Story highlights The current drought is bad, but it's no megadrought NASA: If greenhouse gas emissions don't drastically drop, the nation's West faces droughts that could last decades

No precedent even in the past 1,000 years.

The feared droughts would cover most of the western half of the United States -- the Central Plains and the Southwest.

Those regions have suffered severe drought in recent years. But it doesn't compare in the slightest to the 'megadroughts' likely to hit them before the century is over due to global warming.

These will be epochal, worthy of a chapter in Earth's natural history.

Even if emissions drop moderately, droughts in those regions will get much worse than they are now, NASA said.

The space agency's study conjures visions of the sun scorching cracked earth that is baked dry of moisture for feet below the surface, across vast landscapes, for decades. Great lake reservoirs could dwindle to ponds, leaving cities to ration water to residents who haven't fled east.

"Our projections for what we are seeing is that, with climate change, many of these types of droughts will likely last for 20, 30, even 40 years," said NASA climate scientist Ben Cook.

That's worse and longer than the historic Dust Bowl of the 1930s, when "black blizzards" -- towering, blustery dust walls -- buried Southern Plains homes, buggies and barns in dirt dunes.

The Dust Bowl drought in the 1930s: Dust blows up dunes at Oklahoma farm.

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Risk of American 'megadroughts' for decades, NASA warns