Climate change increasing chance of ‘mega storm’ in California, scientists say

California is no stranger to extreme weather. It typically comes in the form of severe drought and wildfires, but a new study suggests the Golden State should also be preparing for a mega storm it hasn't seen the likes of since 1862. UCLA climate scientist Daniel Swain joined Stephanie Sy to discuss how the potential storm could flood parts of the state with 30 days of continuous rain.

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  • Judy Woodruff:

    As we reported, California is still struggling to deal with record heat, and the state's electrical grid operator warned that rising demand could even lead to outages tonight.

    The heat is particularly challenging right now. But the drought and the high temperatures could be putting California at higher risk for flooding and other complications from superstorms down the line. A recent study suggests the Golden State must also prepare for a large flood event.

    For more on this, here's Stephanie Sy.

  • Stephanie Sy:

    Scientists call it a megaflood or megastorm, 30 days of continuous moderate to severe rain and snow covering wide swathes of land. They haven't had one of these in California since 1862. But a new study says climate change is increasing the likelihood of another one hitting the state.

    The study authors say Californians could expect a monthlong series of storms that could flood parts of the state with more than 100 inches of precipitation.

    Daniel Swain is a co-author of the study and a climate scientist at UCLA, and joins us now.

    Daniel Swain, thank you so much for joining the "NewsHour."

    Just paint a picture for us of what this would look like in different parts of California. If this kind of megastorm were to strike today, what kind of havoc are we talking about?

    Daniel Swain, Climate Scientist, University of California, Los Angeles: Well, thanks for having me.

    As you mentioned last time, California saw month — weeks-long storm sequence of this magnitude was back in the 1860s. So, back then, California was home to fewer than half-a-million people, as opposed to nearly 40 million people today. So it's a very different landscape in which this sort of event would be unfolding. There's a lot more people and infrastructure potentially in harm's way.

    And so this would likely be a very disruptive and destructive statewide event, affecting multiple major urban areas and most of California's major economic sector simultaneously.

  • Stephanie Sy:

    You know, Daniel, I grew up in California, and we have been preparing for the so-called Big One, referring to earthquakes, since I was a kid.

    Put into context how much Californians should worry about this and prepare for it.

  • Daniel Swain:

    Well, coincidentally enough, one of the terms for a modern megaflood in California is California's — quote — "Other Big One," literally the other kind of major disaster that would cause potentially immense destruction and harm for millions of Californians.

    And I think that a lot of folks in California are more focused today on drought and water scarcity and the things that stem from that, like wildfires, and for understandable reasons. We have seen a lot more of that than water overabundance and flooding in recent years.

    But the reality is, this is a part of the world that is still intrinsically susceptible to extremely large flood events periodically. And climate change is raising the odds. Even though they have been rising somewhat quietly and latently in the background, as we have been dealing with all this water scarcity, we would ignore increasing flood risk at our peril.

  • Stephanie Sy:

    And, according to your study, Daniel, climate change, by your estimates, increases the likelihood of a megaflood by two — by twice as high of a likelihood?

  • Daniel Swain:


    We did find that climate change has actually already increased the likelihood of a weeks-long extreme storm sequence capable of producing widespread severe flooding in California. So that's something that's essentially already happened with the warming that's already occurred.

    Further warming is going to increase that risk even more, potentially resulting in a tripling or even quadrupling of that risk relative to what it would have been about a century ago. So that's a pretty large increase in the odds. And it means that, over the next 30 or 40 years, an extreme storm sequence like the one in our study is more likely than not to occur in California.

    And so this is not something that is in the far tail of unlikely probability. This is something that is very plausible and very well could happen sooner, rather than later. So, we really do need to be thinking about this over the next few decades.

  • Stephanie Sy:

    And, beyond California, what should other Western states expect? Do they face a similar possibility?

  • Daniel Swain:

    Yes, I think it's quite likely that the risk of very large flood — extreme precipitation and flood events is increasing broadly, even in places that are currently experiencing a lot of drought and water scarcity.

    In fact, we have even in hints of this past summer in the Desert Southwest currently facing the multidecadal megadrought, as it's been termed, that have seen extreme flash flooding on a localized basis during the summer monsoon. So they're still in a very severe long-term drought.

    But, in the short term, some individual places have experienced very dangerous and destructive flash flooding right in the middle of it. So, of course, that's on a smaller spatial scale than the megaflood event that we're talking about in our own work.

    But it does indicate just how much this risk is potentially widespread across the Western U.S. So this is — in this study, we're focused on California, but the risks are very likely broadly spread across the West, not just along the Pacific Coast, but also in the interior, places that are typically thought of as being drier places.

  • Stephanie Sy:

    Is California and other Western states prepared for this?

  • Daniel Swain:

    I think that's the billion- and perhaps even trillion-dollar question, because the last time that there was an economic analysis done for a flood of this magnitude, it was estimated that it could approach a trillion-dollar disaster in California.

    So, this is a really important question, not just in the context of short-term disaster response and making sure fewer people are harmed by this event when it inevitably does occur, but also from an economic perspective. I mean, this would be a hugely disruptive event.

    And I think there is — this is something we're working to assess for the state of California and other regional and federal agencies where the weak points might be. That's one of the purposes of the ARkStorm 2.0 project, into which this research is just one component.

    What we really want to do is be able to figure out where those weak points are, and hopefully address as many of them as possible before this event actually occurs in the modern world and so, hopefully, California and other regions at similar risk can be more prepared, and not be caught by surprise when these sorts of extreme flood events do eventually arrive at our doorstep.

  • Stephanie Sy:

    Daniel Swain, a climate scientist at UCLA.

    Daniel, thank you so much for joining the "NewsHour."

  • Daniel Swain:

    Thanks again.

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