The drought in the southwestern U.S. isn’t new - it’s actually a couple of decades old now.

But pictures of Lake Mead’s famous bathtub ring, accompanied by headlines signaling its historically low water levels, provide a quick visual aid for just how dire the situation has become.

“The drought in the Colorado River Basin is more dire than it has been because it’s gone on for so long,” says UNLV climate scientist Matthew Lachniet. “The last time it was wet in the Colorado River Basin was in 1998 — so we’re pushing long-term dry conditions for 23 years or so. And we haven’t updated our use of Colorado River water to a significant degree since then.”

Some changes, however, began this week as federal authorities declared a water shortage at Lake Mead, just two months after the reservoir fell to its lowest point — 1,070.44 feet above sea level — since it was filled in the 1930s. As part of the declaration, Nevada will lose about 7% of its allocation, or 21,000 acre-feet of water.

Meanwhile, water levels at Lake Powell, the nation’s second-largest reservoir located on the Utah-Arizona border, last month dipped to their lowest point as well.

Policy must catch up with today’s reality, said Lachniet, who, as a paleoclimatologist and chair of the geoscience department at UNLV, has studied regional climate history extending thousands of years into the past. 

“What we see from looking at the tree ring record is that we can regularly have droughts that are two decades long, sometimes a little bit longer. And if we even go further back in time, we can see that drought can persist for hundreds, if not thousands of years,” Lachniet said. 

His most recent research took him to a cave in the southern Great Basin, located in central Nevada, where ancient climate records - in the form of stalagmites - revealed a true, “worst-case” scenario picture for the Southwest and the Colorado River Basin. During one 4,000 year-period in the ancient past, the Southwest sustained hot, dry and arid conditions which were linked to warm Arctic seas, a lack of sea ice, and warming in the western tropical Pacific Ocean.

“So, it’s not unusual from the natural perspective that we’re having these dry conditions,” he said. But that’s why the policy is out of sync with what we actually know from the science.”

Combine natural variations in climate with human-caused global warming, and it’s like a budget that is way out of balance. 

“The water levels in Lake Mead and Lake Powell are like a bank account,” Lachniet said. “If you spend more than your income, then the water levels go down. And that’s exactly what’s been happening.”

We caught up with Lachniet to understand what the water shortage announcement means for Las Vegas and the broader Southwest, and how anthropogenic climate change has made a dire situation even worse.

How can the current drought be made worse by human-caused climate change?

The Colorado River Basin covers large portions of the Southwest and supplies drinking water to more than 40 million people. It flows down from Colorado to Utah, Nevada, Arizona, and California but most of the snowfall happens up north in Colorado. It ends up going into the river, which we take out here in Las Vegas. 

And as the climate warms — as it is doing — it will create even more stress on the water system. More water evaporates, yet there’s more demand for water during dry conditions. Also less water that falls as snow makes it into the river because the soils get so dry because they’re so hot.

We’ve almost always lived outside of our means here in Las Vegas, because we don’t produce enough precipitation in the form of snow and rain to satisfy the population that we have here. We’re relying on that water that’s coming from very far away, as are Southern California, Tuscon, and other locations.

I think that’s really key to understand: what happens in the Colorado Rockies determines our water supply in the Southwest, including southern California. We’ve had a wet year here and there within that broader, dry interval - but if you look at the history of those 23 years, it’s been consistently more dry than it was in the previous decades.

Lake Mead’s water levels have plummeted to an historic low. And the nation’s second-largest reservoir, Lake Powell, is now at its lowest point since it filled in the 1960s. What do these historic events mean for Las Vegas, Southern Nevada, and the larger Southwest Region moving forward? What can be done to stop the water levels from dropping even further?

Nature’s been giving us a certain amount of water, and we’re taking even more out of the system than nature is putting into it. So effectively that’s a policy question: how do we decide how much water to pull out of these reservoirs, and where does that water go? Does it go to agriculture in Southern California to grow alfalfa that’s shipped overseas? Or does it go into people’s houses and taps, or some other use. 

Those are the questions that policymakers really have to answer. But ultimately, everybody that’s using water has to be thinking about how much water they’re using, and how much they could potentially save.

What can we learn from the most recent heat wave we’ve experienced? Is it a one-off or does it play into the larger drought that we’ve been experiencing?

So the recent heatwave in the Southwest, but also more alarmingly in the Pacific Northwest, certainly plays into drought conditions. And it also is a big warning sign about global warming, because as temperatures warm — whatever the natural drought cycle is — it becomes amplified due to human influence on it.

The climatic phenomenon that made the heatwave possible in the Pacific Northwest is the same phenomenon that we see here. It’s a zone of high pressure that gets very strong and then sits over a region. That zone of high pressure, which we usually see over the Southwest, recently spent some time up in the Pacific Northwest and helped create that very extreme heat wave there. How that’s specifically linked to climate change hasn’t been sussed out yet, but there will be studies for sure coming in the next couple of years.

The Intergovernmental Panel of Climate Change last week released its sixth assessment report, which shows that human influence has warmed the climate at a rate unprecedented in at least the last 2,000 years. Is this a surprise?

What I’ve been seeing in the news fits with what we’ve actually already known for the last decade or two. And that is: the rate of warming is much faster than what we’ve seen over the last couple thousand years, potentially even the last 10,000 years, since the last Ice Age.

It fits with what we’ve known for a long time: as you add carbon to the atmosphere in the form of carbon dioxide primarily, but also methane, the climate is going to warm. 

As a result, we’re seeing more extreme weather events than we did in the past. On a potentially positive note, we’re reporting on it more, and recognizing that these events are linked to human activity. The question becomes: what do we do about it?