- Can we really blame climate change for making the world wetter and drier?

Seems confusing at best, but there has been a lot of flooding and drought these days, so what's up?

Well, since 1900, average annual global precipitation has increased by about a half an inch, and this number is much higher in the United States at about 2.4 inches.

But at the same time, Vegas is flooding, the Colorado basin where Vegas is located is in its worst drought in 1,200 years.

To make sense of it all, I had to totally change the way I think about big precipitation shifts in the coming future.

Maybe the best way to understand the current flooding risk is through the most severe flood in US history.

It happened in a dry place during a dry period before human-caused climate change had truly taken effect.

We're learning that this mega flood will happen again, but now there are far more people, infrastructure, and farmland in its path, and climate change is likely to supercharge it.

- That's a trillion dollars of damage estimated from a big winter on par with 1861, 62 if it were to occur again today.

Bottom line is that it will come.

It's just a matter of when and how much water, and what we're willing to do in the interim to prepare for.

(suspenseful music) - Before we get into the great flood of 1862, let's talk more generally about how climate change affects precipitation and how global patterns make some places wet and some places dry.

- In a warming climate, the overall amount of precipitation on planet Earth does increase by around 2 or 3% per degree centigrade of warming.

The atmosphere is essentially acting like an increasingly large sponge that has both the capacity to soak up more water off of your kitchen counter, but also if you ring it out, you're going to release more water from that ever larger kitchen sponge.

You can think about that as increasing precipitation extremes.

So when it rains, it truly pours.

But on the other end of the spectrum, that larger sponge also has increased capacity to extract more water from the landscape.

And so what this means is that this exponential water vapor holding capacity of the atmosphere in a warming climate, it kind of explains both why in some places we see more severe droughts in a warming climate, but in others, we see more intense extreme precipitation.

- In addition to that global trend, a warmer world is shifting the Hadley cell.

It's a large circulation that makes tropical latitudes around the equator wet like the Amazon rainforest and subtropical latitudes dry like the US Southwest and the Sahara Desert.

Here's how it works.

- [Daniel] Air that comes from the tropics that gets lifted up high into the atmosphere and descends in the subtropics, which is one of the reasons why many of the world's major deserts exist in the subtropics.

And these are the places that are indeed expected to get on average dryer in a warming climate.

- That's because when warm air rises from the tropics, it creates low pressure and sends moisture into the atmosphere resulting in rain and storms.

Then by the time this air reaches the subtropics, it's dropped most of its moisture and creates a zone of high pressure as it descends.

High pressure tends to suppress the creation of storms.

So as the overall temperature of the globe increases, the atmosphere is less likely to release moisture as rain in the subtropics.

And as the globe warms, the Hadley cells expand.

- That slightly pushes the storm tracks.

The jet streams in both hemisphere's a little bit more poleward so northward and the Northern Hemisphere and southward and the Southern Hemisphere.

And so what this means is that the regions that historically received significant precipitation from storms associated with the jet stream, they might shift poleward.

But the interesting thing is that's not happening uniformly everywhere.

But imagine that it's a wavy belt and certain waves in the belt are getting lifted northward faster than others.

This is interestingly one of the reasons why California is not necessarily expected to see much less precipitation on average.

Other places at a similar latitude or in a similar climates to California, like the Mediterranean region in Europe, for example, are expected to get significantly drier on average.

Why is that?

Well, the jet stream over the North Pacific isn't expected to shift northward as much as it is over the Mediterranean basin.

- We know California is in a drought, a long, severe drought, and this is where we have to shift our understanding away from the idea of certain places being dry or wet.

- In the past decade or so, there's actually been a bit of a revision of that sort of framing.

That refinement now suggests that it's more true that the wet times get wetter and the dry times get drier, but it's in time rather than space.

What this means then is that even dry places might get wetter some of the time and wet places might get drier some of the time, but the question is what is that some of the time?

Which times get wetter and which times get drier and which places?

All around the world, the most intense precipitation events are becoming more intense and more frequent.

So even if you're in a desert, if you're in a tropical rainforest, or if you're in the mid latitudes, or if you're in the Arctic, if you're getting heavy precipitation, it's probably heavier than it used to be.

So that seems to be almost a universal truth with very few exceptions globally.

- And of course the timing and intensity can be a problem.

Kentucky experienced rapid flooding that killed nearly 40 people in the summer of 2022.

Just three months later, most of the state is experiencing abnormal dry to extreme drought conditions.

Flash flooding around the same time killed two people in Las Vegas, while the greater Colorado basins melting snow pack couldn't recharge crucial reservoirs for farming.

- So in California right now, we're focused on water scarcity and drought, but I think something that's been a little bit underestimated that's sort of flown in under our radar is the fact climate change also appears to be increasing the risk of a mega flood.

- Daniel calls this phenomenon hydroclimate whiplash.

To be clear, it's not caused by climate change.

It's intensified by climate change.

And that brings us to the historic West Coast mega flood starting in Oregon and Washington, then slamming California.

- Late in 1861, you get the kind of customary seasonal rain season in Pacific Northwest.

But as the winter wore on and as that landscape continued to absorb more and more of this rain, the rains just didn't stop.

These storms just kept kind of crumbling in this kind of conveyor belt, this series of atmospheric river storms as well as some other tropical cyclones.

And you start getting floods.

Pretty much every river system on the inland side of the Pacific Northwest overflows.

And as the winter wears on into December, those storms continue to move south.

And this is when we start getting the big events, the Great Floods of Sacramento.

- Okay, first, let's talk about how this weather happened.

Atmospheric rivers are narrow plumes of moisture suspended in the atmosphere that can hold more water than the entire Amazon River.

They can dump lots of rain when they hit land.

Tropical cyclones don't hit the West Coast very often because the ocean water is too cold to sustain them.

But if they make it far enough north before disintegrating, prevailing west winds can push them on shore as a massive rainstorm.

Back to back, these storms dump more water than most West Coast residents can imagine, especially in Parch, California.

- [William] These are the famous floods where you have a contiguous lake system from Redding all the way to Bakersfield, which is a 4 to 500 mile central valley.

And of course, from the Sierra Nevada to the coast range is just a gigantic body of water.

So around Christmas actually is when the rains really start hitting Southern California, San Diego, San Bernardino to Los Angeles, all three of those towns that today are major cities were all underwater.

- And the consequences were staggering.

- [William] Everybody who lived along a waterway on the West Coast was displaced.

Disease becomes a major issue.

There's a smallpox epidemic.

There's cholera.

The domesticated animals, they're just basically washed off the landscape.

To the moment of the big flood is incredibly disastrous, but it's the kind of subsequent weeks and months and even years, you could argue, that that kind of long disaster continues to affect people's lives.

- Scientists are now finding that these whiplash events have always been a part of California's climate.

- The paleo climate record going back hundreds or even thousands of years suggests that California is a region that's actually quite susceptible to very large flood events that occur rarely on human time scales, but alt with great regularity on geologic and Earth history timescale.

So these are the kinds of events that happen every hundred or 200 years.

- The next mega flood now has a name, Arkstorm.

Because of this strong paleo climate record, the idea is starting to make its way into policy and infrastructure conversations.

- [William] Yes, it will happen again.

And yes, it could be bigger than what we know about 1861, 62.

- [Maiya] Bigger now, of course, because the warmer atmosphere holds more moisture and we're seeing similar events already.

- Just last autumn, we came pretty close to a mega flood event in British Columbia, and the city of Vancouver was cut off from the rest of Canada for weeks.

The most dramatic and probably the deadliest is unfolding still in Pakistan where a significant portion of the country remains underwater weeks to months after some of the extreme precipitation.

How do we adapt to this new climate?

Well, it's quite different if we just have a gradual trend towards dry conditions versus if we have these increasingly wide swings.

- One thing Daniel and many other experts suggest is a move away from the way we've changed rivers: channelizing flows and blocking water to learning natural systems.

- So rather than building more big dams, perhaps this is a question of expanding our capacity for aquifer recharge, for expanding flood plains rather than building ever higher flood walls 'cause we're never gonna be able to contain all of the water in a mega flood, but we can have some control over the places that we really don't want to flood in these events.

- We'd love to hear from you.

What types of flood mitigation projects are present in your area?

Do they mimic natural systems?

Check out our previous episode on floods where we talk more in detail about this kind of solution.

And as always, thanks for watching.

(pensive music)