Intercontinental Dust Transport: The Linkage to Climate and Its Environmental Impact

Mineral dust is a major, often dominant, aerosol constituent in the atmosphere over large areas of the Earth. Indeed dust clouds along with water clouds are the most prominent features in satellite images. There is great interest in atmospheric dust because of the impact that it can have on climate through radiative and cloud microphysical processes. Yet dust is itself affected by climate – increased aridity and increased wind speeds typically translate into increased dust. Thus there is the potential for strong and complex feedback linkages between land-surface processes, dust mobilization dynamics, and climate. Dust source processes are among the most important, yet poorly understood, variables in the dust cycle and the distribution of dust sources is extraordinarily complex and uneven. Here I present an overview of global dust sources and discuss some of the factors that affect their present day activity and which might affect future emissions. A major focus is on African sources because of their dominance in the global budget of long-range dust transport. Every year hundreds of millions of tons of African dust are carried westward across the Atlantic to South America, the Caribbean and to the North America.  Large quantities of African dust are also carried northward across the Mediterranean into Europe and eastward to the Middle East. I focus on the dust record from Barbados, West Indies, where continuous dust measurements began in 1965 and continue to this day.  This 45 year record shows large changes in dust transport that are linked to large-scale climate factors (e.g., El Nino events) and more specifically to climate in North Africa, especially the drought cycle, and possibly to land use changes. However since the late 1990s, the close linkage between Atlantic dust transport and the drought cycle has been disrupted for reasons that are not understood. These changes suggest that it may be difficult to project future dust emissions based on various climate scenarios. Finally I point out that African dust could impact health over large areas. Over the western Atlantic about half of the dust mass lies in the size range below 2.5 um diameter. The atmospheric concentration of these fine dust particles is often so high as to challenge the US EPA's standards for respirable particles (i.e., PM 2.5). Although to date there is no strong evidence that African dust constitutes a health hazard, this possible impact would seem to warrant study especially since some climate change projections show increased dust transport in the future.