Global warming will lead to an intensification of the global water or hydrological cycle through increases in surface temperature and rates of evaporation, and in some regions, increases in precipitation. Changes in the total amount of precipitation and its frequency and intensity directly affect the magnitude and timing of run-off and the intensity of floods and droughts. Such changes will have significant impacts on regional water resources.
It is not certain how individual water catchment areas will respond to changing evaporation rates and precipitation. It is likely however, that currently dry regions will be more sensitive to changes in climate. Relatively small changes in temperature and precipitation could cause relatively large changes in run-off. Arid and semi-arid regions will therefore be particularly sensitive to reduced rainfall and to increased evaporation.
An increase in the duration of dry spells will not necessarily lead to an increased likelihood of low river flows and groundwater levels, since increases in precipitation may be experienced during other seasons. More probably, increased rainfall will lead to an increased likelihood of river flooding. Changes in seasonal patterns of rainfall may affect the regional distribution of both ground and surface water supplies.
Hydrological regimes in high latitude or mountain areas are often determined by winter snowfall and spring snowmelt. Most climate models predict that global warming will reduce the amount of precipitation falling as snow in these regions, increasing the rate of water run-off and enhancing the likelihood of flooding. Climatic effects on tropical hydrological regimes are harder to predict. In the mid-latitudes, including the UK, wintertime soil moisture is expected to increase whilst summertime soil moisture may decrease. There will however, be regional variations.
Freshwater ecosystems, including lakes, streams and non-coastal wetlands will be influenced by changes to the hydrological cycle as a result of global warming. These influences will interact with other man-made changes in land use, waste disposal and water extraction. In general, freshwater organisms will tend to move towards higher latitudes as temperatures increase, whilst extinctions may be experiences at the lower latitudes.
Changes in surface water availability and run-off will influence the recharging of groundwater supplies and, in the longer term, aquifers. Water quality may also respond to changes in the amount and timing of precipitation. Rising seas could invade coastal freshwater supplies. Coastal aquifers may be damaged by saline intrusion as salty groundwater rises. Reduced water supplies would place additional stress on people, agriculture, and the environment. Regional water supplies, particularly in developing countries, will come under many stresses in the 21st century. Global warming will exacerbate the stresses caused by pollution and by growing populations and economies. The most vulnerable regions are arid and semi-arid areas, some low-lying coasts, deltas, and small islands.
Water availability is an essential component of human welfare and productivity. Much of the worlds agriculture, hydroelectric power production, water needs and water pollution control is dependent upon the hydrological cycle, and the natural recharching of surface and groundwater resources. Changes in the natural water availability as a result of global warming would result in impacts which are generally most detrimental in regions already under existing climatic stresses. Even in more benign climates, the effective management of water resources will receive increasing attention as climate change increases the level of competition between potential users for water.