Geologists measure time differently from the rest of us. Millennia for them have the weight of a season, centuries that of a fortnight or two. Every once in a while, however, they discover something about the earth's slow shiftings that warrants attention now - in our terms.

So it is with the Great Plains.Geologists have known for years that sand dunes underlie major stretches of the Plains; they're associated especially, though not exclusively, with rivers such as the South Platte, the Cimarron and the Arkansas.

Although there are a few areas in which sand is exposed or, as in Nebraska's Sand Hills, close to the surface, the dunes are for the most part stable and unnoticeable. Until recently, it had been thought they would probably stay that way, at least for the foreseeable future.

No longer. Where geologists had once thought that the dunes' last period of activity was at least 10,000 years ago, evidence gathered in the past decade suggests that there were some areas of desert as recently as 3,000 years ago.

Some scientists believe that sand may even have been spreading during prolonged periods of drought within the last few centuries. And they have begun to wonder whether parts of the Plains may be in danger of turning into desert again within our lifetimes.

"What you have to realize is that the ecosystem on the Plains is very fragile - drought is a normal part of it," says Steven Forman, a geologist at Ohio State University who is studying the dunes using satellite imagery.

The implications would be severe for the agricultural economies of the states most likely to be affected: Colorado, Kansas and Nebraska, along with parts of Oklahoma, Texas and a few others.

Although "desertification," the term geologists generally use to characterize the process, isn't yet taking place, some geologists suggest that public officials should be paying attention now to whether current land-use patterns, farming practices, and grazing and water policies could hasten its arrival.

The primary focus of the scientists' concern is greenhouse warming, which could eventually turn the semi-arid Plains into the arid Plains. What keeps the dunes still is the vegetation that covers them. If there is less water available, there will be less vegetation and, presumably, more exposed sand.

"The winds are strong enough, the sand is there - all we're missing to have good blowing sand today is we have too much vegetation," says Jim Swinehart, a research geologist with Nebraska's geologic survey. "As I understand the global climate people, we could approach the conditions that existed on the Plains 3,000 years ago in the order of a lifetime. Then we would say we're going to have some major reactivation of those dunes."

As a result of all this concern, the U.S. Geological Survey, NASA and others have launched a series of studies to look at historical cycles of dune activity and to pinpoint the areas that would be most susceptible in the future.

"Our concern is, what would it take for it to really catch on? What are the thresholds for desertification to take over? That's still a big unknown," says Chris Schenk, a co-leader of the USGS project.

Although it may well be that the process is entirely out of human hands, Forman suggests that there is room for policymakers to address the matter. "If you can understand (the Plains') natural history and sensitivity to change," he says, "then you want to develop agricultural water use practices that don't take advantage of the propensity to aridification; you want to develop strains of crops that are drought-resistant; you want to watch overgrazing; and you want to watch how much water is being siphoned off of agricultural areas - which keeps them nourished and alive - and being sent to urban areas."

The point, Forman argues, is that the Plains are not as immutable as they seem to be. "You can use the land out there," he says, "but you have to use it judiciously, understanding there are natural constraints you're dealing with."