Thursday 5 January 2023

The causes and impacts of dust and sand storms on the steppes of Northeast Asia.

The Eurasian Steppes stretch from Manchuria in the east to Hungary in the west, and can be loosely divided into three areas, the Mongolian Steppe, the Kazakh-Russian Steppe, and the Ukrainian-Hungarian Steppe. These are dry environments, which rely on small amounts of water reaching them from distant seas by repeated precipitation-evaporation cycles (i.e. the re-evaporation of water that has already fallen as rain once or even several times), and have suffered repeated cycles of desertification and rehabilitation with changes in the climate cycle, as fairly small changes in atmospheric moisture can lead to the advance and retreat of desert regions. This is in part due to the feedback loop between vegetation cover and heat distribution, with the air heating more rapidly above patches of ground denuded of vegetation, promoting gustier, windier conditions.

The peoples of these steppes have developed a nomadic pastoralist system which has persisted since at least the Bronze Age, which balances livelihoods with grassland stability by never staying in one place to long, thus preventing overgrazing and the desertification which would follow. While the livestock remove Grass from the system during grazing, they also contribute to the nutrient cycle by the production of manure, which returns nutrients to the soil. 

However, while over long periods of time the movements of nomadic groups are an essential part of the ecology of the steppes, over shorter periods this link can be broken by either natural forces or Human nature. From the 1970s onwards both crop cultivation and livestock farming were increased in the Chinese province of Inner Mongolia, resulting in a series of droughts and desertification events in the 1980s and 1990s. Between 1924 and 1990 the Mongolian People's Republic viewed all livestock in Mongolia as property of the state, and maintained livestock at low densities in keeping with traditional practices. However, following the peaceful democratic revolution of 1990, the nation's herds were privatised, following which the number of Animals being raised in the grazed in the country rose from 25 million in 1991 to 56 million in 2015, despite two severe winter livestock disasters within this period.

These severe winter events are called 'dzud' in Mongolian, and are periods when the steppes are visited by waves of extreme cold, accompanied by frozen ground and sometimes heavy snowfall. These events can lead to large-scale die-offs of livestock, but do not always do so; livestock die-offs generally only happen in years when there has been a summer drought, causing the Animals to enter the winter season with low fat reserves. On average, a dzud event happens every 4-5 years, causing significant hardship for nomads who lose their stock, but also enabling the grasslands to recover by reducing the number of grazing Animals.

Southward of the Mongolian steppes lie the deserts of the Gobi and the Hexi (or Gansu) Corridor, which are major suppliers of material to the dust and sandstorms of Northeast Asia. These storms are largely driven by weather conditions over the distant Pacific, Indian, and even Atlantic oceans, but are a major source of public health concerns, due to the high density of particulate matter in the air. For example the Korean warning threshold of 800 μg of particulate matter per square meter of air was last surpassed in the spring of 2021, when northern China was also affected by severe dust storms.

Northeast Asia suffered a string of these dust storm events in the early 2000s, leading to changes in agricultural policy in Inner Mongolia, which were thought to have alleviated the problem, but the storms returned in 2010, 2015, and 2016, underlining the difficulty of trying to predict such events, and their complicated relationships to other weather phenomena. Notably, dust storms appear to be connected to desertification and dzud events on the steppes, with dust storms and dzuds often occurring in the same years.

In a paper published in the Journal of Ecology and Environment on 26 November 2022, Sinkyu Kang of the Department of Environmental Science at Kangwon National University, Sang Hun Lee of the Center for Global Cooperation at the Korea Environment Institute, Nanghyun Cho and Casmir Aggossou, also of the Department of Environmental Science at Kangwon National University, and Jungwha Chun of the Forest ICT Research Center at the Korean National Institute of Forest Science, present a review of the current understanding of the causes and consequences of dust storm events in Northeast Asia, taking account of the geographical, climatic, and Human influences on these events.

The dust storms of Northeast Asia receive material from the shifting sands of Inner Mongolia and Mongolia, the Loess Plateau of north-central China, and the Gobi, Taklamakan, and Hexi Corridor deserts, with the majority of the dust reaching as far east as Korea and Japan coming from the Gobi Desert, Hexi Corridor, and Inner Mongolia.

A conceptual map of the geography and climate for major sources of dust and sandstorms in Northeast Asia. Major sources of dust and sandstorms are italicised. The straight line shows the approximate boundary of the dry area. The block arrows indicate the inflow path of moisture, and the stars are sample cities selected to demonstrate long-term climatic variability: Ulaanbaatar, Dalanzadgad, Yinchuan, and Lanzhou in the southward direction. Kang et al. (2022).

The contribution of different areas to dust and sandstorms from different areas varies over time, with the amount of dust and sand coming from the Taklamakan Desert and Loess Plateau having fallen steadily over the past few decades, while the amount of material coming from the Hexi Corridor and Mongolian Plateau fell off in the 1980s and 1990s, then rose again in the 2000s. In particular, the amount of dust coming from the southeastern Mongolian Plateau and Manchurian Drylands has risen during this time. 

The dust and sandstorms reach Korea are driven by the prevailing westerly winds coming from Central Asia. These events are most likely to happen in spring, with 80% of days affected by dust storm events since records began in 1960 having occurred between March and May. The frequency of such events appears to have varied on both five and thirty year cycles, and risen sharply after 1990, with an average of 2.6 dust storm days before 1990 and 7.1 after, rising to 9.7 in the 2000s and falling back to 6.1 in the 2010s. The worst affected year was 2001, when there were 23 dust storm days. At the same time, the these events shifted earlier in the year, with April being the most affected month in the twentieth century, but events in March becoming as common as those in April in the twenty first. By May 2021 the number of days with dust storm events recorded in that year had reached 9.3, exceeding the average for the 2010s (6.1).

A dust cloud moving from China over Korea and towards Japan on 21 March 2001. SeaWIFS Project/NASA/Godard Space Flight Center/Wikimedia Commons.

During dust and sandstorms sediment particles are swept up by winds, often reaching hundreds of metres into the atmosphere. Larger particles generally fall back to Earth close to the source, but fine dust particles, in the size range 1-10 μm, can be carried for hundreds of kilometres on the westerly winds, often falling on Korea or Japan two or three days after being picked up by the storm.

These storms only develop when strong winds blow over dry sediments, and a combination of high and low pressure centre's generates a strong updraught. As air is swept from an area of high pressure to one of low pressure it is deflected by the Coriolis forces generated by the Earth's rotation, at a direction tangential to the boundary between the two systems, forming what meteorologists refer to as a trough. The greater the difference between the pressure centres, the higher the wind-speed, and when the wind-speed reaches the 'dust point' it is able to lift dust into the atmosphere. In Asia this dust is then carried eastwards by the prevailing westerly winds. Thus, as the Sun more readily heats exposed soil, leading to higher air pressures, the dust storms are driven by the combination of strong sunlight and dry conditions.

This means that dust is easily generated in areas of semi-desert and desert grassland; as the vegetative cover increases, the roots of Grass and Shrubs hold the soil in place more effectively, and the amount of organic matter, which tends to bind the soil together increases. Eventually the ground vegetation becomes dense enough for leaves and branches to slow the winds, effectively preventing dust formation.

Plants also cool the ground through the transpiration process, further hampering the development of low pressure systems, effectively stopping dust formation. Whereas exposed soil absorbs heat from the Sun, then radiates it back into the atmosphere above, fuelling the generation of low pressure systems. Eventually this will manifest as strong gusts or even a whirlwind as a new trough forms.

A dust storm sweeps across Beijing in March 2021. Todd Lee/Zuma Wire/Rex/Shutterstock

A straight line can be drawn across a map of East Asia, separation the lowlands to the south and east from the highlands to the north and west. North of this line lie the Mongolian and Tibetan plateaus, and the drylands between them. To the south lie the forests and agricultural lands of China. This line also marks the effective limit of the Asian Monsoon climate.

During the Northern Hemisphere summer, high evaporation over the Indian and Pacific oceans form rain clouds, which are driven north and east by the prevailing westerly winds, depositing rain across East Asia. An atmospheric system known as the North Pacific High blocks the eastward migration of these rain systems, pushing them northward onto land. However, as these systems move north they meet the prevailing westerlies of Central Asia at the margins of the plateaus, preventing them from carrying their moisture onto the steppes, and creating a permanently arid climate across the interior of Northeast Asia.

However, this system is not immutable, and has changed throughout the Holocene. Around 8000 years ago, during the Holocene Climatic Maximum, global temperatures were significantly higher than today, leading to a much stronger Monsoon, which pushed further inland, causing the drylands to shrink. As the climate cooled after this event, the Monsoon limit retreated to something like its current mark, and the drylands moved eastwards into formerly fertile areas. However, the Monsoon line has continued to shift to a lesser extent throughout Human history, with the rise of the nomad empires of East Asia occurring during the Medieval Warm Period, when the drylands retreated and the plains of Mongolia became more fertile.

A small amount of moisture does make its way northward to the Asian steppes from the East Asian Monsoon, although most is pushed back by the prevailing westerly winds. Similarly, a small amount of moisture from the Sount Asian Monsoon, makes it over the Himalayas to the Tibetan Plateau, and the lands to the northeast. However, the largest source of moisture to this region is the remote Atlantic Ocean; the prevalent westerly winds blowing across Europe and Asia can carry moisture as far as Siberia and the drylands to the east, making this the biggest single source of rainwater in Mongolia and western China.

The annual formation of two weather systems over the North Atlantic, the Azores High and the Icelandic Low, creates a strong pressure trough which directs a stream of rain clouds towards northern Europe in the summer and central Europe in the winter, with some of these rain clouds moving onwards through eastern Europe and Siberia into Mongolia. This is not a simple process, with a series of high and low pressure systems along the way serving to keep pushing the moisture eastwards. The Mediterranean on the other hand, appears to make no contribution to the climate of Central and East Asia.

Thus the three largest contributors of moisture to the drylands of Northeast Asia are the Atlantic, Pacific, and Indian oceans, and the climate of these drylands is influenced by climatic events over these oceans, such as the El Niño/La Niña oscillations, the Atlantic Multidecadal Oscillation, and the Pacific Decadal Oscillation. Thus desertification cycles and dust storm events can be driven by climatic oscillations far from the drylands where they occur.

Rainfall records exist since 1940 for the cities of Ulaanbaatar and Dalanzadgad in Mongolia and Yinchuan and Lanzhou in China, to the north and south of the Gobi Desert, respectively. These records show that rainfall in Mongolia follows a roughly twenty year cycle, while in the two northern Chinese cities it follows a roughly five year cycle. This is likely to be connected to the arrival of dust storms in Korea, which has been shown to vary on both five and twenty year cycles. In addition, a general climatic warming in the region from 1970 onwards appears to have fuelled a series of droughts across Northeast Asia, by increasing the rate of evaporative water loss.

Schematic illustration of the recurrent circumglobal teleconnection (CGT) and Atlantic-Eurasian teleconnection (AEA) patterns in summer (June-July-August). The six ellipses denote the six centres of action of the circumglobal teleconnection. The shades areas denote the five centres of action of the Atlantic-Eurasian teleconnection. The solid black and green curves with arrows denote the wave paths of the circumglobal teleconnection and Atlantic-Eurasian teleconnection, respectively. Li & Ruan (2018).

Natural cycles are not the only cause of dust storm events in Northeast Asia. A range of Human activities, including excessive livestock farming, inappropriate crop cultivation in dryland environments, and large-scale mining operations can all reduce vegetative cover, and lower groundwater reserves, leading to droughts and desertification. Nevertheless, desertification does not automatically lead to dust storm events, it simply increases the size of the area in which these conditions can develop. This may mean that global climate change has a greater impact on the formation of dust storms than local desertification events. In the 1980s and 1990s, the drylands of northern China experienced extensive deretification, but this did not lead to a rise in dust storms due to a fall in wind speeds over the same period.

Desertification is the process by which non-desert areas are converted into deserts, a process with is accompanied by a loss of vegetative cover, leading to more exposed soil areas where dust storms can form. Furthermore, this process promotes the development of low pressure systems, and the accompanying winds, by leaving the ground exposed to direct sunlight.

The Ordos Plateau of Inner Mongolia, the area which includes the Kubuqi Desert, has had a 2000 year climatic record reconstructed from lake core samples. This shows that high levels of dust formation were associated with periods when the levels of Human settlement in the area were the highest. This in turn coincided with a period when the Asian Monsoon was strong, and higher levels of moisture were reaching the Plateau, leading to more vegetation growth, indicating that Human actions were overwhelming the natural processes in the region a thousand years ago.

Pastorialism has occurred for a long time on the steppes of Asia. Livestock grazing crops the the grass close to the ground, lowering the wind speed needed for dust uplift, while at the same time increasing ground temperatures by leaving more soil exposed to the direct sun.  Thus livestock farming has an inherent tendency to drive desertification and dust storm generation, particularly if to many Animals are raised densely in a small area of the dryland.

In the 1980s and 1990s attempts to convert parts of the drylands of northern China, including Inner Mongolia, into arable land led to widespread desertification. Then in the early 2000s a severe drought led to a series of massive dust storm events, leading to a change in policy. Since this time efforts have been made to reverse this desertification and restore natural vegetation to the area. These have been successful in some places, but the extent to which the dry grasslands of Inner Mongolia have recovered remains uncertain.

Livestock farming has also increased rapidly in Mongolia since 1991, when the country transitioned from a socialist to a market economy. This has been partly driven by the herding of Goats for cashmere, a valuable commodity. Overgrazing of the grasslands has accompanied this expansion in herding, particularly in areas around cities, and there are concerns that this may lead to a major desertification event. At the same time, a change in rainfall patterns has led to a recovery of the grasslands of northern and eastern Mongolia, which had previously been affected by desertification, which has caused many to question the link between overgrazing and desertification within the country.

Mongolia suffered severe dzud events in 2000-2002 and 2009-2010, during each of which periods about 30% of the country's livestock was lost. These years were also severe dust storm years. The term dzud can refer to any severe winter, but livestock die-offs generally happen when a summer drought is followed by a severe winter. Ecologically speaking, this is an effective control mechanism, reducing the number of grazing Animals, and therefore enabling the grasslands to recover from overgrazing. However, this caused severe social problems for the people involved, destroying the livelihood of nomads, causing migrations of impoverished people to the cities, and widening the gap between the rich and the poor.

Over the past few decades, a variety of efforts have been made to prevent desertification events on the grasslands of Northeast Asia, some of which have been quite successful. However, the difficulty in proving a direct connection between Human activities and climatic events, in this case desertification, can place pressure on policy makers to pursue other priorities. Nonetheless, the reduction of desertification and dust storm genesis hinges in maintaining vegetative cover in the drylands, although this will often be complicated by far away events beyond the control of policy makers.

A herder with her goats in southern Mongolia.. Adam Oswell in McLaughlin (2019).

Dust storms in Northeast Asia are driven by the geography, climate, and vegetation cover of the grasslands of the Asian Steppes, which in turn are driven by global climatic events. Thus any future global climate change will have an impact on the climate of these grasslands, and the occurrence of dust storms across Northeast Asia.

Rising spring temperatures appear likely to cause earlier onsets of the spring dust storm season, while alterations to the Siberian High pressure system may increase the number of dust storms in the autumn, and effect the spring weather in unpredictable ways. Intensification of the El Niño/La Niña oscillation could potentially destabilise the climate of Northeast Asia, by causing larger fluctuations in annual rainfall in the drylands. A decrease in the Arctic ice cover is likely to increase the range of temperature fluctuations in the region, with adverse affects for both livestock and vegetation.

Desertification, dust storms, and dzuds are all the result of a combination of events hitting the grasslands of Northeast Asia. The most obvious factors are droughts and vegetation cover, but these in turn are driven by a variety of other factors, including distant climate systems and grazing management by Humans. Ultimately, all other factors are probably less important than global climate patterns, which fluctuate on a scale of decades, and currently considered to be under threat of severe modification by Human actions.

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