China has recently faced drastic water shortages. Here, a worker shields young trees at a nursery on the edge of a desert. (Agence France-Presse via Getty Images)

Around the world, 168 countries are undergoing “some level of desertification,” Dan Baum reports in the June issue of Scientific American.

California, source of many fruits and vegetables, is in a three-year drought. Australia just emerged from nine years of drought. Brazil, China and much of the Middle East and South Asia have recently faced drastic water shortages.

Where can the world find more fresh water? In the air, some scientists say.

Cloud seeding — infusing clouds with chemicals that help water vapor coalesce into rain droplets — was invented in 1946 by atmospheric scientist Bernard Vonnegut (brother of novelist Kurt). Its success has been mixed. But Baum reports that “a fleet of new NASA weather satellites, advances in radar and . . . growth of computing power have combined to let scientists say with considerable certainty — for the first time — that yes, under the right circumstances and in limited ways, cloud seeding works.”

From the Chinese government’s “weather army” of 50 planes, 7,000 rocket launchers and 7,000 cannons to the seven-county West Texas Weather Modification Association, Baum explains how seeding works, how hucksters have given it a bad name and how it offers some hope of greening our problematic planet.

A dried up river filled with sand winds its way across the desert near Gos Beida in eastern Chad. (Finbarr O'Reilly/Reuters)

He also explains what scientists cannot do: They can enhance rain, but they can’t make something out of nothing. “Think of pulling a sponge out of a bucket of water,” one expert says. “You can hold it up and let it drip, or you can squeeze it. What we do is squeeze.”