1 Water as a global resource

Freshwater is a natural resource that is vital for human survival and health. The Earth is a very wet planet, but only 2.53% of its water is fresh; the rest is seawater (UNESCO, 2003). There is currently much concern about the capacity of the Earth's freshwater resources to sustain human life and health in the near future. One estimate suggests that, if current trends continue, by 2050, when the global human population will reach almost nine billion people, seven billion people in 60 countries will be short of water unless action is taken (UNESCO, 2003). Half the human population will be short of water by 2025. (Note: 1 billion = 1000 million.)

As well as being concerned about the quantity of water available for humans, governments and international agencies are much concerned with its quality. Naturally occurring water is never pure, but contains a wide variety of dissolved substances, some of which are harmful to health. You will return to health issues related to the pollution of water by chemicals in Section 4.

If predictions about a shortage of water for half the human population in 2025 seem alarming but far away, it is important to point out that, for many people, a water crisis is already a daily experience. As you will see later, many people in the world already face the severe adverse consequences for their health of having insufficient water and water that is also polluted. This is particularly true in Africa (Figure 1).

A global water crisis is already apparent to those who look beyond humans and consider what is happening to other species. Planet Earth is at the beginning of a mass extinction event that is eliminating species at a faster rate than at any time in the history of the planet. This is the sixth mass extinction event in Earth's history; the fifth saw the extinction of the dinosaurs, around 70 million years ago. While much media attention is focused on the destruction of tropical forests around the world, it is in fact biodiversity in the world's freshwater habitats that is declining the fastest. You will return to this in Section 4.

Planet Earth contains an enormous amount of water, but only a tiny fraction of it is available as freshwater to plants and animals, including humans, that live on land. As Figure 2 shows, only about 0.01% of the world's total freshwater is readily available to terrestrial life.

Here are some more facts about the world's freshwater resources to bear in mind as you study this unit (Lannoo et al., 2006):

• Freshwater is unevenly distributed throughout the world, e.g. Canada has 30 times as much freshwater available to each of its citizens as China.

• Freshwater is being contaminated by saltwater influxes (tidal waves, rising sea levels), human waste and other by-products of human activity (e.g. industrial chemicals, acid rain), as well as agricultural fertilisers, pesticides and herbicides.

• Since 1950, the number of people on Earth has increased from 2.5 to 6.5 billion, and the per capita use of freshwater (i.e. the amount each person uses annually) has tripled. By 2050 the human population is predicted to reach 8.9 billion; per capita water use is also expected to continue to increase.

• More than 60% of all freshwater used in the world is diverted for agriculture.