Climate change, high water demand and even tourism are putting unprecedented pressures on the world's desert ecosystems, according to a new report.
The Global Deserts Outlook, produced by the UN's Environment Programme, is described as the most authoritative assessment to date of desert regions.
Its authors say too much water is being frittered away on water-intensive agricultural crops.
But, they add, deserts have huge economic benefits if managed sensibly.
Far from being barren wastelands, deserts are biologically, economically and culturally dynamic, the report says.
Desertification is the theme of World Environment Day on Monday when ecologists plan to plant trees to slow erosion, or deliver talks in schools.
Among the WED events:
- A group in Mauritius plans to plant vegetation on dunes to protect beaches from erosion
- Activists in Churchill, Australia, is collecting computer parts for recycling
- A group in Zambia holds a "Miss Environment" beauty pageant.
- Activists in Vadodara, India, encourage local schools both to plant trees and build sandcastles to "get a closer connection to the topic of deserts and desertification"
"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.
Solar energy source
According to the report, while many changes are likely to occur in the next 50 years, some are surprisingly positive.
There are new economic opportunities such as shrimp and fish farms in Arizona and in the Negev Desert in Israel offering environmentally friendly livelihoods for local people.
Similarly, desert plants and animals are being seen as positive sources of new drugs and crops.
Even the problems of global warming could be tackled by better use of deserts: some experts say that an area of the Sahara 800km by 800km could capture enough solar energy to meet the entire world's electricity needs.
However, most of the 12 desert regions whose climate has been modelled are facing a drier future.
There are also problems caused by the melting of the glaciers whose waters sustain deserts in South America.
The impact of humans continues to cause difficulties.
In the United States and in the United Arab Emirates more and more people are choosing to live in desert cities creating further pressures on scarce water resources.
Mountainous areas in deserts face particular threats to their wildlife and ecosystems - all of which could be lost in 50 years without urgent action.