When you live in Las Vegas, it can be difficult to understand drought. Las Vegas sits in the ultra-dry Mojave Desert. It doesn't rain much here. You might say we're always operating under drought conditions.

But that's not quite right. What Las Vegas has is aridity, not drought.

Drought is a temporary climate shift, not a constant condition. It might not be apparent in a place like Las Vegas, where it's hard to tell the difference between 3 inches and 5 inches of rain per year. But the reality is that the American West has been in an extreme drought since 2000.

It's much more obvious in, say, Colorado, where drought has a more profound impact. Drought in Colorado leads to water shortages for towns and farms alike. And we Las Vegans should be watching Colorado's drought situation a lot more closely than we do.

Las Vegas' primary water supply, the Colorado River, depends on climatic conditions in the Rocky Mountains. The river is supplied by snowpack in those mountains. When that snowpack shrinks, which happens during a drought, it reduces the amount of water flowing into the river.

And if the drought persists, that could affect how much water Las Vegas can draw from the Colorado River.

The drought was severe from 2000 to 2004, the worst since records started being kept in 1906. The driest year was 2002, when the river flow was 25 percent of normal.

Then 2005 was a wet year, and some folks took a deep breath, thinking the drought was over. But 2006 is proving to be another dry, hot year, suggesting the previous year was merely a blip amid a prolonged drought period. (Incidentally, last month was the warmest June ever recorded at McCarran International Airport.)

How long will the drought last? No one can say for sure, obviously. But historical records suggest it could go on for a while.

"The Southwest is capable of ginning up a drought that could last 15 to 20 or 25 years," says Kelly Redmond, regional climatologist for the Western Regional Climate Center, based in Reno. "That's a known thing. We've seen this in the records."

The records Redmond refers to are studies of tree rings. Tree rings are a pretty good indication of the water situation dating back 500 to 1,000 years.

If the drought lasts 25 years, Las Vegas could be in trouble, plain and simple.

The Colorado River is supposed to provide about 15 million acre-feet of water per year. That's the average over the past 100 years or so. It's also the figure used to decide how much each state can draw from the river.

If the Colorado doesn't deliver 15 million acre-feet, there are consequences. In reality, the river hasn't delivered that much lately. It's been closer to 13 million acre-feet. This has been manageable because of ample storage in Lake Powell and Lake Mead, among other clever techniques to maximize the water supply. But if the Colorado delivers significantly less than that, officials will have to cut back allocations to the states.

There's yet another problem: global warming.

Put aside for a minute the politics of global warming. I'm not going to deify Al Gore or try to make anyone feel guilty about their Hummer today.

There is no disagreement on this: Over the past 30 years, the average temperature in the West has risen about 2 degrees. Regardless of why this has happened -- natural cycles, greenhouse gases or both -- it has happened. And the implications for water are huge.

Warmer temperatures mean shorter winters. Shorter winters mean shorter snow seasons, and thus less snowpack in the mountains. Less snowpack means less snow melting into the Colorado River. In addition, with a longer warm season, humans use more water.

"Warmer temperatures exaggerate drought effects," Redmond says. "So if we have similar precipitation but in a warmer setting, that, in effect, is like having less precipitation."

Predicting the future of the climate is a dicey business. We have enough trouble predicting whether it's going to rain tomorrow. But Redmond says the best evidence right now suggests that over the next 30 years, Western temperatures will increase another 2 degrees. That's a lot -- especially in the Rocky Mountains.

It's even more troubling for the Sierra Nevada, which is the main source of water for the Reno area. Sierra Nevada snow isn't as frozen as snow in the Rockies, which means rising temperatures are likely to have a more immediate effect of less snow and more rain. For a host of reasons, from evaporation to plant transpiration, rain doesn't have the same benefits as snow. It's definitely not good for skiing.

The Southern Nevada Water Authority didn't conceive its rural water importation plan because it was worried about a prolonged drought. Back in the moist 1990s, the main goal was to find additional water resources to serve fast-growing Las Vegas.

Local officials still want to secure plenty of water to serve a growing community. But now they look at the rural water project a little differently. Today, they see the possibility that in a prolonged drought, the Colorado River may not be able to handle our immediate needs. And they worry that global warming is going to exacerbate the problem.

They see a pressing need for a backup plan, known in government parlance as a "more diversified water portfolio."

"We need to be prepared for the future," says Kay Brothers, deputy general manager of engineering and operations for the Southern Nevada Water Authority.

The water authority has some work to do to convince White Pine County that it won't be another Owens Valley disaster if Las Vegas runs a $2 billion pipeline up there and taps the groundwater. Las Vegas advocates must understand that for White Pine, the issue is only partly about water. It's also about history and state politics and the economy. In the rest of Nevada, Las Vegas is more likely to be sneered at than embraced, a byproduct of Las Vegas' growing economy and political power while once-prominent communities such as Ely and Tonopah have been left behind.

Still, while White Pine residents are divided on the issue, it's likely that a majority of them will come around eventually -- if Las Vegas officials provide set-in-stone assurances that they won't suck White Pine dry.

Meanwhile, Las Vegas has made significant strides in conserving water in recent years. A fair number of residents, as well as builders and business owners, have finally recognized that we live in the Mojave Desert.

But we can and should do more. We can't predict how long the drought will last, but we know temperatures will rise and we know the population will increase. We know the days are long past when we can re-create Vermont or Wisconsin in this arid climate.

Despite these considerable challenges, Redmond is optimistic that we will figure out how to solve them.

"But we have a habit as human beings of constantly taking ourselves right up to the edge of a cliff before we decide to do something," he says. "Most of the time we work it out, because we're still here. That speaks for itself."

Geoff Schumacher (gschumacher@reviewjournal.com) is Stephens Media's director of community publications. His column appears Sunday.