Water Supply
Planting trees has long been an expression of intent to do something of substance in the climate change debate ; scientists have found a new rationale for this
atmosphere to which trees and wetlands contribute importantly and in quantities that can be measured , and ii ) a starting point for condensation of vapour into cloud droplets and rain drops . Trees are a source of volatile compounds that can become cloud condensation nuclei and trees are also a source of bacteria that form ice nuclei . “ In clean , dust-free air , cloud droplets may cool down to -40 ° C , high up in the atmosphere , before freezing occurs if there are no ice nuclei present that can catalyse freezing ”, says Dr . Cindy Morris , one of the coauthors . “ But trees and forests release ‘ ice nuclei ’ into the atmosphere including certain fungal spores , pollen and bacteria that can initiate rainfall at much warmer temperatures , sometimes as warm as -4 ° C . This means that rain initiation can take place more readily in lowaltitude clouds .”
As forests modify and contribute to the atmospheric flows of moist air they influence downwind rainfall . While coasts derive most of their rainfall from oceanic evaporation , downwind continental surfaces are increasingly dependent on upwind terrestrial sources of atmospheric moisture . On average 40 % of rainfall over land is recycled from evapotranspiration over land surfaces .
Long-distance dependencies
Important examples of long-distance dependencies have been documented between the Congo basin and East Africa providing rain to the Ethiopian Highlands and the Sahel ; the Amazon supporting rain in NW Argentina ; and mainland Southeast Asia feeding atmospheric moisture to China . In all these cases , major changes in tree cover can break the chain and reduce precipitation in downwind basins .
“ Where most water studies have focused on the ‘ blue water ’ in rivers and the ‘ green water ’ used by plants , water in the atmosphere is now recognized as ‘ rainbow water ’”, says Dr Meine van Noordwijk , co-author and Chief Scientist with the World Agroforestry Centre ( ICRAF ). “ The policy arena may have to adjust to the idea that rainfall is not simply the result of large scale air mass movements , but depends importantly on how upwind neighbours care for their forests . “ Reliable rainfall in the continental interiors of Africa and South America , as well as in other downwind locations , may depend on maintaining relatively intact and continuous tree cover from upwind coasts . The geopolitics of these relations can become a source of conflict , but can also lead to new types of cooperation .”
With these and more fascinating “ cool insights ” and evidence from research , the authors point out that there is a strong basis for a hydro-climate policy that involves forests and trees . This policy would be much wider than what has so far been shaped by scientific understanding of the greenhouse-gas dominated climate and been incorporated in international agreements .
The review concludes with a cry to action on forests , water and climate : “ Climate policy must take these waterprocessing , cooling and rainfall-generating effects of trees and forests more explicitly into account .”
Significant revision of national , regional and continental climate change mitigation and adaptation strategies are urgent as next steps .
Water resources
Water resources are sources of water that are useful or potentially useful to humans .
It is important because it is needed for life to exist . Many uses of water include agricultural , industrial , household , recreational and environmental activities .
Virtually all of these human uses require fresh water . Only 2.5 % of water on the Earth is fresh water , and over two thirds of this is frozen in glaciers and polar ice caps .
Water demand already exceeds supply in many parts of the world , and many more areas are expected to experience this imbalance in the near future .
It is estimated that 70 % of world-wide water use is for irrigation in agriculture .
Climate change will have significant impacts on water resources around the world because of the close connections between the climate and hydrologic cycle .
Due to the expanding human population competition for water is growing such that many of the worlds major aquifers are becoming depleted .
Many pollutants threaten water supplies , but the most widespread , especially in underdeveloped countries , is the discharge of raw sewage into natural waters .
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