Climate change is already and will continue to affect the availability of water. The amount of water available for agriculture is crucial to feed a growing world population. Agriculture competes for water with industrial companies that are growing in size and number in developing countries in particular.
There are five primary categories: 
- oceans (96.5 %)
- liquid freshwater (frozen freshwater in glacier and permafrost, 2 %)
- non-seasonal frozen water (depends on season)
- groundwater (1.6 %) and
- water vapor, lakes and rivers (< 1 %).
The amount of water, neither absolute nor relative, is not constant. Scientists believe that hydrogen is lost into space, leaving less H for H2O on earth. Water of course also moves between the different categories. Generally, the earth has become drier over the last 10.000 years.
Effects of Climate Change on the Quantity of Water
Climate change however adds a number of new aspects to the quantity and availability of water. There has been scientific discourse about an intensification of the water cycle, but empirical evidence does not consistently support the idea.
Climate change results in a change of precipitation patterns that differ substantially from region to region. It is likely that rainfall increases in high latitudes and the tropics. This is, among others, related to an increase in the average river runoff. Subtropic and low-latitude regions (esp. the Mediterranean basin, western USA, southern Africa and north-eastern Brazil) will probably experience less precipitation. It is also likely that seasonal rainfalls change in timing and intensity. Heavy storms have been happening more frequently. It has been widely noticed that dry regions tend to dry out while wet regions become even wetter.
It is by now common knowledge that increased anthropogenic emissions of green house gases result in rising temperatures. Warmer air in the atmosphere can hold more evaporated water, thus reducing the amount of liquid water available in water bodies or as soil moisture.
Soil moisture is affected by global warming, too. Back in the 1970ies half of the area that is today classified as very dry was fertile.
Climate change substantially contributed to the melting of mountain glaciers as well as the ice at the poles. The availability of water in regions that are traditionally supplied by glacier water is lowered, affecting the lives of about 15 % of the world population. Also, seasonal differences in water supply are affected: The ratio of winter to annual water flow is lower, too. 
The changes in water quantity that climate change brings about are very likely to negatively affect food security and public health. Additionally the existing water infrastructure (power plants, flood defences, drainage, and irrigation systems) will face major challenges when dealing with the expected quality and quantity of water. Climate change will further aggravate the current issues with regard to urbanisation, land management, population growth and economic intensification. The water management systems applied today don't sufficiently prepare for the upcoming changes. 
Watch Water and Global Change
- ↑ 1.0 1.1 Peter H. Gleick (editor), 1993, Water in Crisis: A Guide to the World's Fresh Water Resources (Oxford University Press, New York).
- ↑ Pielou, E. C. (2008). After the Ice Age. Chicago: University of Chicago Press.
- ↑ Huntington, T. G. (2005). Evidence for intensification of the global water cycle:Review and synthesis. Journal of Hydrology 319 (2006) 83–95. http://www.ic.ucsc.edu/~mdmccar/ocea213/readings/discuss_1_Oki_Huntington/Huntington_2006_JHydrol_Evidence_intensification_Hydrologic_cycle.pdf [03 June 2013]
- ↑ OECD (2013). Water Resources in Agriculture: Outlook and Policy Issues. Available online under http://www.oecd.org/agriculture/48498988.pdf [03. June 2013]
- ↑ 5.0 5.1 5.2 Intergovernmental Panel on Climate Change (2008): Technical Paper on Climate Change and Water. Finalized at the 37th Session of the IPCC Bureau. http://www.ipcc.ch/meetings/session28/doc13.pdf [accessed 31 May 2013]
- ↑ Intergovernmental Panel on Climate Change (2008): Technical Paper on Climate Change and Water. Finalized at the 37th Session of the IPCC Bureau. http://www.ipcc.ch/meetings/session28/doc13.pdf [accessed 31 May 2013]