Excerpts from an interview with Mark Dimmitt, associate director of science at the Arizona-Sonora Desert Museum in Tucson, Ariz.
Are the world's deserts growing or shrinking?
Deserts are not changing except over spans of tens of thousands of years, which humans will not see.
Have some places always been deserts?
Deserts are fairly new habitats. The earth's climate has been getting drier since the age of the dinosaurs, marking a slow trend toward aridity.
Where does all the sand in a desert come from?
The main source of sand is rivers. [The current grinds rocks together, and this makes sand - the way waves create sand on ocean beaches.] The sand dunes all around the Gulf of California [in Mexico] came from the Colorado River. The sand was carried down from the mountains, then the river drastically changed course and the area dried up.
Are deserts good for anything?
It's a matter of their intrinsic value. The biggest value to me is that it is open space: I can walk or drive great distances and not run into traffic.
Wouldn't all the animals that live in the desert rather live somewhere else?
That is one of those interesting human perspectives - that the desert doesn't have enough water [for animals to thrive]. The desert is defined as a place where plants and animals have adapted to seasonal drought, an adaptation that is required for their survival. The dryness keeps out other animals and plants that might otherwise shade them out or eat them.