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LOS ANGELES — Already dealing with parched conditions, the US Southwest faces the threat of megadroughts this century as temperatures rise, says a new study that found the risk is reduced if heat-trapping gases are curbed.

Oppressive dry spells lasting at least two decades have gripped the Southwest before, but scientists said future megadroughts would be hotter and more severe, putting a strain on water resources.

The study, published Wednesday in the journal Science Advances, is the latest to find that droughts more extreme than what is currently being experienced could become more common as the planet warms.

Using computer modeling, researchers calculated there’s between a 70 percent and 90 percent chance the Southwest will experience a megadrought later this century.

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If precipitation is below normal, the risk jumps to 99 percent — ‘‘virtually certain,’’ said lead researcher Toby Ault of Cornell University.

If countries follow through with the Paris climate agreement to reduce emissions from the burning of coal, natural gas, and oil to limit global temperature rise to well below 3.6 degrees Fahrenheit, then the risks are cut nearly in half, according to calculations.

‘‘The likelihood of a megadrought is already increasing, and that risk will continue to go up as long as temperatures keep rising,’’ Ault said in an e-mail.

The Colorado River basin, which spans parts of Arizona, California, Colorado, New Mexico, Nevada, Utah, Wyoming, and Mexico, has been in the midst of a historic drought for the past 16 years. Lake Mead is now surrounded by a distinctive white mineral ‘‘bathtub ring’’ showing the 130-foot drop in water levels since the beginning of the regional drought.