The humid tropics encircle the Earth’s equator and extend over 2,000 Km to the north and to the south, roughly paralleling the Tropics of Capricorn and Cancer.

This region contains approximately 20% of the world’s land area, contains parts of as many as 60 countries and is characterized by warm temperatures, abundant rainfall and extreme weather conditions, including cyclones and monsoons.

Although the average annual rainfall is high in most tropical regions, often exceeding several metres per year, its distribution in time and space can be highly variable. Madagascar, for example, has an annual average rainfall of 2-4 metres along the east coast, whereas the west coast frequently receives less than one metre.
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Eutrophication is a slow ageing process during which a lake or estuary evolves into a bog or marsh and eventually disappears. During eutrophication, the lake becomes so rich in nutritive compounds (especially nitrogen and phosphorus) that algae and other microscopic plant life become superabundant, thereby choking the lake and causing it to eventually dry up.

Eutrophication is accelerated by discharges of nutrients in the form of sewage, detergents and fertilizers into the ecosystem.
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Acid rain occurs when sulphur dioxide and nitrogen oxides are emitted into the atmosphere, undergo chemical transformations and are absorbed by water droplets in clouds. The droplets then fall to earth as rain, snow, or sleet.

The major causes of acid rain are the sulphur dioxide and nitrogen oxides produced when fossil fuels such as coal, oil and gas are burned and sulphur dioxide and nitrogen oxides are released into the atmosphere where they can be absorbed by the moisture and become weak sulphuric and nitric acids, sometimes with a pH of around 3.
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It is estimated that there are presently some 30 million environmental refuges and a further 17 million other refugees and displaced persons from wars, persecution and other causes. The former have fled from resource scarcity, from deforestation and environmental degradation, climate change impacts, overpopulation, displacement by development projects, etc.

It has been suggested that the number of environmental refugees could rise to 150 million by 2050 as one of the results of climate change.
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The water footprint of an individual, business or nation is defined as the total volume of freshwater that is used to produce the foods and services consumed by the individual, business or nation. A water footprint is generally expressed in terms of the volume of water use per year.

Since not all goods consumed in one particular country are produced in that country, the water footprint consists of two parts: use of domestic water resources and use of water outside the borders of the country. The water footprint includes both the water withdrawn from surface and groundwater and the use of soil water (in agricultural production).
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