Twice a week, the Heart of the City Farmers Market transforms San Francisco’s gritty United Nations Plaza with dozens of white canopies and truckloads of fresh produce. But on a recent sunny winter Wednesday, the abundance of sweet-smelling fruits and vegetables are contrasted by a gloomy point.

It didn’t rain once here last January. Not in this spot, nor in all of San Francisco.

In California, January historically gets more rain than any other month. But lack of precipitation and high temperatures have made the previous three years of drought the worst in 120 years on record.

Environmental scientists are racing to understand what’s causing this drought and whether global warming is playing a role. NASA, for example, recently warned that in the next century there could be water shortages that last for decades.

This unprecedented extreme weather also raises a question for businesses that rely on water. Specifically, what’s the game plan?

Research meets reality

Vang’s Farm is a regular fixture at the farmers market. Today, the family-owned operation is selling mustard greens, cauliflower, broccoli, and more. Mike Vang says about 13 varieties of tomatoes are on the way.

But two items are missing: winter greens and root vegetables. They need colder weather and lots of water and just wouldn’t grow this season. This is where research and reality meet.

A few stalls away, Far West Fungi sells an array of mushrooms. They have a mushroom expert on staff who is developing ways to grow the same amount of mushrooms with less water.

“Most of the mushrooms, like especially the white or the brown sort, are 60 percent water actually, so those ones need quite a bit,” says Rob Desanto.

Desanto says they grow up to 40 beds of typical white mushrooms at a time, so that variety alone needs about 220 gallons of water per day. That’s why the farm is adapting.

Far West Fungi is buying water from the Santa Cruz Water Department and conserving what’s in their private well for the time being. Otherwise, if the extreme drought continues, their well will run dry in a couple of years.

The climate change connection

The drought isn’t shriveling crops in the Bay Area yet. But unusual weather is impacting the farmers here. There are honey sellers from Napa County whose bees haven’t had enough flowers and citrus producers from the far East Bay whose oranges were bitten by severe frost. Environmental scientists are wondering whether all these problems are caused by the same thing.

“I often get asked, is the drought caused by global warming, or is a particular heat wave caused by global warming,” says Stanford climate scientist Noah Diffenbaugh.

When it comes to the drought, Diffenbaugh says there’s a high probability that it is. His research team believes a region of persistent high atmospheric pressure deflecting storms from California was caused by human emissions of greenhouse gases. In other words, global warming.

Diffenbaugh uses a metaphor that the storm track is a stream across the Pacific Ocean, and the atmospheric pressure is a giant boulder in the stream that’s preventing storms from getting to California.

The atmospheric boulder has a nickname – the “ridiculously resilient ridge” – and it’s been disrupting the storm track and causing drier, warmer weather in California. The perfect recipe for a severe water shortage.

That’s how scientists are putting a human fingerprint on the drought, or any extreme weather event. By proving that it was more likely to occur because of the climate conditions humans created -- mainly by burning fossil fuels.

Not only did the state have a truly dry January, California also just experienced the hottest February on record. That’s driving scientists to question what’s next.

“The reason that we're interested in understanding whether or not global warming is influencing individual extreme events is so that we can understand the risks of climate change,” Diffenbaugh says.

Managing water scarcity

Like how changing weather can really impact people. Take the drought, for example. That affects agriculture, which is our food. And of course drinking water.

Steve Ritchie, from the San Francisco Public Utilities Commission, manages the entire Hetch Hetchy Regional Water System. That means he’s responsible for the water of 2.6 million customers in the Bay Area.

Standing on a peaceful bank of Calaveras Reservoir, tucked into the Alameda Watershed lands east of Fremont, Ritchie talks about the point when someone in his position starts freaking out.

“I have to admit that last January, of 2014, when we weren’t getting any precipitation I was scared,” Ritchie says.

Around the Bay Area, some reservoirs aren’t where they should be this time of year, but in general the region is in better shape than drier parts of the state. At Calaveras, the water level is low, about 30 percent of capacity. There’s a deep bathtub ring up to around the vegetation line along the rim.

But in this case the low water is by design, while a larger dam is being built downstream that will be able to withstand strong earthquakes and expand to hold more water.

As it stands now, Ritchie says the customers relying on the Hetch Hetchy system have enough water to survive two to three more years if it never rains again.

Drought or no drought, much of California would be desert if not for human disturbance.

“It's a mediterranean climate, it's dry most of the year, one of my colleagues said one day, who else is in a business where you sell a product you have no idea how much is going to show up,” Ritchie says. “I said that’s an interesting way to look it.”

That’s what farmers and scientists and water agencies have in common. They’re trying to find certainty in an already uncertain area. As science races to gain perspective on global warming and extreme weather, industries have no choice but to adapt. Plant fewer crops, use less water, build a bigger dam and pray that it rains.