Vast swathes of Southwestern United States — a region already grappling with parched conditions — may now have to brace for another threat — decadeslong “megadroughts.”

According to a new study published Wednesday in the journal Science Advances, there is roughly a 70 to 90 percent chance that the Southwest would experience a megadrought — an extreme dry spell lasting 35 years or more — before the end of the century. And, if precipitation is below normal, it's 99 percent certain that such an event will occur.

“Megadroughts are rare events, occurring only once or twice each millennium. In earlier work, we showed that climate change boosts the chances of a megadrought, but in this paper we investigated how cutting fossil fuel emissions reduces this risk,” lead author Toby Ault, a professor of Earth and atmospheric sciences at Cornell University, said in a statement. “The increase in risk is not due to any particular change in the dynamic circulation of the atmosphere. It’s because the projected increase in atmospheric demand for moisture from the land surface will shift the soil moisture balance.”

The present-day Earth is 1 degree Celsius hotter than it was 150 years ago. The Paris climate agreement, which is to come into force next month, aims to restrict this rise to below 2 degrees Celsius — a target that many experts now believe is almost certain to be breached.

If, or when, this happens, megadroughts in the Southwest would become “very probable,” Ault said.

“A megadrought in the American Southwest would impose unprecedented stress on the limited water resources of the area, making it critical to evaluate future risks not only under different climate change mitigation scenarios but also for different aspects of regional hydroclimate,” Ault and his colleagues wrote in the study. “Furthermore, business-as-usual emissions of greenhouse gases will drive regional warming and drying, regardless of large precipitation uncertainties.”

If, on the other hand, we manage to somehow restrict the rise to “well below” 2 degrees, the risk of a megadrought would be cut nearly by half.

“Our findings have important implications for both mitigation and adaptation. With regard to mitigation, the dependence of megadrought risk on mean temperature highlights a relative advantage of keeping GHG emissions low,” the authors wrote in the study. “If regional warming remains below 2°C, megadrought risks will correspondingly remain below 66% for a wide range of precipitation changes. Further emission reductions, and hence smaller temperature increases, would have even greater benefits in reducing megadrought risks.”