Sunday, November 17

Natural atmospheric dust and its consequences

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A super sand storm blows through Arizona in the United States in July 2011. — Photo by Roxy Lopez

RECENTLY, I focussed on the effects of volcanic ash on short term climate change. Today I am exploring the effects of desert dust upon the behaviour of our atmosphere. Dust storms are mainly experienced in arid and semi-arid regions where wind lifts grains of sand or tilth from a dry surface. Dust storms are really desert storms and with ever increasing frequency these days we see photos in the press of dim skies at noon in cities such as Cairo and Beijing with vehicle headlamps on full and street lights illuminating the dusk-like atmosphere – Saharan dust affecting the former and Gobi dust the latter.

Such atmospheric phenomena have been in existence for millennia but occur more frequently today. The 20th century saw the expansion of the African Sahel or Sub-Saharan areas so often featured in the media in recent years leading to Bob Geldorf’s noble campaign to feed the world. One has only to read John Steinbeck’s novel, ‘The Grapes of Wrath’ to appreciate the human suffering caused by 1930’s Dust Bowl in the American Mid-West.

Desertification

This has been defined as “the loss of agricultural productivity and income in drylands caused by unsustainable land management and climate change”. As recently as 2017, 821 million people worldwide were without a secure source of food and of these, 31 percent were Africans. Sub-Saharan Africa saw 28 per cent of the world’s undernourished people. In Eastern Africa, 31 per cent of the population are undernourished. These horrific statistics have been the result of the 10 per cent expansion of the Sahara Desert in the 20th century.

Saharan dust

This desert is the major source of our world’s mineral dust amounting to 60 to 200 million tonnes a year. Usually this fine dust, containing silica, aluminium, iron, calcium, and other oxides together with calcium and magnesium carbonates, is lifted by convection currents to great heights into the atmosphere, from whence it is transported over thousands of kilometres by our planet’s wind systems.

One normally associates plumes of Saharan dust with Mediterranean and Western European countries but, owing to the Trade Winds, large concentrations of this dust ‘fall out’ in the Caribbean and the eastern seaboard states of the USA and beyond in Amazonia. The very ecosystem of the Amazon rainforest depends upon the annual input of mineral nutrients from Saharan dust. Strong winds are associated with steep pressure gradients between high and low pressure areas. With high atmospheric pressure over the Sahara Desert and low pressure systems in the Mediterranean Sea to the north and the Gulf of Guinea to the south the desert dust is well distributed by locally known winds such as the Simoon, Haboob, or Sirocco. This summer, 2019, has seen plumes of Saharan air carried by the Sirocco bringing very high temperatures to the UK with the highest ever recorded temperatures of 37.8 degrees Centigrade reached in Cambridge Botanic Gardens on July 25. A month earlier, a new highest mainland temperature of 45.9 degrees Centigrade was recorded for France at Gallargues-le-Montueux.

It has been observed that Saharan dust storms have increased tenfold since 1950, thus causing massive losses of top soil in Niger, Chad, Burkina Faso, and Northern Nigeria. Mauritania once recorded, in the 1960s, an average of two dust storms a year but more recently these have increased to 80 dust storms per annum.

A Nasa Aqua satellite image shows a dust plume spanning hundreds of kilometres in western Africa in April 2010. — Nasa image by Jeff Schmaltz

Saharan dust fall out

Nasa observed that, between June 2005 and June 2006, the chilling effect of Saharan dust upon the North Atlantic sea-surface temperatures reduced the number of hurricanes from 15 to five within one year. Warm surface water temperatures promote hurricane activity. In the Mediterranean Sea, plankton growth is boosted by the mineral enrichment of the Saharan dust which, alas, carries a fungus, Aspergillus sydowii, infecting coral reefs. Since 1970, dust events have been linked to a decline in coral reef growth in the Caribbean Sea.

The minute mineral particles in Saharan dust cool down upon entering higher altitudes and act as condensation nuclei for millions of water droplets suspended in clouds. Eventually the condensation nuclei fall out as rain drops. In the last five years, in South West England, I have had to wash my car more frequently than ever before, after rain from Sirocco plumes of Saharan air dumped their reddish-grey dust everywhere.

Asian dust storms

Springtime sees Asian dust storms from the Gobi Desert in Mongolia and Northern China carried eastwards by the prevailing winds only to be deposited over China, Japan, Korea, and even as far afield as the western USA states. In all places visibility is inevitably decreased and health problems increase, especially in urban areas where this dry air containing minute mineral particles mixes with toxic industrial plant emissions. Sore throats and respiratory difficulties, to include silicosis, occur as well as a form of conjunctivitis or dry eyes.

Such dust storms have prevailed over China for millennia, creating vast deposits of loess, hundreds of metres deep – such fertile soils are so rich in mineral nutrients. The intensive cultivation of the long, meandering Yellow River Valley bears witness to this. Even today, this river is coloured by the thick loess deposits it has dumped in former years, all eroded from upstream. The fertile loess deposits of the North European Plain of Germany came from wind-blown dust of the glacial outwash deposits of the Pleistocene glaciations over 11,000 years ago. Desert dust takes many forms.

Economic punches

Enormous quantities of top soil from drylands are removed by dust storms thus depriving these areas of mineral nutrients. Hence, agricultural productivity is decreased. Young plants are sandblasted and destroyed or stunted in growth. As volcanic ash has often closed airports for fear of the ash being sucked into jet turbines and melting and recrystallising thus causing engine shutdowns, so desert storms affect all aircraft as well as flying and road transport visibility. Add to these equations time lost from work through respiratory illnesses, delays caused by road traffic conditions, and so productivity is seriously affected.

Not only are local climates affected by desert dust storms but man is only now really beginning to study the effects of these storms on global climate patterns in terms of their effects on rainfall distribution and average annual temperatures. Climate, man, and the land certainly interact and influence each other. This is no better illustrated than in an analysis of the source areas of refugees trying to cross the Mediterranean Sea from North Africa in search of perceived ‘greener pastures’ in Europe. Desertification has been, only too often, the ‘push factor’ in their lives in leaving their homelands.