This article was contributed by Atreyee Bhattacharya
I remember being told in my college sedimentary geology class that “dust” is not just the stuff that dirties our hands and clothes. Before it comes to us, dust has already had an interesting history. To me that was a revelation. Dust is, in fact, the finest fraction of very old rocks. These rocks have been broken down by fluctuations of extreme temperatures, by action of ancient glaciers, by deep-rooted trees and burrowing activity of animals. The rock particles have been carried away by rivers and glaciers that may or may not exist now, and finally deposited in plains where rainwater and bacteria act upon them, breaking them down to a size that can finally be windblown. So dust as we know it has had quite an interesting life, extending back for millions of years, before soiling our clothes.
Since then however, I have learnt quite a lot more about dust. More specifically, dust that is blown away by strong winds.
Windblown dust has recently been credited with some of the worst climactic calamities of the last century, from long, sustained droughts in sub-Saharan Africa, to mortality of corals in the Caribbean, air quality disruption in the USA, and hurricane activity in the Atlantic Ocean. All of these events seem to be concomitant with spikes in dust in the northern hemisphere. The north-east trade winds that blow roughly from the north-east towards the equator carry with them dust from the Sahara. The Sahara desert is by far the biggest source of dust in the world, contributing an estimated 50% of the world’s airborne dust. Once airborne, it takes dust particles roughly a week of piggybacking on these trade winds to cross the Atlantic Ocean to the shores of the Americas – to the corals of the Caribbean, the islands of the Bahamas, and the Amazon forests of South America. In fact, a good portion of the soil in the Bahamas and the Amazon is composed of African dust. Incredible if you start to think about it.
But why should we be so bothered about dust?
The Intergovernmental panel on climate change (IPCC) is a body of scientists from several countries, whose goals are to understand and report changes in climate, predict the possible ways in which different climactic components could change in the future, and predict the effects of these changes on the human population. The IPCC has recently released a report in which they have shown that while natural dust constitutes the single biggest component of aerosols in the atmosphere, the scientific understanding of the magnitude and nature of its impacts on climate are not well understood.
A very disturbing fact
While in air, dust particles do not have a passive journey. They actively modify the atmosphere and the properties of the ocean and land over which they pass. Research has shown that depending on their size, dust particles are transported at different altitudes – the bigger and/or heavier particles at lower altitude for shorter distances and the smaller and/or lighter particles at higher altitudes over long distances. It is the second group, with a longer residence time in the atmosphere, that affects the climate system. A layer of dust in the upper atmosphere (more than 2.5 miles from the earths surface) interferes with both the radiation from the sun (short wave radiation) and radiation reflected back from the Earth (long wave radiation). This interference of the dust with incoming and outgoing radiation is believed to modify surface temperatures over the land and ocean, in ways scientists do not yet fully understand.
Dust particles act as points on which water vapor in the atmosphere condenses to generate cloud-forming nuclei. Eventually, the water particles coalesce to grow big – up to a particular size, beyond which the vapor cannot remain suspended as clouds, and has to rain down. That is how rain-bearing clouds form. However, if there is too much dust – and scientists’ debate about how much is too much – in unit volume of the atmosphere, sufficient number of water droplets do not condense on individual dust particles. Each one tries to grab on to its own dust particle to settle on. Hence water droplets cannot grow to the particular size which is needed for it to rain down. And thus we have a sky full of clouds which cannot rain.
So while you need dust to have rains, you also cannot have too much of it … for then the rains stop and a region becomes even dustier, feeding back into the dryness of a region – a potentially vicious cycle. Is that what is happening in marginally dry areas, like sub-Saharan Africa? Is increased dust emission from these regions is impeding the regional monsoon system?
Over the last fifty or sixty years, it has been shown that human activities – over-grazing, over-farming, basically the demands of converting a vast nomadic community into an agro-based community – may have increased the dust flux from these semi-dry regions. This may have dire consequences: we know for a fact that dust does have an impact on regional climates by modifying cloud properties, and on global climates by affecting sea surface temperatures. This is truer for those countries just south of the Sahara, which have economies vulnerable not only to a burgeoning population demand and unstable political systems, but are also intimately linked to a natural cycle of dry and wet conditions.
A delicate balance
Has the balance been tipped to the wrong side by an increase in dust emission from these countries alone? Or is there a natural cycle of dust, controlled by factors external to North Africa – like sea surface temperatures and atmospheric phenomenon that control rainfall and wind speeds in the Sub-Saharan countries among other thing – which regulates dust emission from the region? Or is it the former stoking the latter, especially in a scenario of escalating green house gas emissions that affects every single facet of global as well as regional climate?
The jury is out on this one.
For more information, please see:
On coral mortality and African dust: http://coastal.er.usgs.gov/african_dust/
Press release from University of Wisconsin-Madison on the impact of dust on the temperature of the Atlantic Ocean: http://www.news.wisc.edu/16472
The Intergovernmental Panel on Climate Change: http://www.ipcc.ch/
A New York Times article on drought in Kenya: http://www.nytimes.com/2009/09/08/world/africa/08kenya.html?_r=2
Primary research referenced in this article:
Tegen, I., Fung, I., (1995). Contribution to the atmospheric mineral aerosol load from land surface modification. J. Geophys. Res.100, 18707–18726.
Prospero, J.M and Lamb, P.J (2003). African droughts and dust transport to the Caribbean: climate change: Implications .Science 302, 1024-1027
Harrison ,S.P., Kohfeld ,K.E., Roelandt ,C., Tanguy ,C.,(2001).The role of dust in climate changes today, at the last glacial maximum and in the future.Earth.Sci.Rev.54, 43-80