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DOES AN ACTIVE SUN TRIGGER EXTREME DROUGHT IN SOUTHERN AFRICA ?
By
Stephen Belbin
Department of Geography
National University of Lesotho.
Drought is an ancient problem for Africa which shows no sign of going away. Coping with drought is part of the struggle for an African renaissance for all. Although droughts have social as well as natural causes, extreme droughts usually involve rainfall being much less than expected often over a period of more than one year. Given their impact, what can modern science tell us about how and why extreme droughts
occur?
For much of Southern Africa south of latitude 20 ° S, a link has already been made between periods of strongly reduced rainfall and a stronger atmospheric circulation in mid-latitudes with a shift of important rain bearing winds away from the subcontinent. But the main reason for such disturbances remains unclear.
One possible course being considered in the Department of Geography at the National University of Lesotho is an active sun or more precisely a strong solar wind. The solar wind consists of low density (10
particles/cm3), high speed (400km/s) streams of ionized hydrogen and helium that break away from the sun’s outer layer, the corona, in different directions and flow out through the solar system. The solar wind varies in strength and speed, and is usually stronger when the sun’s surface is more active with more explosions and solar flares. Solar surface activity doesn’t vary randomly but increases and decreases over the well known sunspot cycle which lasts on average about 11 years. So when the sun is more active at and around the sunspot cycle maximum, the solar wind tends to be stronger.
Some of the particles of the solar wind can be intercepted by the outer atmosphere where they come into contact with the Earth’s magnetic field and can be guided down to the magnetic poles to form spectacular aurora – the northern and southern lights. But the real climatic significance of the solar wind lies in how it affects another set of particles coming from space towards our planet, Galactic Cosmic Rays (GCR). GCR are not really rays but mainly energetic hydrogen ions spiraling around our Galaxy at speeds of over half the speed of light guided by interplanetary magnetic fields. GCR can also be intercepted by the Earth and because of their high energy can penetrate the atmosphere. But how many enter depends in part on the strength of the solar wind. A strong solar wind can intercept and slow GCR and can even strengthen the Earth’s magnetic field. The overall effect being to reduce the GCR intensity in our atmosphere.
This shielding effect of the Solar wind on GCR has long been known but an exciting recent discovery is that GCR may influence the formation of clouds in the lower atmosphere. This is significant because clouds play important roles both directly and indirectly in weather and climate. During the last 20 years, the annual amount of cloud in the sky, particularly low clouds in the higher, ice bound latitudes (above 60 ° N and S) has varied with GCR intensity. More GCR means more cloud cover, less GCR means less cloud. Low clouds generally shield the earth’s surface from warming solar radiation also coming through the atmosphere. If so, then in years when the sun is active, the solar wind strong, GCR intensity weak and low clouds in higher latitude fewer, one might expect more solar radiation to reach the surface and summer melting of ice to be more pronounced. The influx of cold water produced by such melting is then likely to cool the overlying air in the higher mid latitudes at the edge of ice centers such as Antarctica. Cooling of air in such latitudes in the Southern hemisphere has already been associated with the expansion and strengthening of the mid latitude wind system known as the circumpolar vortex and the displacement of rain bearing winds from Southern Africa. So while the sun remains highly active over 2 or 3 years, rainfall can be expected to be reduced over the subcontinent triggering an extreme drought.
Although this is still only a hypothesis and comments on it and any other points in this article will be welcomed by the author, some numerical evidence already exists in its favour. During the last 60 years, there have been 5 years when the GCR intensity in the atmosphere has been at a minimum: 1948, 1957, 1967, 1981 and 1991. All of these occurred at or just after a sunspot maximum when the sun was very active. Furthermore, all of these were followed by droughts in Southern Africa with more than one year of strongly reduced rainfall viz 1948 and 1950, 1958/59, 1968/69/70, 1982/83 and 1992/93. Also, between these periods e.g. in the early ‘ fifties and during the ‘seventies when the sun was less active and the GCR flux was higher, Southern Africa was generally wet. Not all these droughts were equally severe, but even this can be linked to the GCR flux. The early ‘nineties drought was more intense than that of the late ‘sixties possibly because the GCR flux was much lower in the former than the latter.
It should again be stressed that droughts are not just due to reduced rainfall. They also depend on how well prepared people are. But if Science can help make us aware of why and when major droughts are likely, then being unprepared is not nature’s fault. Furthermore, droughts have occurred at other times than those mentioned above. The Solar wind/GCR hypothesis is likely to be only one cause, although a prominent one accounting for many of the serious droughts in recent years. The other droughts may have been triggered by other mechanisms working at different times.
One final word. Beware! The sun is very active now.
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