18 Precipitation: more extremes. Scenarioes.
When global surface temperatures increase, changes in precipitation and atmospheric moisture are very likely to increase: the hydrological cycle will be more active, and the atmosphere will increase its water holding capacity. Atmospheric water vapour is a climatically critical greenhouse gas, and more of it leads to a stronger greenhouse effect through natural feedback systems.
( Houghton et al 2001 )
As a rule of thumb, precipitation will increase in areas that already has much precipitation, while it might decrease in already dry areas. "Precipitation during the 20th century has on average increased over continents outside the tropics but decreased in the desert regions of Africa and South America”. ( Houghton et al 2001)
The IPCC scenarios projects the impacts of various projected concentrations of CO2 in the year 2100, fluctuating between 540 and 970 ppm, compared to about 280 ppm in the pre-industrial era, and 375 in the year 2003. The A2 scenario implies focus on economic growth and increasing population, while the B2 scenario focuses on environmental sustainability. ( Watson and the Core Writing Team 2001 )