enhanced dust wet deposition and suppressed dust emissions cause atmospheric dust reduction over the Middle East. Vegetation greening in the Middle East as the climate warms has no contribution because there is no obvious trend found in observations. Plain Language Summary: Atmospheric dust absorbs and reflects solar and terrestrial radiation, and alters cloud lifetime and cloud albedo, which affects the earth's radiative budget and the hydrologic cycle. As one of the major dust source regions over the globe, dust emissions over the Middle East from wind erosion have a large contribution to the global dust loading. Anthropogenic land cover changes (e.g., deforestation) directly perturb the underlying land surface, thus affecting dust emissions and transport. However, the impacts of anthropogenic land use such as irrigation on dust are unclear. During recent decades, it is observed that dust optical depth is decreasing over the Middle East, simultaneously with an increase of irrigated areas over the Middle East and South Asia. Whether there is a linkage between them is explored in this study. By analyzing observations/reanalyses and global climate model simulations with and without irrigation, irrigation expansion over the northeast Middle East and Pakistan does reduce dust burden over the Middle East by increasing dust wet removal and decreasing dust emissions. The altered dust processes result from increased precipitation, soil moisture, and decreased surface wind speed by irrigation. Key Points: A decreasing trend of dust over the Middle East from 2007 to 2019 is found in observationsObserved significant decreasing dust in the Middle East can be partly attributed to irrigation expansionEnhanced dust wet deposition and suppressed dust emissions by irrigation are responsible for atmospheric dust reduction [ABSTRACT FROM AUTHOR]