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Megadroughts, persisting for decades at a time, parched the Southwestern U.S. centuries ago between 800 CE and 1600 CE. Then, the extreme droughts stopped.

But with temperatures today both exceeding the warm climes of past droughts and now relentlessly rising, the return of the Southwestern megadroughts is almost assured. New research, published Wednesday in the journal Science Advances, illustrates how a timely confluence of warm temperatures and changes in the ocean stoked a cluster of 14 potent Medieval-era megadroughts.

Similar events could unfold again.

"This is what we would expect to happen in the future too," said Nathan Steiger, an associate research scientist at the Lamont-Doherty Earth Observatory of Columbia University and lead author of the study.

Since 1901, average temperatures have increased across nearly the entire Southwest, in many places by over 2 degrees Fahrenheit. Record high temperatures have been repeatedly broken around different regions of the Southwest since 2012. Meanwhile, over the greater globe, 18 of the 19 warmest years on record have occurred since 2001.

To gauge how previous Southwestern megadroughts arose, Steiger and his team employed an advanced reconstruction of the past climate over the last 2,000 years, which uses evidence stored in tree rings, ancient glacial ice, lake sediments, coral reefs, and beyond.

During one of the peaks of crippling dryness around 800 years ago, the once flourishing Chaco Culture abandoned their great houses and intricate societies (though other factors may have played a role, too). Thousands of miles away, there was an increase in periods of colder than usual sea surface temperatures over the Pacific Ocean, known today as La Niña events. La Niña has the well-documented effect of pushing rain-bearing storms farther north, above the Southwest, which deprives the region of rain. To boot, a warming trend in Atlantic ocean waters also contributed to forcing storms north of this region. "It can then dry out the Southwest," noted Steiger.

But that's not all. Steiger also noted that a slight uptick in the sun's activity warmed the region while a lack of volcanic activity (which reflects sunlight) allowed more heat to reach Earth.

For some 800 years, then, the right conditions came together to foster and sustain a series of decades-long megadroughts.

"[The researchers] are essentially listing the ingredients you need to have these megadroughts," said Flavio Lehner, a climate scientist focused on hydroclimate at the National Center for Atmospheric Research. Lehner had no role in the research.

"Their story makes a lot sense to me -- it makes a compelling case," he added. "But as often occurs in science, it's probably not the last word," Lehner said, noting that climate researchers don't fully understand longer term La Niña fluctuations in the Pacific Ocean and the probabilities of another megadrought -- or cluster of megadroughts -- occurring.

But one thing is clear. The primary "ingredient" of warming and record-breaking temperatures will be present this century -- if not well beyond (depending on how or if civilization slashes heat-trapping carbon emissions). "It's by far the most certain part of the equation," Lehner said.

A series of La Niñas could then come along and "trigger" a megadrought, explained Steiger.

Though, a sustained drought -- an event even flirting with the designation of "megadrought" -- might occur just from the current global warming trend alone. It's threatening to happen now as warming climes dry out Southwestern lands and watersheds: The Southwest is mired in a 19-year drought, the worst in Colorado River history. "There’s no analog, from when humans started gauging the river, for this drought," Brad Udall, a climate research scientist at Colorado State University, explained last year.

An exceptionally rainy last winter (the wettest winter on record in the Lower 48), however, may finally signal an end to the drought. Or not.

"If [the drought] continues to be severe, it would be within the realm of a historical megadrought," said Steiger.

"If the next five years are dry again then you can treat it as one 24-year drought," agreed Lehner. "We're coming to a point whether this drought is like a megadrought of the past.

Overall, one of the most straightforward and predictable consequences of climate change is amplified drought, especially in already drought-prone regions. Hotter climes mean more evaporation from the land.

"Most future model simulations show a strong increase in drought in the Southwest," said Jessica Tierney, a geoscientist at the University of Arizona who had no role in the study. "That's going to be a problem for all the arid lands."

And, expectedly, amplified dryness is a big ingredient of future megadroughts. Though, another big component is what transpires in the tropical Pacific Ocean -- the place where La Niña brews. "The other half of the equation is precipitation, which depends on what the tropical Pacific is going to do," Tierney emphasized.

Right now, the future behavior of the ocean, such as an uptick in La Niñas, isn't predictable. Neither, then, are megadroughts. "It remains difficult to say if and when we should expect these megadroughts to occur in the next 100 years," said Lehner.

But, with the ongoing warming trend and the highest atmospheric levels of heat-trapping carbon dioxide in at least800,000 years, megadroughts could be closer on the horizon.

"This study has paleoclimatic evidence supporting that future possibility," said Steiger.