The Earth is threatened by an increase in desert land caused by human activity and climate change, according to a report released on Monday, World Environment Day.

The United Nations Environment Programme released its 148-page Global Deserts Outlook on scarce water supplies in the Earth's most arid regions.

Dead palms on a deserted golf course near the Salton Sea in California.
(Photo: Michael Field)

"Across the planet, poverty, unsustainable land management and climate change are turning drylands into deserts, and desertification in turn exacerbates and leads to poverty," UN Secretary General Kofi Annan said in a statement.

"There is also mounting evidence that dryland degradation and competition over increasingly scarce resources can bring communities into conflict."

Desert areas make up almost one-quarter of the earth's land surface — 33.7 million square kilometres, home to more than 500 million people, the report said.

Over-exploitation of groundwater and salinization are causing arid lands to become dryer, threatening unique wildlife, said study author Andrew Warren, a geography professor at University College London.

Water tables are already rising under irrigated soil in western China, India, Pakistan, Iraq and Australia, salinizing fresh water supplies.

Elsewhere, glaciers are melting more quickly than desired, threatening future water supplies. In the southwestern U.S., Central Asia and South America, people use water for domestic and agricultural purposes that comes from glaciers and snow-covered mountains, the report's authors noted.

Dams on large rivers that feed deserts can lead to water losses downstream. For example, the Colorado River in the southwestern U.S. was dammed to provide water and electricity for Arizona and California, but Mexico has lost water.

Water scarcity

Climate change is expected to reduce rainfall by up to 20 per cent by the end of the century, experts predict.

More comphrensive water policies could help manage the world's limited supplies. If nothing is done, Chad, Iraq, Niger and Syria could experience water scarcities by 2050, the report's authors warn.

Wildlife under threat include several species of gazelle, Barbary sheep and the houbara, a favourite quarry of falconers.

Deserts also hold opportunities. Certain plants in Israel's Negev desert are said to have anti-cancer and antimalarial properties, and desert plants from countries such as China and India are being exported for herbal treatments and medicines.

Eco-tourism — which in the past has been shown to promote conservation — is attracting people to deserts. Popular sites include Joshua Tree National Park in North America, St Catherine’s Monastery in Sinai, and Uluru, also called Ayers Rock, in Australia.