If global warming is essentially perceived as an increase of the average temperature on the Earth’s surface, one of the most visible consequences is the change in types of precipitation. Evaporation on the surface of oceans and continents is increasing. As it is warmer, the atmosphere can also contain more water vapour which causes more abundant and intense rainfall. We are therefore heading towards a more humid world with more precipitation.
But a hotter more unstable climate also goes hand in hand with more frequent and more extreme events – floods and droughts – regardless of latitude. Repeated droughts could make all forms of agriculture impossible on currently cultivated land. This is the case in Australia in the Murray-Darling basin. Major floods can kill and make plains uninhabitable. The causes of these disasters are not all climate-related. The development of river watercourses and their drainage basins and the deforestation and overexploitation of water tables are also factors that aggravate these meteorological phenomena.
Even in areas where the overall rate of precipitation is not increasing much, its distribution over the year just has to vary to create recurring disasters. In a Mediterranean climate for example, summer droughts set in from the end of spring and last until autumn, whilst more abundant violent winter rain causes erosion and unusual flooding. In mountainous regions, a smaller proportion of precipitation falls as snow; a sixth of humanity depends on the water released by this snow as it melts. There is a lack of it during the dry season. The water regimes of rivers are thus changed.
In other cases, water levels are decreasing. This is the case in subtropical regions in the Northern hemisphere which have recorded a 10% decrease in precipitation since 1990. The surface area of regions that are thus subjected to extreme droughts like the Sahel during the 1970s and 1980s could increase 10- to 30-fold. Up to 30% of continents will experience such phenomena before the end of the century, compared to 1 to 3% today.