The need of fresh water for humans and terrestrial animals, and the high cost of desalinization have put an increasing demand on global supplies of fresh water. In addition to its uses for direct consumption by humans and animals, and for cleaning, agriculture, and power production, fresh water bodies provide a source of food and ecosystem services previously described. River valleys running through dry regions form riparian zones that have much higher biological diversity than the arid surroundings. Rivers offer an inexpensive form of transportation: a single barge tow can carry the equivalent amount of grain or coal as 900 large trucks . The role of rivers and lakes for recreation and aesthetics should not be overlooked. Their contribution to a high "quality of life" is difficult to quantify but is increasingly recognized in societies having substantial leisure time.

Marshes and wetlands are bodies of fresh surface water that have particularly rich ecological diversity. These bodies support not only their own diversity but provide an ecological service in their ability to break down anthropogenic chemicals such as agricultural fertilizers and pesticides. As we saw in the unit on the Carbon Cycle, these areas also are substantial reservoirs of carbon.

5. Subsurface Water

Soil moisture, permafrost, groundwater, and water in deep aquifers compose different forms of subsurface water that impact or are impacted by global change.

Soil moisture provides the basic nutrient for plant growth and a reservoir for precipitation storage. By supplying moisture to the soil surface and to plants at a regulated rate depending on soil characteristics, soil moisture also influences surface evaporation and evapotranspiration (water loss by plants) and hence the global energy budget. By slowly delivering water to the surface and to plants over a period of weeks to months, soil moisture introduces a seasonal timescale into the climate system. A climate model with an inappropriate soil moisture submodel may allow its simulated soil to dry out too fast or too slow and cause, respectively, excessive or insufficient heating at the soil surface.

Permafrost plays an important role in global change because of the plant material it has locked up in ice. Melting permafrost exposes new plant material to temperatures that accelerate the decay process. This melting, from whatever cause, will lead to new releases of CO₂ and methane, both greenhouse gases, that lead to further global warming (a positive feedback to the climate system).

Ground water includes near surface aquifers that frequently are tapped by wells for human consumption, agricultural irrigation, and industrial uses. Their proximity to the surface and occasional direct connection to surface water, leave ground water supplies vulnerable to contamination from surface pollutants.

Deep aquifers represent water bodies that have been created thousands or more years ago by slow geo-climatic processes, and their water is sometimes referred to as "fossil water". The large Ogallala aquifer in the Central Plains of the US is an example of a deep aquifer.

Volcanic eruptions cause episodic additions of rather small amounts of subsurface water to the atmosphere, but large eruptions put combinations of sulfates and water vapor into the stratosphere where they contribute to a transient (1-2 year) cooling effect on the global climate.

PREVIOUS: --Ice Masses and Snow

NEXT: --Biospheric and Atmospheric Water and Clouds