THE CHANGING FREQUENCY OF DUST STORMS THROUGH TIME

Dust storms are major, but under-studied actors in the world's drylands. Not only are they an important manifestation of desertification and land degradation, but they also have a whole suite of important environmental impacts, including possible rainfall suppression (Maley, 1982), fertilization of offshore areas, and disturbance to satellite communications. It is therefore important to ascertain whether their frequency and extent is changing. An increasing dust-strom incidence could be both a manifestation of and a contributor to global change. By analysing long-term meteorological records for a large number of areas (the Great Plains of the USA, the USSR, Morocco, The Arabian Gulf, Australia, the Sahel-Sudan zone of Africa, China, Mongolia and Mexico) certain conclusions can be drawn. The first of these is that there is no one global pattern of dust-storm frequency trend. Some stations (e.g. in the Sahel) show a clear upward trend of great severity, others show a downward trend (e.g. Mexico City), while others show a more cyclical pattern. In many cases it is evident that essentially natural processes (precipitation totals, snow cover, wind strength) determine the frequency of dust events in any one year. It has also been possible to show the importance of runs of drought years (e.g. in the High Plains in the 1930s, and in the Sahel zone of Africa in the 1970s and 1980s). Elsewhere, however, various human activities have been significant in determining dust-storm frequency variations: the introduction of centre-pivot irrigation in the High Plains, the abstraction of water from the Owens and Mono basins in California, the disruption of surfaces by construction activity and vehicle use (e.g. in Ulan Bator, Mongolia), and the deliberate stabilisation of susceptible surfaces (e.g. the Lake Texcoco scheme in Mexico).