What's a Megadrought? Are We in One?

Listen to all six episodes of our State of Drought series on Spotify.

When summer is hot and dry, Californians have reason to think about drought — what it means to be in one, and what it means to come out of one.

In 2017, the state emerged from a nearly six-year seasonal drought. Then we had a few years that saw more water across much of the state. Now in 2021, we’ve hit another significant drought year.

That led Bay Curious listener Nicholas Hardy to ask: Are we in the start of a drought or are we actually in the middle of a megadrought with some wet years thrown in?

When we examine the long-term history of California’s relationship with water, or the lack thereof, we can start to piece together what’s going on.

How do we know we’re in a drought?

Weather conditions, such as day-to-day rainfall and heat, tell us if we’re currently in a drought. What little snow fell in the Sierra Nevada melted and quickly soaked into the ground this year, never reaching reservoirs. That plus meager rainfall over the winter set the stage for bone dry summer conditions in California and across the West. By the end of August, the national Drought Monitor, showed the entire state of California was at least in moderate drought, with a significant swath ranked in “exceptional drought,” the highest level.

Oregon and Nevada had similar conditions. The Drought Monitor is a map that federal agencies and university scientists pull together from actual weather data such as temperatures and rainfall and also on-the-ground observations, such as those made by UC Cooperative Extension agents throughout the state.

What is a megadrought, and are we in one?

Over the long-term, it’s climate science that paints the picture of how long droughts in the past have lasted, and to some extent how severe they were. While “drought” refers to one season or one year’s conditions, climate scientists use the term “megadrought” when drought conditions last for decades.

“In the southwest, it’s been overall drier since the late 1990s. So we’re talking about a 20 year dry period here now,” said Richard Seager, a climate scientist at the Lamont Doherty Earth Observatory at Columbia University, “and using the term megadrought is justifiable because it stacks up in terms of the severity and the length with the ones that we’ve inferred from tree ring data back in the Medieval period.”

Last year, a study published in the journal Science described how tree rings allowed researchers to conclude that the last time the West experienced sustained arid conditions over decades was a 28-year dry spell that ended in the year 1603.

“We now have a full, uninterrupted record of soil moisture across western North America that extends from 800 A.D. all the way up to the near present,” Park Williams, who led the study, told KQED at the time. He’s now a bioclimatologist at UCLA. “That allows us to compare for the first time this drought event to the big megadroughts.” Williams said the current megadrought looks “indistinguishable from those big megadroughts.”

The study looked at California, eight other states and Northern Mexico and it corroborates what scientists have long feared and warned policymakers: Extreme warming will exacerbate any dry spell making it longer, severer and more widespread, and this will bake states in the Western U.S. and areas of Mexico with a punishingly long drought.

While the occasional wet period punctuates a long run of below-average precipitation, for the past two decades the region has been consistently dry and hot. In response, thirsty farms and cities have drawn down the Colorado River, putting the water supply for 40 million people in California alone at risk.

For the first time, in August 2021 the federal government declared a water shortage on the river and some farmers, particularly in Arizona, are already feeling the impact.

California Boomed During Unusually Wet Times

The 20th century saw California grow to be the most populous state in the nation, says Scott Stine, a professor of geography and environmental studies at Cal State East Bay, and that coincided with an especially wet century.

“We built this phenomenal infrastructure, second to none in the world, here in California and in other parts of the West,” he said. “All of it based on the diversion of water and all of it based on the assumption that the 20th century was normal. And the 20th century is not normal.”

“The problem of the present day and the coming droughts is that we are set up to need far more water than we should expect at any time in the coming decades and centuries,” he said. “Particularly with the higher temperatures. We’ve created a monster that we have to continue to feed with water, and the water is just not going to be there.”

That likely means changes to the ways water is consumed in California.

“A lot of agriculture in the United States is going to have to move from California and the southwest to other water rich areas,” Seager said.

But already farmers across the state are finding different ways to use their land and water, by choice, circumstance or legal requirement. Researchers at UC Merced hope the “smart farm” they are building on campus will help them develop strategies to improve the ways agriculture makes use of scarce natural resources, including water.

John Abatzoglou, a climate scientist at UC Merced, who collaborated on the Science paper, said people may yet innovate their way through the new conditions.

“Some of these changes could actually lead to beneficial climate adaptation strategies, being more efficient, like revolutionizing how we work with land, how we work with water,” Abatzoglou said. “So there may be some good things that come out of dealing with hardship, which is what we’re going through in California.”

An example would be custom micro-irrigation so that sensors determine exactly how much water a plant needs and only that amount is slowly dribbled out into the soil.

When will this megadrought end?

One challenge for residents, farmers and water managers alike is that neither weather nor climate forecasting is very good at giving us advanced drought warnings or predictions for when megadroughts will end.

Abatzoglou says he and others are refining their ability to forecast into a coming “water year” (October through September), which would help people make smarter decisions. He says those could range from farmers switching up what they plant in the fall to reduce water demand in the winter and reservoir operators being better informed as they balance how much water to reserve and how much to distribute.

Because drought is a creeping phenomena, he said. “Usually you don’t know you’re in a drought until it’s a little bit too late.”

Longer-term, climate scientists are developing tools to better forecast ocean temperatures in the tropical Pacific, Seager says, because they have a big influence on drought conditions on land. Right now, they can only see about a year into the future, but with a better system, scientists hope to predict when overall California drought conditions might change.

“There are big international research efforts to do that,” Seager said. “But in the Pacific Ocean, that research to date has not met with a tremendous amount of success. [It] has been rather disappointing in progress.”

Gov. Gavin Newsom has already said that another dry winter could lead him to impose mandatory water reductions, which will be a tricky thing to impose unilaterally as in the state’s driest regions (think Imperial Valley as one example), many people have already made changes to reduce their water use and they won’t take kindly to being asked to make more significant reductions when residents of wetter areas are just taking their first major steps toward water conservation.

This post is adapted from an earlier post published in 2020.