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How climate change impacts atmospheric rivers

SACHA PFEIFFER, HOST:

Heavy rain and strong winds are thrashing California. Hundreds of thousands of people are still without electricity in the northern and central parts of the state, and Los Angeles is dealing with flooding and mudslides. Rebecca Hersher from NPR's climate desk is here with details. Hi, Rebecca.

REBECCA HERSHER, BYLINE: Hi.

PFEIFFER: What is making this particular storm so damaging?

HERSHER: So the storm is very large. It's what's called an atmospheric river. So it's basically a river of moisture in the air flowing from the area around Hawaii over to California. This one is lingering over southern California, especially Los Angeles, dumping half a foot of rain or more. And that's why there's so much flooding in that part of the state. The other thing about this storm is that it got really powerful really quickly right before it hit land around the San Francisco Bay area yesterday. That meant really damaging wind. There were gusts of upwards of a hundred miles an hour. Those are hurricane-force winds. So that's why you saw so many power outages. You know, a lot of trees and power poles came down.

PFEIFFER: We know that a lot of extreme weather events are being affected by climate change. What about this one?

HERSHER: So it's an active area of research, actually, for atmospheric rivers. You know, scientists expect that atmospheric rivers, which are basically big rainstorms - right? - will get more intense as the planet warms. That's what climate models suggest. That said, atmospheric rivers are very complicated, and scientists can't say for sure that they're already getting more intense. What we do know for sure is that climate change makes heavy rain more likely around the world. And that's because as humans burn oil and gas and coal, it obviously emits carbon dioxide and other greenhouse gases that trap heat, and that heat heats up the air. Warmer air can hold more moisture, which then falls as heavier rain.

PFEIFFER: It's obviously alarming to see these images of flooded roads across LA, this major metropolitan area. How ready is California for this type of rain?

HERSHER: Well, yeah, a lot of rain in a short period of time - it does put strain on infrastructure. Things like roads and dams, storm drains, retaining walls - you know, these are things that offer important protection for people from flooding. And they're built to withstand a certain amount of water. When you get more water than that, that's when you have problems. Climate researcher Daniel Swain says the current infrastructure is not necessarily built for the current or future climate.

DANIEL SWAIN: We built our infrastructure to particular thresholds. We don't have storm drains that magically get 5%, 10% bigger per degree of warming. We built them decades ago, and they're there.

HERSHER: So right now, for example, storm drains in some parts of LA can't keep up with all the water, which leads to those flooded roads and neighborhoods.

PFEIFFER: Rebecca, you recall that last winter California also got hit with atmospheric rivers. Does that indicate that we should be prepared for this to happen every year?

HERSHER: No, not necessarily. Remember; rainstorms are a part of California's normal weather, especially this time of year. Some years forecasters expect there to be less intense rain, some years more intense. But with climate change, it is true that heavy rain is getting more likely. So really intense events like what happened last winter with the flooding in many parts of California or what LA is experiencing locally right now - those are getting more common. One way to think about it is that in the past, maybe you could expect to experience extreme rain and the flooding that goes with it a few times over the course of your life. Now it's more like every few years, so that's a big change. And it's good to remember, you know, that's true in California, but it's also true in much of the rest of the U.S., where climate change is also causing heavier rain and more flooding.

PFEIFFER: Rebecca Hersher from NPR's climate desk. Thank you.

HERSHER: Thanks. Transcript provided by NPR, Copyright NPR.

NPR transcripts are created on a rush deadline by an NPR contractor. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.

Rebecca Hersher (she/her) is a reporter on NPR's Science Desk, where she reports on outbreaks, natural disasters, and environmental and health research. Since coming to NPR in 2011, she has covered the Ebola outbreak in West Africa, embedded with the Afghan army after the American combat mission ended, and reported on floods and hurricanes in the U.S. She's also reported on research about puppies. Before her work on the Science Desk, she was a producer for NPR's Weekend All Things Considered in Los Angeles.