Global climate change will have major impacts on freshwater ecosystems. This will affect availability, as well as quality, distribution, and form. In certain regions, where stream flow and precipitation increase, flooding could threaten the structure and functions of aquatic systems, leading to increased pollution of freshwater ecosystems, especially in areas where human-mediated activities might have altered the landscape. The impacts of climate change on the Earth's freshwater resources have the potential of affecting international relations, especially at continental or country borders, where shared wetlands can generate local and international political and geographical disputes.
Impacts of climate change on water resources will have a wide range of consequences for coastal ecosystems. The health of the Earth's ecosystems will be affected by changes in the quality and quantity of freshwater runoff into coastal wetlands, higher water temperatures, extreme runoff rates or altered timing, and the ability of watersheds to assimilate pollutants and wastes.
Global climatic changes could pose a serious demand on the water supply. In most regions of the world where there are observed and projected declines in per capita average of annual freshwater availability, coupled with attendant population growth, the possibility of increased demand for water will likely lead to increased withdrawal of water, which will invariably reduce the recharging time of the water tables. These changes may influence a wide range of water-system components, including reservoir operations, water quality, hydroelectric generation, and navigation. In some regions, where large volumes of water are channeled for non-consumptive purposes such as agriculture demand, particularly for irrigation, water supply will be particularly sensitive to climate conditions;
demand for irrigation water tends to increase as conditions become hotter and drier. A possible change in field-level climate may result in altering the timing of, and need for, irrigation.
In-stream water uses such as hydroelectric power generation, navigation, recreation, and ecosystem maintenance are also sensitive to changes in the quantity, quality, and timing of runoff stemming from greenhouse warming. Potential negative implications of climate change include reductions in dilution flows, increased storm surges, and higher water temperatures. Low flows in many rivers will lead to increases in salinity levels to downstream water users.
On the other hand, higher flows could help reduce some water quality concerns. Warmer water could threaten aquatic life directly, as cool-water habitats disappear, and indirectly, as dissolved oxygen levels decline with higher temperatures. An increase in days with more intense precipitation could increase the agricultural and urban pollutants washed into streams and lakes, further reducing oxygen levels. Heavy rainfall is primarily responsible for soil erosion, leaching of agricultural chemicals, and runoff of urban and livestock wastes and nutrients into water bodies.
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