Climate is another changing input into the equation, if a more dispassionate one. It has the potential to seriously threaten water supply, the most critical of desert resources. Climate could change at two scales. On a long time-scale, climate has and could again change without human intervention. The paleoclimatic record shows that radical change has happened within a decade (or even more quickly). The Akkadian civilization in Iraq and the Indus Valley civilization in Pakistan were brought down by sudden climatic change about 4 000 years ago (Staubwasser 2003). Only 6 000 years ago, Lake Chad, the northern basin of which is now in a literally howling desert, was a freshwater lake bigger than the present Caspian Sea (Drake and Bristow 2006).

Faster climate change is already happening and most climatologists believe that its acceleration is inevitable, whatever the cause, and almost whatever the response: it may be too late to intervene to change the trajectory of the next few decades (IPCC 2001). Temperatures, and with them evaporation (and hence aridity), will almost certainly rise further, which may or may not be compensated by increased rainfall. The deserts whose own climate is most vulnerable to change are in southern Africa. Projections for decreases in run-off in southern African rivers are of the order of 10-30 per cent (Milly and others 2005). Deserts that will most certainly suffer (and perhaps badly) are those that get their water from alpine meltwater (see Chapters 3 and 6). Climate change could adversely affect human health, both through rising temperatures or through increases in rainfall, or its variability. The virulence of plant or domestic animal pathogens may increase, or crop yield could decrease (say after drought), leading to malnutrition. Some climatic effects are well proven, as in the correlation between ENSO events and plague and hantavirus pulmonary syndrome in the U.S. Southwest; child diarrhoea in Lima; and the effects of increased ozone levels in urban areas, brought on by higher temperatures (Patz and others 2005).