During the 20th century, the world’s urban population increased more than tenfold, while rural population increased but twofold.

Today, half of the world’s population lives in urban centres, compared to less than 15% in 1900.

Human population growth and the expansion of economic activities are collectively placing huge demands on coastal and freshwater ecosystems. Water withdrawals, for instance, have increased sixfold since the 1900s, which is twice the rate of population growth.

In 1900, ‘million cities’ (cities with more than one million inhabitants) were unusual and cities with over 10 million unknown; by 2000, there were 387 million cities and 18 with more than 10 million inhabitants.

In most urban areas in low- and middle-income countries, between 25% and 50% of the population lacks provision for water and sanitation of a quality that greatly reduces the risk of human contamination with faecal-oral pathogens.
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The right to water is already recognized in several legal or political instruments. It guarantees access to water, without discrimination, in a permanent and sustainable manner – and at a socially and economically acceptable cost. It also addresses the issues of subsidiarity, solidarity, and cooperation.

Increasing recognition is accorded to the right to water, in terms of a human right to a supply of safe water, the role of water rights in helping to deal with local competition for water and in dealing with social, economic and environmental problems.
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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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