The American West is in the middle of a historic “megadrought” — an ongoing stretch of extended arid conditions not seen in centuries — according to a major new study released Thursday.

In the nine Western states from California to Colorado, 2000 to 2018 ranks as second-driest 19-year period in the past 1,200 years, according to scientists at Columbia University who led the study published in the journal Science.

The scientists, who studied tree rings from roughly 30,000 trees in 1,586 locations to measure the amount of rainfall and soil moisture over the centuries, found that the only time when conditions were drier in the West than they are now was between 1576 and 1603, when the Pilgrims had yet to set sail for Plymouth Rock and Spanish conquistadors first ventured into New Mexico and Arizona.

The current megadrought is still underway, and while its causes are natural, it is being made worse by warmer temperatures from climate change, the researchers said. And although there have been wet years, such as in 2017 and 2010, they noted, those are exceptions in the longer two-decade pattern, similar to days when the stock market drops in value, even though the longer overall trend in prices is upward over decades.

“Across the broad American West, the last two decades have looked as bad as the worst two-decade period of the last millennium. This is an event of millennial significance,” said Park Williams, a bioclimatologist at Columbia University’s Lamont-Doherty Earth Observatory, and lead author of the study.

“The severity varies throughout the region,” he said. “But when you look at the region as a whole, this is really truly a monumental event.”

It’s understandable that people don’t always realize when they are living amid historic shifts, said Bill Patzert, a retired oceanographer and research scientist for 35 years at NASA’s Jet Propulsion Laboratory in Pasadena.

“The thing about droughts in the West is that they are large. And they are long,” Patzert said. “They don’t go for a few months. They wax and they wane. They are on-again off-again. We have a couple of years with good rain and snowpack — like 2005 and 2010 and 2017 — and you think you are out. But it’s like the Godfather. You are back in again. Everybody is too quick to terminate droughts.”

Patzert studied ocean patterns for decades and concluded nearly 20 years ago that a shift was underway in which wetter conditions seen during the 1980s and 1990s would give way to an extended drought.

He said the best way to measure whether a drought is over in the West is to look at the level of Lake Mead, the vast reservoir behind Hoover Dam on the Colorado River, which flows through seven Western states. The lake, a critical water source for the West, is currently just 44% full.

“Lake Mead has been low for a long time,” he said. “It’s still low. And there’s no quick fix.”

Ominously, the researchers in Thursday’s study found that the 20th century was the wettest century in the entire 1,200-year record.

In other words, people in expansive swaths of California, Las Vegas, Phoenix and other areas that are largely built in deserts have constructed vast cities, water systems and farms around expectations for rainfall and snowfall which are well above the long-term historical realities for the region.

California suffered through a five-year drought from 2012 to 2016. Former Gov. Jerry Brown declared it over when reservoirs filled after huge storms in 2017 that caused flooding in downtown San Jose and wrecked the spillway at Oroville Dam.

But the longer trend underway over the past 20 years shows that California needs to accelerate reforms that came out of that drought, said Felicia Marcus, former chairwoman of the State Water Resources Control board.

Those include building more off-stream reservoirs to capture water in wet years, expanding conservation programs like paying people to replace lawns with water-efficient landscaping, recycling more wastewater for irrigation and other uses, capturing storm water, and other solutions, she said. Because even when it seems like a drought may be over, it will return, she said.

“We’ve already had the wakeup call of the century in our drought, and this study is just more evidence of the fact that we need to light a fire under our efforts,” Marcus said. “We are living in something of a dreamworld. Modern California — our economy, agriculture and our ecosystems — are built around water. This is just one more alert that business as usual just won’t cut it. How many reminders do we need?”

Over the past 20 years, California has had three stretches of short-term drought, Williams said. They were 2000-2003, 2007-2009 and 2012 to 2016.

“In California it looks more like three individual droughts, but when we look at the larger scale, we can see it’s really one,” Williams said.

The most severe conditions over the past two decades have come in Arizona and Southern California, the research shows. Overall, a record number of wildfires, hundreds of millions of dead trees in Western forests, declining groundwater levels and drying soil moisture levels are all evidence of the current megadrought, the researchers said.

The idea of decades-long droughts isn’t new.

At least four major megadroughts have been well documented over the past 1,200 years in the American West. Some lasted a few decades, others a century or more. The most severe stretches ran from 867 to 898 AD; 1136 to 1172 AD; 1218 to 1310, and 1576-1603. They all caused major upheavals in wildlife and in human settlement. The third helped lead to the collapse of the Anasazi civilization in the Southwest, for example.

Scientists — who measure tree rings to the thousandth of a millimeter using high-powered microscopes to compare rainfall patters over centuries — believe those extended droughts were caused by naturally changing cycles of the ocean which result periodically in more La Niña conditions. During such conditions, ocean waters off South America are cooler than normal, affecting weather patterns that reduce rainfall in California and the West.

That’s what’s underlying the current megadrought, the researchers said. But based on those natural conditions, the past two decades would have ranked 11th worst in the last 1,200 years — significant but not historically extreme. Due to climate change caused by the burning of fossil fuels trapping heat in the Earth’s atmosphere, this event has been more severe.

Temperatures now are about 2 degrees Fahrenheit warmer than they would have been without climate change, Williams noted. Those warmer temperatures have further dried out soils and vegetation. Wet periods will return, he said, but if climate change continues to warm the Earth, droughts will worsen.

“We need to be educated enough in science and trusting enough of science to learn from this type of study,” he said. “We are constantly reminded — most recently by the coronavirus — how much better off we’d be if we can interpret scientific information and understand what projections mean. Epidemiologists have been warning about the coronavirus for decades. Here we have a similar situation.”