Atmospheric Rivers Threaten Once-in-a-Millennium Flooding

By Amy Lupica, ODP Staff Writer

In 2020, many Californians prayed for rain as wildfires in the region destroyed millions of acres of land. But they got more than they bargained for when what weather forecasters described as an “atmospheric river” set a course for the state, causing storms that triggered catastrophic mudslides and washed out a section of Highway 1. Experts say as climate change worsens, the Western United States will continue to see fluctuation between these extremes: devastating drought, and deadly floodwaters.

Why This Matters: Atmospheric rivers can carry as much water through the atmosphere as land rivers, and at similar speeds. Between 1978 and 2017, just 10 atmospheric rivers caused over half of all flood damage in the Western U.S.   Daniel Swain, a climate scientist with the National Center for Atmospheric Research (NCAR) explains that, like hurricanes, these rivers will only get worse as temperatures rise, “one of the things that’s really clear, the intense ones are going to become much more intense.” Experts expect these stronger systems to damage infrastructure, impact food supply chains, and affect water supplies for agriculture and communities.  We need to better prepare for more events like the one last week that destroyed part of a major highway.

What is an Atmospheric River?

Atmospheric rivers are exactly what they sound like — twisting trails of water in the atmosphere that span miles. Atmospheric rivers can be 1,200 miles long, 620 miles wide, and 1.8 miles deep, and carry tropical moisture north and particularly strong rivers can carry more than twice the water volume of the Amazon river. Atmospheric rivers aren’t always bad. They are the largest transporters of freshwater, refreshing reservoirs and bringing good rains. They also help redistribute heat from equatorial regions to the poles.

The issue arises as temperatures do; for every increase of one degree Celsius, the atmosphere can hold 7% more water, which means these rivers are more likely to form and more likely to grow in intensity. Rains that once nourished the land will cause massive flooding, damaging infrastructure and crops. “What we are seeing is, the rainy seasons are getting shorter, and the precipitation will become more extreme, and the extremes will become more frequent,” said University of Lisbon climate scientist Alexandre Ramos.

Washed Away

Much like the Highway 1 incident, which resulted in detours nearly 200 miles longer than the original route, Swain says our infrastructure is incredibly vulnerable to these rivers. “This is a problem in a society where we build our infrastructure to last many decades, on the assumption that our climate does not change over 50 years,” he said. To prevent this, existing infrastructure must be strengthened and new anti-flooding infrastructure must be built.

That’s easier said than done. States like California have been hit incredibly hard with COVID-19 and devastating wildfires; like Southern states, which struggled to recover and rebuild between repeated Hurricanes, Western states will struggle to recover and prepare for the “feast or famine” cycle of drought and storm.

Experts say these trends will only continue, and that Western states need to prepare for mega-storms. “By 2060, there is a 50-50 chance of seeing a mega-storm on the scale that swamped the Central Valley [in 1862],” said Swain. “Given that would be a catastrophic flood event, I don’t think we’re fully prepared for that. For me, that almost means that, by late century, it’s almost an inevitability. That’s pretty amazing for an event that happened 5 times per millennium up to now.”

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