• The megadrought that has parched southwestern United States and parts of Mexico over the last two decades is the worst to hit the region in at least 1,200 years, according to a new research paper.

• A megadrought is defined as a prolonged and severe drought spanning over a period of two decades or longer.

• The study, published in the journal Nature Climate Change, showed that human-caused global heating accounts for more than 40 per cent of the dry spell’s intensity.

• Over the last decade, California and other western states in the US have experienced severe water shortages, triggering periodic restrictions on water usage and forcing some communities to import bottled water for drinking.

• Occasional heavy snow or rainfall have not been enough to compensate.

• Drought covers more than 87 per cent of the nine western US states, according to the US Drought Monitor. 

• California’s drought has caused massive wildfires as well as water shortages around the most populous US state.

• Last summer, two of North America's largest reservoirs — Lake Mead and Lake Powell — reached their lowest recorded level in more than a century.

• The research found that warmer temperatures and increasing evaporation are drying out soil and vegetation.

• According to the findings, the odds are high that the current dry spell will continue for at least a couple of years, probably longer.

• Running simulations based on soil moisture records stretching back 1,200 years, the researchers calculated a 94 per cent chance that the drought would extend through 2022.

• There is a three-in-four chance it will run until the end of decade, it said.

• Researchers used tree rings to track soil moisture over the centuries. Tree-ring analysis shows that the area west of the Rocky Mountains from southern Montana to northern Mexico was hit repeatedly by so-called megadroughts — lasting at least 19 years — between the years 800 and 1600.

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