Editing IPCC:AR6/SRCCL/Chapter-3 (section)

=== 3.3.1 Sand and dust aerosols ===

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Sand and mineral dust are frequently mobilised from sparsely vegetated drylands forming ‘sand storms’ or ‘dust storms’ (UNEP et al. 2016 <sup>[[#fn:r457|457]]</sup> ). The African continent is the most important source of desert dust; perhaps 50% of atmospheric dust comes from the Sahara (Middleton 2017 <sup>[[#fn:r458|458]]</sup> ). Ginoux et al. (2012) <sup>[[#fn:r459|459]]</sup> estimated that 25% of global dust emissions have anthropogenic origins, often in drylands. These events can play an important role in the local energy balance. Through reducing vegetation cover and drying the surface conditions, desertification can increase the frequency of these events. Biological or structural soil crusts have been shown to effectively stabilise dryland soils. Thus their loss due to intense land use and/ or climate change can be expected to cause an increase in sand and dust storms ( ''high confidence'' ) (Rajot et al. 2003 <sup>[[#fn:r460|460]]</sup> ; Field et al. 2010 <sup>[[#fn:r461|461]]</sup> ; Rodriguez-Caballero et al. 2018 <sup>[[#fn:r462|462]]</sup> ). These sand and dust aerosols impact the regional climate in several ways (Choobari et al. 2014 <sup>[[#fn:r463|463]]</sup> ). The direct effect is the interception, reflection and absorption of solar radiation in the atmosphere, reducing the energy available at the land surface and increasing the temperature of the atmosphere in layers with sand and dust present (Kaufman et al. 2002 <sup>[[#fn:r464|464]]</sup> ; Middleton 2017 <sup>[[#fn:r465|465]]</sup> ; Kok et al. 2018 <sup>[[#fn:r466|466]]</sup> ). The heating of the dust layer can alter the relative humidity and atmospheric stability, which can change cloud lifetimes and water content. This has been referred to as the semi-direct effect (Huang et al. 2017 <sup>[[#fn:r467|467]]</sup> ). Aerosols also have an indirect effect on climate through their role as cloud condensation nuclei, changing cloud radiative properties as well as the evolution and development of precipitation (Kaufman et al. 2002 <sup>[[#fn:r468|468]]</sup> ). While these indirect effects are more variable than the direct effects, depending on the types and amounts of aerosols present, the general tendency is toward an increase in the number, but a reduction in the size of cloud droplets, increasing the cloud reflectivity and decreasing the chances of precipitation. These effects are referred to as aerosol-radiation and aerosol–cloud interactions (Boucher et al. 2013 <sup>[[#fn:r469|469]]</sup> ).

There is ''high confidence'' that there is a negative relationship between vegetation green-up and the occurrence of dust storms (Engelstaedter et al. 2003 <sup>[[#fn:r470|470]]</sup> ; Fan et al. 2015 <sup>[[#fn:r471|471]]</sup> ; Yu et al. 2015 <sup>[[#fn:r472|472]]</sup> ; Zou and Zhai 2004 <sup>[[#fn:r473|473]]</sup> ). Changes in groundwater can affect vegetation and the generation of atmospheric dust (Elmore et al. 2008 <sup>[[#fn:r474|474]]</sup> ). This can occur through groundwater processes such as the vertical movement of salt to the surface causing salinisation, supply of near-surface soil moisture, and sustenance of groundwater dependent vegetation. Groundwater dependent ecosystems have been identified in many dryland regions around the world (Decker et al. 2013 <sup>[[#fn:r475|475]]</sup> ; Lamontagne et al. 2005 <sup>[[#fn:r476|476]]</sup> ; Patten et al. 2008 <sup>[[#fn:r477|477]]</sup> ). In these locations declining groundwater levels can decrease vegetation cover. Cook et al. (2009) <sup>[[#fn:r478|478]]</sup> found that dust aerosols intensified the ‘Dust Bowl’ drought in North America during the 1930s.

By decreasing the amount of green cover and hence increasing the occurrence of sand and dust storms, desertification will increase the amount of shortwave cooling associated with the direct effect ( ''high confidence'' ). There is ''medium confidence'' that the semi-direct and indirect effects of this dust would tend to decrease precipitation and hence provide a positive feedback to desertification (Huang et al. 2009 <sup>[[#fn:r479|479]]</sup> ; Konare et al. 2008 <sup>[[#fn:r480|480]]</sup> ; Rosenfeld et al. 2001 <sup>[[#fn:r481|481]]</sup> ; Solmon et al. 2012 <sup>[[#fn:r482|482]]</sup> ; Zhao et al. 2015 <sup>[[#fn:r483|483]]</sup> ). However, the combined effect of dust has also been found to increase precipitation in some areas (Islam and Almazroui 2012 <sup>[[#fn:r484|484]]</sup> ; Lau et al. 2009 <sup>[[#fn:r485|485]]</sup> ; Sun et al. 2012 <sup>[[#fn:r486|486]]</sup> ). The overall combined effect of dust aerosols on desertification remains uncertain with low agreement between studies that find positive (Huang et al. 2014 <sup>[[#fn:r487|487]]</sup> ), negative (Miller et al. 2004 <sup>[[#fn:r488|488]]</sup> ) or no feedback on desertification (Zhao et al. 2015 <sup>[[#fn:r489|489]]</sup> ).

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==== 3.3.1.1 Off-site feedbacks ====

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Aerosols can act as a vehicle for the long-range transport of nutrients to oceans (Jickells et al. 2005 <sup>[[#fn:r490|490]]</sup> ; Okin et al. 2011 <sup>[[#fn:r491|491]]</sup> ) and terrestrial land surfaces (Das et al. 2013 <sup>[[#fn:r492|492]]</sup> ). In several locations, notably the Atlantic Ocean, the west of northern Africa, and the Pacific Ocean east of northern China, a considerable amount of mineral dust aerosols, sourced from nearby drylands, reaches the oceans. It was estimated that 60% of dust transported off Africa is deposited in the Atlantic Ocean (Kaufman et al. 2005 <sup>[[#fn:r493|493]]</sup> ), while 50% of the dust generated in Asia reaches the Pacific Ocean or further (Uno et al. 2009 <sup>[[#fn:r494|494]]</sup> ; Zhang et al. 1997 <sup>[[#fn:r495|495]]</sup> ). The Sahara is also a major source of dust for the Mediterranean basin (Varga et al. 2014 <sup>[[#fn:r496|496]]</sup> ). The direct effect of atmospheric dust over the ocean was found to be a cooling of the ocean surface ( ''limited evidence, high agreement'' ) (Evan and Mukhopadhyay 2010 <sup>[[#fn:r497|497]]</sup> ; Evan et al. 2009 <sup>[[#fn:r498|498]]</sup> ) with the tropical North Atlantic mixed layer cooling by over 1°C (Evan et al. 2009 <sup>[[#fn:r499|499]]</sup> ).

It has been suggested that dust may act as a source of nutrients for the upper ocean biota, enhancing the biological activity and related carbon sink ( ''medium'' ''evidence, low agreement'' ) (Lenes et al. 2001 <sup>[[#fn:r500|500]]</sup> ; Shaw et al. 2008 <sup>[[#fn:r501|501]]</sup> ; Neuer et al. 2004 <sup>[[#fn:r502|502]]</sup> ). The overall response depends on the environmental controls on the ocean biota, the type of aerosols including their chemical constituents, and the chemical environment in which they dissolve (Boyd et al. 2010 <sup>[[#fn:r503|503]]</sup> ).

Dust deposited on snow can increase the amount of absorbed solar radiation leading to more rapid melting (Painter et al. 2018 <sup>[[#fn:r504|504]]</sup> ), impacting a region’s hydrological cycle ( ''high confidence'' ). Dust deposition on snow and ice has been found in many regions of the globe (e.g., Painter et al. 2018; Kaspari et al. 2014 <sup>[[#fn:r505|505]]</sup> ; Qian et al. 2015 <sup>[[#fn:r506|506]]</sup> ; Painter et al. 2013 <sup>[[#fn:r507|507]]</sup> ), however quantification of the effect globally and estimation of future changes in the extent of this effect remain knowledge gaps.

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