Why West Coast weather will be chaotic in the future, according to a climate scientist

When I moved to San Francisco in 2013, the state of California was in a drought. As a transplant from the Midwest, I discovered that this manifested itself often at restaurants. Accustomed to water being excessively offered at a restaurant table, I remember waiters telling me that, because of the drought, they were only serving water upon request and in very small quantities. At that moment, I began to understand why Californians bring their own water bottles everywhere.

This week, water is not hard to come by in California. In fact, it's overflowing in the streets around my house as I write this very sentence, flooding my neighbors' houses and businesses. Earlier this week, my power went out because of flooding around electrical equipment; this scenario might have seemed unthinkable a decade ago.

Indeed, California's series of "atmospheric river" storms have splashed across national headlines. From flooding, knocked out trees, power outages and closed highways, the series of storms has caused over $30 billion in damage, according to Bloomberg.

While atmospheric rivers are not a new weather phenomenon in California, the density of such storms this winter is certainly surprising. And given the ways in which climate change has upset normal weather patterns, an obvious question to ask is whether these unusually powerful and destructive west coast storms are connected to the continued emissions of greenhouse gases from human industrial civilization.

To better understand if this is the "new normal" in California— as in weeks of heavy rain that cause damage to much of the state's infrastructure — I interviewed Christine Shields, a climate scientist at The National Center for Atmospheric Research (NCAR). This interview has been condensed and edited for clarity.

Can you explain to people who aren't familiar with meteorology what an atmospheric river is?

Shields: Atmospheric rivers are these weather features that transport a lot of water in the atmosphere. So if you think of a river on land, you like to think of it like the Amazon River or the Mississippi River, there's a certain amount of water that goes through these rivers, right? So this is sort of similar except it's water vapor and it's in the sky. And they can hold just as much water as the Amazon or Mississippi rivers. In fact, a typical atmospheric river actually has as much [water] as twice the Amazon River.

So these are really big ways of moving water from lower latitudes to higher latitudes. And for the Western U.S. a very common type of atmospheric river is called a Pineapple Express. And this is called the Pineapple Express because it moves water from like the Hawaiian Island region, which is where you get the word pineapple from, and you move the water from the subtropical region where Hawaii sort of lives across the Pacific Ocean and north to the west coast of North America. And California (and Southern California in particular) get a lot of these Pineapple Express atmospheric rivers.

Fascinating.

Yeah and there are two ingredients to an atmospheric river: the water is one ingredient and the wind is another ingredient and the way we measure atmospheric rivers usually takes these two components and sort of condenses it into one metric. And we can quantify how intense these atmospheric rivers are by looking at this combination of wind and water and so it's just a retrofit actually. They're also long and narrow. When you look at it from a satellite picture, you can really pick it out because you can see the clouds associated with the atmospheric river, this narrow band of clouds that are thousands of miles long and hundreds of miles wide.

What is making this specific series of atmospheric rivers really newsworthy right now?

When you have one atmospheric river, it can hold a lot of water content. And even one atmospheric river can actually lift California out of a drought. But what's happening now is what we call families of atmospheric rivers, where it's one right after the other. And the overall weather pattern in the atmosphere is basically the jet stream is just bringing one storm after the other across the Pacific Ocean. We have this jet stream that's just barreling into California. This is something that happens, actually, pretty commonly, maybe not every year, but definitely, you know, there's definitely different instances of this. For example, the year that the Oroville dam collapsed in February of 2017. We're just seeing a really great example of this jet stream in the right position and these families of atmospheric rivers that are just coming one right after the other.

Why have we been hearing more about atmospheric rivers?

The term was actually coined just from an academic standpoint relatively recently, like in the 1990s. These things have always been around. But what we're calling them and how we understand them has changed.

Do you think this is the 'new normal' for California?

As I said, these things have happened in the past and we definitely expect them to happen in the future.

One of the things that I do in terms of climate change research is to try to understand what's going to happen to these types of things in the future. And so if we just separate this out into water and wind again, we know very clearly what's going to happen with atmospheric rivers in terms of the water content. As the global temperature increases, the amount of water that we can evaporate into the atmosphere will also increase. So just by the fact that we have warmer surface temperatures in the troposphere— which is the lower part of the atmosphere — is just guaranteeing that we'll have more water available to atmospheric rivers. So the atmospheric rivers will tend to be definitely wetter, with potentially more intensive rain periods.

But one of the things that is really ongoing research is whether or not the numbers of atmospheric rivers — if there will be more or if there will be less. We're seeing there's research out there, not mine, that suggests that you're going to have more of these, you're gonna have more drought and then more intense rain periods. And so we might be oscillating from more severe drought to super wet, super dry, super wet, super dry — these swings that can be potentially destructive.