At least 17 people have been killed by extreme mudslides in California -- here's why the situation has gotten so deadly

Severe mudslides in Southern California have left 17 dead, and rescue crews still searching for at least 8 more people.

The mudslides came on the heels of one of the regions’ worst spat of wildfires in recorded history, and the two cataclysms, like many processes on our planet, are related. Because the fires charred the landscape around the Montecito area, the lack of vegetation on the hills made them unstable as they got pounded with torrential rain.

Santa Barbara County Sheriff Bill Brown told reporters the scene looked like a “a World War One battlefield.”

Mudslides can come suddenly, with little to-no-warning, but the genesis of these slides began in early December, with the largest wildfire in California’s history.

Wildfires such the Thomas Fire, which burned hundreds of thousands of acres in Santa Barbara, Los Angeles, and Ventura Counties in December, don’t just burn structures. They burn vegetation and the ground itself.

In the hilly regions around Santa Barbara, vegetation, including scrub, grass, and trees, play a crucial role in keeping the steep slopes stable. But when the Thomas Fire burned more than 280,000 acres in the region in December, biological, carbon-rich material in the topsoil also burned.

When this topsoil – which is highly porous and contains life-giving things like bugs, plant roots, and decomposing vegetation – burns away, what’s left are nonporous layers of clay and rock. If there are no bugs left to aerate the fire-hardened soil, the ground loses its ability to absorb rainwater.

This is what the landscape looked like after being burnt by the fires. The vegetation has not had time to reinstate itself into the soil since the December blazes.

Some research has shown that when water hits charred soil, it can make the ground more dense and water repellent, according to The Washington Post.

Because of the steep topography and fire-hardened soil of the hillsides, the whole region was susceptible to flooding and mudslides at the first sign of rain. The ground became impenetrable and almost unable to absorb the water – like a clay pot dried in a kiln.

On Monday night and Tuesday morning, Santa Barbara experienced a torrential downpour, its first significant rainfall in weeks.

This wasn’t an average rain shower. According to the National Weather Service, almost one inch of precipitation fell every 15 minutes.

With little vegetation to stop the hills from collapsing, roughly 100 single-family homes were destroyed and 300 damaged.

Santa Barbara issued a mandatory evacuation order for 7,000 people in neighbourhoods below the fire-scorched hills on Monday night, but many residents chose to stay in place. Although authorities can anticipate which regions are mudslide-prone during a storm, those predictions aren’t perfect and there’s little warning as to when the land will actually give out.

Santa Barbara County will be susceptible to mudslides for years to come, since it can take decades for a region to recover from the effects of a disastrous fire.

What’s more, wildfires like the ones that raged through Southern California in December are expected to get more frequent and severe as the world gets hotter.

Already, 15 of the 20 largest fires in California’s history have occurred since the year 2000.

More devastating wildfires in the future will likely mean more big mudslides.

Because of climate change, the average wildfire season lasts at least 2 1/2 months longer than it did in the early 1970s. And the amount of land burned in the US since 1984 is double what would have been expected without the effects of climate change.

Weather-related natural disasters cost the US a total of $US306 billion in 2017, the most expensive year on record.

And the planet is only getting hotter: According to a recent study in Nature, the planet may get 15% hotter by the end of the century than scientists’ highest projections,which means extreme weather and costly disasters are likely to get much worse.

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