California hasn't had a drought this bad since at least 1895

It's no secret that California is in the grips of an absolutely brutal drought. But how does it compare historically?

According to this week's figures from the US Drought Monitor, some 82 percent of the state is now suffering either "extreme" or "exceptional" drought — the first time that's happened since the monitor was set up in 1999.

(US Drought Monitor)

But we can go back even further, as Andrew Freedman of Mashable points out.

Yesterday, the National Weather Service Hanford posted a chart suggesting that — according to a different metric — California's drought is now the worst since at least 1895:

(National Weather Service Hanford)

The chart above shows the Standardized Precipitation Evapotranspiration Index (SPEI), which takes into account both rainfall levels and the amount of water that's evaporating — say, due to high heat.

The drought has already caused $2.2 billion in damages

By this metric, this year's drought in California is now the worst since records began in 1895, eclipsing the previous "worst-ever" drought in 1958-59.

So how did we get here? Much of California was already in drought in 2012 and 2013, but conditions worsened considerably this year. A blocking ridge of high pressure kept California hot and dry during the normally wet winter. That means there was far less snow falling on the Sierra Nevada mountains — and hence less snow melt providing water during the spring. (There's a fair bit of dispute about what role global warming might have played in causing the drought, although warmer temperatures have likely exacerbated the dry spell.)

The drought has already caused an estimated $2.2 billion in damages in California — including $1.5 billion to the state's massive agricultural industry. All told, California's farming industry is worth some $44.7 billion per year and the state is responsible for half the fruits and vegetables grown in the United States.

There's also more subtle damage: A recent study from UC Davis found that the drought and lack of rainfall were spurring farmers to pump more and more freshwater out of the ground. That's troubling because that groundwater isn't easily refilled — which means that farmers will have less of a buffer in future droughts (or if the current drought extends for a long period of time).

It's still not clear how long California's current drought will last. Some experts were holding out hope that a strong El Niño event in the Pacific Ocean might bring wetter weather this winter. But forecasters are now scaling back their predictions for El Niño — and it's less certain how much precipitation a weaker event might bring.

There's also evidence this is the worst drought since 1580

A sign is posted near an almond farm on February 25, 2014 in Turlock, California. (Justin Sullivan/Getty Images)

By the way, Scientists can also go back even further than 1895 by studying tree-ring samples of drought-sensitive trees (since tree growth is very much affected by droughts) and sediment records.

The historical record shows evidence of 200-year 'mega-droughts'

This historical evidence suggests that California may be suffering its worst dry spell since a massive drought in 1580.

That historical record also brings horrible news. Evidence suggests that even the most severe recent droughts in California are nothing compared to the 200-year "mega-droughts" that occurred between 850 and 1090 or between 1140 and 1320.

It's hard to say if California is currently facing one of these extended "mega-droughts" — some experts think yes, some no — but an extended drought would obviously be extremely destructive.

Earlier this year, Paul Rogers of the San Jose Mercury News asked a variety of experts what would happen if California's drought did extend another 10 years, say. "Cities would suffer but adapt," he found. But farmers — who use some 80 percent of the state's water — wouldn't fare so well:

In a megadrought, there would be much less water in the Delta to pump. Farmers' allotments would shrink to nothing. Large reservoirs like Shasta, Oroville and San Luis would eventually go dry after five or more years of little or no rain.

Farmers would fallow millions of acres, letting row crops die first. They'd pump massive amounts of groundwater to keep orchards alive, but eventually those wells would go dry. And although deeper wells could be dug, the costs could exceed the value of their crops. Banks would refuse to loan the farmers money.

The federal government would almost certainly provide billions of dollars in emergency aid to farm communities.

Agriculture is only 3 percent of California's GDP, so the state would survive if farming moved elsewhere — but it would certainly heavily affect some communities, particularly in the Central Valley.

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