Water exists throughout the biosphere, inside living organisms, and as ice, vapour or liquid throughout their environments. There are about 1.4 billion cubic kilometres (km3) of water on Earth, of which almost all is sea water. Of the 36 million cubic km of fresh water, two thirds is frozen in ice caps and glaciers, and almost all the rest is held underground in rocky aquifers. The remaining fraction of about 200,000 cubic km is above ground or nearly so, 45% in lakes, 45% in soils and permafrost, and the other 10% is divided between atmospheric water vapour, swamps, rivers, and inside living organisms.
As I describe in my book on the ecology of the global water crisis, humanity now uses about three trillion tonnes (3,000 km3) of fresh water each year. Irrigation takes 70% of it, with a trillion tonnes being used for export crops such as cotton, but industry’s needs are growing towards 25%, leaving little for domestic use in the cities where half the world’s population now live. The regular supply of fresh, clean, affordable water is essential to life and is set jointly by the rules of ecology and decisions made by society. Undamaged catchment ecosystems such as forests and grasslands absorb rain and release it steadily into rivers and wetlands, and into aquifers where it is accessed by wells and boreholes. People living harmoniously with these systems can prosper indefinitely, their economies sustained by their water supplies.
But catchments can be damaged and water easily over-pumped from aquifers. Whether from the ground or from lakes and rivers, when that water is sluiced across irrigated fields it poisons the soil as it evaporates, by drawing salt out of the earth. Hence, what were once the breadbaskets of nations are fast losing their fertility, and rivers, such as the Indus in Pakistan and the Colorado in the USA and Mexico, are running dry into salty dust long before they reach the sea. Meanwhile, careless industries and great densities of people create wastes that contaminate what fresh water remains, and the total supply of fresh water is being competed over by agriculture, industry and ordinary people. Between pollution, over-use and competition, the share of clean, fresh water available to families is faltering.
If water is diverted to irrigation and industry, or withheld for hydroelectricity, or polluted, then the price of clean, fresh water will go up and the poor will suffer. Competition for useful water can then become harsh, as shown by the fact that up to a billion people lack a secure water supply and 2.5 billion have poor sanitation, causing millions of illnesses and deaths every year, mostly among children. There is a huge waste of human energy in an endless quest for water, a burden falling hardest on women. This is a global water crisis. It means that thousands of communities are living out of balance with the forests, grasslands and wetlands that sustain springs and wells. It can be described as tens of thousands of local water crises, all simultaneous and all caused by ecosystem damage and the diversion, over-use, maldistribution and pollution of fresh water. The failure of our leaders to understand, anticipate and correct these problems, and also the additional risks driven by climate change, can and should be judged as political negligence.