'Mega-drought': Could California's drought last years? A century?

It's dry. It's been dry. 

And forecasters say despite some rain here and there, it looks like California will stay dry for some time.

This prompted Gov. Jerry Brown to declare a drought emergency last month, but the state has seen less rain than usual for about three years now.

So how long could this dry streak last?

RELATED: Drought: Snowpack in Sierra Nevada at record low levels

The state's last drought stretched from 2007 through 2009.

From 1987 to 1992, the region suffered a particularly severe dry spell as well, and in the late 1920s and early '30s California endured a whopping six years without average rainfall.

These examples pale in comparison though to California's mega-droughts, says Lynn Ingram, professor of earth and planetary sciences at UC Berkeley and author of the book "The West Without Water."

She says a mega-drought lasts anywhere from several decades to more than a century.

"We haven't seen that since the  Middle Ages," Ingram said.

By studying tree rings and examining geological clues, researchers determined that in the period between 900 AD and 1400 AD, such mega-droughts were common in California.

Some lasted more than 200 years, Ingram said, affecting the state in dramatic ways.

"The lakes were substantially drying up, fire frequency increased and the size of fires increased."

She adds that bones from early civilizations in the region show signs of malnutrition and increased violence, possibly due to fights over scarce food and water.

Drier future?

Richard Seager is a climate scientist with the Lamont-Doherty Earth Observatory at Columbia University. He says scientists don't know what caused these mega-droughts.

He notes, though, that the Middle Ages were a warmer period for much of the globe.

Seager says current models suggest California could see rainier winters, and the rain that falls is likely to evaporate quickly and not add as much to the mountain snowpacks the state relies on for most of its water. 

"So all of those shifts in the seasonal cycle are gong to stress California's water."

Because of this, he says that it's worth planning now for a potentially much drier future.