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The state of California is currently experiencing one of the worst droughts ever. It's draining the local water supply and turning reservoirs into empty bathtubs. But there's a worse threat lurking around the corner -- a megadrought. A more extreme version of a drought that could last for decades.

Prehistoric megadroughts are said to have toppled ancient civilizations in Cambodia, South America and North America. They've turned entire prairies into sand dunes. 

In February, a study from NASA suggested that Western North America could enter a megadrought within the next few decades, propelled in part by climate change. But what are megadroughts? How are they caused? And what can we do about them? 

Well, first lets just talk about plain old regular droughts. Basically a drought is an extended period of lower than average rainfall. It's often called a creeping phenomenon because unlike other natural hazards like earthquakes and hurricanes, droughts come on slowly. They can take anywhere from a week to a season to develop, deepening in severity with every passing day, and leading to crop failure and water shortages. Normally they last up to a few years. 

Then there are megadroughts, which as I said can last up to a few decades. The terms was first popularized in 1998 by two American environmental scientists named Jonathan Overpeck and Connie Woodhouse in the paper they wrote reconstructing the climate history of the American Great Plains. 

They wanted to put the terrible droughts of the 20th century, like the dust bowl of the 1930s, into context. Prior to this study, there were all these smaller regional studies that suggested there was a really bad drought in the Great Plains between 900 and 1400 CE, also known as the medieval period. 

For instance, historians in the early 1930s suggested that a great drought swept through the southwestern United States and coincided with the abandonment of ancient Anasazi villages and the redistribution and reorganization of populations across the west. 

And in 1974, a study of the number of bison bones found in archaeological dig sites across the Southern Great Plains found that there were far fewer bones dating back to this period compared to the eras before and after.

Then, in the early 1990s a geographer carbon-dated dead tree stumps that were found at the bottoms of lakes and river beds in the Sierra Nevada mountains in California. He discovered that most of them were alive during the medieval period.

The trees couldn't have grown underwater, so he theorized that the climate must have been so dry that these lakes and rives used to be much smaller.

But these studies were all like individual puzzle pieces in a much larger picture of the great plains during this medieval dry spell. So in the late 1990s Overpeck and Woodhouse reconstructed the entire great plains' historic climate, going back to 1 CE.

To do that they used paleoclimate data, data derived from ice cores, tree rings, coal, soil sediments, anything natural that's been around for a really long time.

Take trees for instance. In temperate areas, like much of North America, Asia and Europe, trees only grow during part of the year - the growing season. Near the beginning of the growing season the tree grows faster, expanding and forming paler wood, but toward the end the tree grows slower, forming more condensed and darker wood. This process repeats itself every year, and the contrast between the light and dark wood forms growth rings, which is why counting a tree's rings can help you figure out how old it is.

In good growing seasons, ones with favorable temperatures and more precipitation, the trees grow faster and form wider growth rings. But in poor growing seasons, ones with extreme temperatures and less precipitation, like during a drought, the trees form thinner growth rings.

But there are issues with tree ring chronologies, as they're called. For instance, trees only grow in warmer months, so the rings can't show us what climate was like in the winter. Tree growth can also be affected by stressors, like competition with other plants or poor soil nutrients. That's why Overpeck and Woodhouse used many different types of paleo-climatic data, including tree rings, to figure out what happened in the Great Plains.

They discovered a series of major, severe droughts. We're talking worse than the Dust Bowl. The scientists called these droughts "megadroughts," and the term stuck.

Subsequent studies showed that these megadroughts lasts for more than 40 years, and they could be incredibly persistent in their severity. Between 1140 and 1165, for example, droughts as dry as the Dust Bowl happened year after year after year. And from 1276 to 1313, sediment studies have shown that the state of Nebraska, which is now covered in prairie grass, was full of massive sand dunes.

It wasn't all bad every year during these periods. Occasionally, a year or two were actually wetter than average. But the norm was drought. That changed the environment dramatically, which is why studies kept finding things like absent bison bones, sand dunes, abandoned villages, and tree stumps beneath rivers.

But megadroughts aren't unique to the medieval Great Plains. A 2009 study published in Science magazine suggested that sub-Saharan Africa had suffered from megadroughts in the past, including one as recently as 300 years ago. And south-east Asia experienced megadroughts in the 14th and 15th centuries, interspersed with heavy monsoon seasons.

But how do these megadroughts happen? Well, again first we have to understand how regular droughts happen. There are three main contributors to a drought: soil moisture, atmospheric circulation patterns, and land and sea surface temperatures.

Soil moisture is the amount of moisture in the soil, and when the ground is wet, that water evaporates as the day heats up. It rises, eventually interacting with cold air high in Earth's atmosphere. That cold air causes the moisture to condense back into a liquid, and that's how you get rain.

But those rain clouds don't always stick around. They can be carried hundreds of miles away from the source by atmospheric circulation patterns, which are giant, worldwide air currents powered by heat and evaporation from the ocean. Earth is basically like a huge, complex, water recycling machine.  But things can go wrong.

Sometimes these circulation patterns shift due to the cooling or warming of ocean surface temperatures, and rain might stop arriving in a region as frequently. When this happens, the soil moisture drops. Each day, more moisture is evaporated into the air without being returned to the surface. The hotter it is on the ground, the faster it evaporates. Eventually it turns into a drought. But when these temporary shifts in atmospheric circulation patterns become the prevailing patterns, you end up with a megadrought.

Take those medieval Great Plains droughts, for instance.  Scientists aren't entirely sure why they happened, but they do know that the surface of the tropical Pacific cooled, altering the path of atmospheric circulation patterns over North America for centuries.  And now, NASA researchers are suggesting that a megadrought might be returning to the North American west, and this time, they know what would be causing it: climate change.

California's current drought problem is a big deal, the worst the area has dealt with in 1200 years, according to a recent study.  But that drought only began three years ago in 2012.  It's still a regular drought, though a prolonged one.  The NASA team wanted to see what was in store for both California and the rest of the North American west on a much larger scale over the course of the next century.  So they used a wide set of metrics including soil moisture data, as well as current and projected greenhouse gas emissions to model the future climate.  And they came up with two forecasts.
 
The first showed how a drought developed if greenhouse gas emissions continue as they are now, while the second looked at what happened if those emissions were much lower.  In both cases, megadroughts were predicted for the latter half of this century.  But unlike those previous megadroughts in the American west, which were caused by a decrease in rainfall, these new megadroughts would have a different trigger: an increase in the evaporation due to warmer temperatures. 

Essentially, this region, which already receives less than 250 millimeters of rain per year is going to get hotter, which will dry out the land more quickly, and this starts a vicious cycle.  When rains do come, mostly between April and September, they can't replenish the amount of moisture that was lost, deepening the drought each year.  With current greenhouse gas emissions, the forecasts say there's an 80% chance that these megadroughts will happen, and they'll be more severe and prolonged than the medieval megadroughts, soil moisture will be twice as low as it was back then.  But if greenhouse gas emissions stop increasing by the mid-21st century, the chance of a megadrought decreases to 60%, and if it does happen, it won't be as bad.

Either way, a future megadrought in the North American west would be challenging.  For the past 150 years or so, human populations in the area have needed more and more water.  A megadrought could lead to widespread crop failure, as well as the further depletion of nonrenewable groundwater resources and reservoirs.  The good news is that it's still early in the game.  There are ways to mitigate these risks, like alternative methods of farming that don't use as much irrigation, or by reusing water.  Mostly, it would also help if we could figure out how to live on less water, and one way to do that is by re-using what's called 'grey water', wastewater from clean-ish activities, like showering or laundering clothes.  Grey water makes up about 60% of all the water used in a household.  Nowadays, when it goes down the drain, it's often mixed with black water, which is highly contaminated water that comes from toilets.  But if we could separate that grey water, it could be recaptured and used to water gardens and fill toilet bowls, basically anything that doesn't involve eating or drinking it.  Reusing grey water could reduce a household's daily water needs by up to 40%. 

Then there are more extreme measures, like desalination plants.  At a desalination plant, you pump seawater at high pressure through screens that filter out the salt.  One proposed in Carlsbad, California would provide the state with 50 million gallons of fresh water per day, but there are drawbacks.  Modern desalination plants are expensive to build.  The Carlsbad project would cost a billion dollars, and they can have a hefty environmental footprint.  The plants use, on average, 15,000 kilowatts of power to filter one million gallons of seawater.  By comparison, it takes only 3,400 kilowatts to pull one million gallons of water from an aquifer or a reservoir.  So even though a desalination plant could provide clean water to a drought-stricken region, it would also make the environmental problems worse.  One way to compensate is by funding efforts to fix the environment, like buying carbon credit or helping to restore wetlands, but still, it's not exactly ideal.  So climate scientists and environmental engineers have their work cut out for them, but hopefully, by the time a megadrought hits, we'll be ready.

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