The world is running out of sand

Sand doesn't sound like something the world could run short of, with whole deserts full of it. But after water, sand and gravel are the materials people use most, and in some places the supply is under real pressure. Here are answers to the questions readers ask most often.

What do we use it for? Mostly concrete. Every building, road and bridge depends on it. Glass is made from sand too, and so is the silicon inside your phone. Each year, people take tens of billions of tonnes of sand and gravel from the earth. A UN report once worked out that this was enough to build a wall 27 metres high and 27 metres wide around the equator. Much of it goes into fast-growing cities in Asia and Africa.

Isn't there plenty in the desert? Unfortunately, desert sand is almost useless for building. The wind has rounded its grains, so they don't bind together well. Builders need the rougher sand found in rivers, lakes and along coasts, and that's exactly where the damage is happening. Even Dubai, surrounded by desert, has imported sand for its building projects.

What kind of damage? When too much sand is dredged from a river, the riverbed drops and the banks can collapse, taking farmland and houses with them. Vietnam has seen this in the Mekong Delta, where years of uncontrolled mining have helped riverbanks fall into the water. Taking sand from beaches and the seabed leaves coasts more exposed to storms and destroys the homes of fish. Because sand is cheap and easy to move, illegal mining is common in some countries, and local officials often lack the staff to stop it.

Are there alternatives? Some, though none is perfect. Crushed rock can replace some river sand, and old concrete can be broken up and reused. Engineers are experimenting with sand made from recycled glass and from the ash left by power stations. The simplest option is to build less and make buildings last longer. On a human timescale, sand isn't a renewable resource, and experts say it should be managed as carefully as water.