Soil Atmospheric Dust
Unsustainable land use may increase global dust emissions
Sand- and dust storms (SDS) occur when wind mobilises exposed, loose soil. These conditions are common in semi-arid and arid regions. Sandstorms typically occur relatively close to the ground surface, but fine dust particles may be lifted high into the atmosphere (several kilometres) where strong winds can transport them vast distances across oceans and continents. About 75 % of global dust emissions arise from natural sources. Topographic depressions in arid regions, mainly dry lake beds with little vegetation cover, are a major source. Disturbances to the sediment or soil surface, e.g. removal of vegetation cover and the destruction of biological crusts by vehicles, increases their susceptibility to SDS.
Although anthropogenic sources currently constitute only 25 % of global dust emissions, the potential for increasing this is great. Anthropogenic sources stem largely from ephemeral water surfaces. Human-induced hydrological changes, often driven by demand for water in areas adjacent to natural sources, can lead to the desiccation of wet or ephemeral water bodies and thus an increased risk in SDS. Unsustainable land use and land degradation, especially in arid and semi-arid areas, poses a corresponding increased risk of SDS. Major SDS events in various parts of the world, such as the American Dust Bowl, have occurred due to a combination of prolonged drought and unsustainable land management practices.
SDS are characterised and tracked using a combination of satellite imagery, ground monitoring and numerical modelling. The largest areas with high SDS are located in the Northern Hemisphere, mainly in a broad “dust belt” that extends from the west coast of North Africa, over the Middle East, Central and South Asia, to China. Although it can have large local impacts, the Southern Hemisphere contributes comparatively little to global atmospheric dust, with small concentrations in central Australia, southern Africa and the Atacama in South America.
Simulations suggest that total global annual dust emissions have increased by 25 % to 50 % over the past century due to a combination of land use and climate change. SDS frequency and severity have increased in recent decades in some areas but decreased in others. There have been few major changes in SDS over the past three decades over North Africa, the Middle East and South America, whereas there have been substantial changes in the US high plains, central Asia and Australia. There is evidence to support positive impacts of improved land management on reducing SDS, particularly in northern China.
Climate change projections suggest regions that are currently sources of dust are likely to become drier; this includes most of the Mediterranean areas of Europe and Africa, the northern Sahara, central and west Asia, southwest USA and southern Australia. Precipitation has increased in mid-latitude land areas in the Northern Hemisphere since 1950, which might help to reduce dust emissions in the mid-latitude belt. Dusty regions that are likely to become wetter include East Africa and East Asia, whereas large model uncertainties preclude projections for the Sahel-Sudan, the Gangetic basin and the Lake Eyre region.
Dust deposition has positive and negative environmental impacts. Dust affects and interacts with the climate system in a variety of different ways, e.g. influencing the radiative balance of the Earth by scattering and absorbing incoming solar radiation and indirectly by changing the optical properties of clouds. There is increasing concern in the international community because SDS have damaging effects on human health through air quality (especially in arid and semi-arid regions) and damage agricultural land, infrastructure and transport. Inhalation of fine particles can cause or aggravate diseases such as asthma, bronchitis, emphysema (damage of the air sacs in lungs) and silicosis (lung fibrosis). In Sahelian countries there is a strong correlation between dust loads from the Sahara and meningitis outbreaks. On the positive side, global dust deposition provides nutrients to terrestrial ecosystems and oceans, thereby boosting primary productivity, which in turn affects the global carbon cycle. Saharan dust fertilises the Amazon rainforest, providing a phosphorus input comparable to the hydrological loss from the basin. Similarly, Hawaiian rain forests receive nutrient inputs via dust from central Asia. On the other hand, dust from Africa and Asia may have harmful effects on coral reefs in the Americas.
SDS have wide-ranging economic impacts, both immediate and long-term. In addition to the environmental and health impacts, short-term costs include crop damage, livestock mortality, infrastructural damage (e.g. buildings, power, communications), interruption of transport and communications, air and road traffic accidents and costs of clearing sand and dust. Longer-term costs include chronic health problems, soil erosion and reduced soil quality, soil pollution through deposition of pollutants and disruption of global climate regulation. Economic losses from a single SDS event can be in the order of hundreds of million dollars.
The global impacts of local and regional dust emission
Climate processes and human actions combine to generate dust. Climate change, especially increasing aridity, leads to an increase in naturally occurring dust particles and, hence, dust storms. Dust affects climate through various direct and indirect mechanisms, including its effect on:
- the carbon cycle via the redistribution of soil organic carbon and as a moderator of carbon sources and sinks,
- marine primary productivity and surface ocean cooling,
- radiative effects.
Dust-induced climate change, especially in drylands, can increase the frequency, duration and intensity of droughts.
Particle transport processes
Sand and dust storms are atmospheric events that result from the erosion and transport of mineral sediments from the ground surface. They are typically associated with arid and semi-arid areas, but can occur anywhere where there are dry and unprotected sediments.
The process involves three phases: the entrainment or emission of surface material, its transport through the atmosphere and its deposition. Entrainment of particles occurs when the wind shear stress exerted on the surface (wind erosivity) exceeds the ability of the surface material to resist detachment or transport (sediment or soil erodibility).
Wind transport can cause particle movement through the processes of creep, saltation and suspension. Particles larger than about 500 microns in diameter will creep across the land surface. Saltation is when wind transports particles of between 63 and 500 microns and usually at a height of less than 1.5 metres above ground level. Suspension refers to the longer-range transport of particles of diameter of less than 63 microns. Sandblasting is a process whereby saltating particles bombard soil aggregates, causing aggregate fragmentation and the release of fine particles that are then entrained.
Particle transport processes.
Source: WAD3-JRC, 2018.