Mudslides, Wildfires, and Drought—California's Deadly Weather Explained

First came the drought. Then came the fire, and then the flood.

Mudslides plowed through southern California in early January, killing more than a dozen people as coursing rivers of mud drowned the landscape. The fatalities could increase as hundreds more are still in danger and awaiting rescue.

But what triggered these devastating landslides? And while extreme cold has gripped the East Coast, what's happening in California this winter?

Below, find those and other questions about these unusual events explained:

How Did the Mudslides Start?

Heavy rains caused tons of mud and debris to cascade down hillsides and through homes in southern California, and misplaced detritus has cut people off from escaping the area. Accumulating more than five-and-a-half inches, this was the first major winter storm the region had seen this season. (Read: "Tsunamis More Likely to Hit California Than Thought?")

Wildfires that raged through the area late last year have possibly made the mudslides more devastating. Vegetation that could have held soil in place was burned away by the rampant flames, rendering the area barren and more flood-prone. Without hardy roots to keep dirt in place, nothing stopped torrential rain from sweeping away stretches of the landscape. (See: "Landslides in the United States Since 2007")

In 2014, a deadly landslide killed 44 people in Washington state and obliterated a riverside neighborhood. Years of chopping down trees had stripped the area of strong roots, rendering it barren and vulnerable to heavy rains. Researchers said the excessive logging made the landscape more prone to floods, and if trees had been in place, maybe the mudslide wouldn't have been as disastrous. (Read: "Mudslides Explained: Behind the Washington State Disaster")

Extensive logging hasn't taken root in southern California recently, but widespread wildfires did leave part of the land treeless.

How Did the Wildfires Start?

Wildfires need three ingredients: oxygen, fuel, and a heat source. Before wildfires blazed through the state in November, California was a natural disaster waiting to happen.

Along with greenhouse gases like carbon dioxide and methane, there's oxygen in our atmosphere. California's ongoing drought parched its landscape, making the state tinder for potential fires. This fall, southern California boiled in extremely hot temperatures. All this was enough to satisfy the dangerous trifecta. (Watch: "California Farmers Without Water")

So wildfires sparked up across the state, fanned by strong Santa Ana winds that made the blazes difficult to extinguish. The devastating Thomas Fire ripped through Ventura County and burned through more than 280,000 acres of land. In retrospect, 2017 saw the worst wildfires in California's history, and the Thomas Fire was the state's largest blaze yet. (Read: "Devastating Wildfire Can Be Seen From Space")

Although determining the exact cause of a fire can take months, four out of five wildfires can be traced back to people. Extreme heat this season was a spark for the flames, but California has been parched for months.

How Did the Drought Start?

Along with much of the semiarid western United States, California relies on snowpack for its water supplies. Each spring, meltwater has historically flowed from the region's mountains to refill streams and reservoirs. These flows have also helped to replenish California's water reserves, one-third of which have been supplied by snowpack.

But with climate change heating the planet, the Cascades and Sierra Nevada ranges are becoming too warm for snow. Flurries aren't sticking, so there's no meltwater left to trickle down and resupply the region's waterways. California has turned to pumping groundwater to make up the difference, but aquifers are depleting too quickly to replenish themselves. (Read: "Could California's Drought Last 200 Years?")

The 2011 drought that plagued the state is officially over, but Santa Barbara and Ventura counties—two regions hit hard by extreme weather this season—took the longest time to recover from drought conditions.

Researchers have reported that low precipitation started the state's drought, but unyielding heat has perpetuated it. Human-induced greenhouse gas emissions continue to rise, which make for warmer, drier conditions across the region. And these warming temperatures are melting Arctic ice, contributing to unusual weather patterns on a global scale.

It doesn't seem like California will quench its thirst anytime soon. Researchers say a "megadrought" could sweep across the American West by the end of the century if greenhouse gas emissions increase at their current rate. (Read about California's dry environment in National Geographic magazine.)

A previous version of this story misstated facts on the Thomas Fire. The story has been updated.