Climate change, high water demand and even tourism are putting unprecedented pressures on the world's desert ecosystems, according to a report.
The Global Deserts Outlook, produced by the UN's Environment Programme, is described as the first comprehensive look at the Earth's driest regions.
It highlights the problems - and also the potential - in arid areas.
The authors call for more careful use of scarce water resources to safeguard the futures of desert populations.
"Far from being barren wastelands, [deserts] emerge as biologically, economically and culturally dynamic while being increasingly subject to the impacts and pressures of the modern world," said Shafqat Kakakhel, from Unep.
"They also emerge as places of new economic and livelihood possibilities, underlining yet again that the environment is not a luxury but a key element in the fight against poverty and the delivery of internationally-agreed development goals."
The report defines deserts in three ways:
Taken together, these areas give a composite definition of global deserts, occupying almost one-quarter of the Earth's land surface - some 33.7 million sq km (13 million sq miles) - and inhabited by over 500 million people.
- Climatologically, as the arid and hyper-arid areas of the globe
- Biologically, as ecoregions that contain plants and animals adapted to an arid existence
- Physically, as those areas with ample extensions of bare soil and low vegetation cover
Most of them live at desert margins and it is here that some of the pressures threatening ecosystems in arid areas are at their greatest.
Population growth and inefficient water use are, by 2050, set to move some countries with deserts into water stress, or even worse, water scarcity, the report says. Examples include Chad, Iraq, Niger and Syria.
Renewable supplies of water which are fed to deserts by large rivers are also expected to be threatened, in some cases severely, by 2025.
Examples include the Gariep River in southern Africa; the Rio Grande and Colorado Rivers in North America; the Tigris and Euphrates in south-western Asia and the Amu Darya and Indus Rivers in central Asia.
One of the report's author's, Professor Andrew Warren from University College London, said that without careful management in the future, the unique landscapes, ancient cultures, flora and fauna in these areas were at risk of disappearing.
"What alarms me now is that they are threatened as never before by climate change, by over-exploitation of groundwater, salinisation and the extinction of wildlife," he said.
However, the report concludes that although some trends are worrying, other changes likely to occur in the next 50 years could be positive.
There are new economic opportunities, it says, such as shrimp and fish farms in Arizona in the US and in the Negev Desert in Israel, offering environmentally friendly livelihoods for local people.
Desert plants and animals are being seen as positive sources of new drugs and crops.
Desert regions are becoming a major draw from tourism
Nipa, a salt grass harvested in the Sonoran desert of north western Mexico at the delta of the Colorado River by the Cocopahs people, thrives on pure seawater, producing large grain yields similar to wheat.
"It is a strong candidate for a major global food crop and could become this desert's greatest gift to the world," the report says.
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 (500 miles by 500 miles) could capture enough solar energy to meet the entire world's electricity needs.
Nonetheless, climate change is seen a major hurdle for desert ecosystems in the years ahead.
Most of the 12 desert regions whose climate has been modelled are facing a drier future.
The overall temperature increase in desert regions of between 0.5 and 2C over the period 1976-2000 has been much higher than the average global rise of 0.45C, the report says.