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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 megaflood 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 it.
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- 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 wring it out,
you're gonna 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
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, drier
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 hemispheres
a little bit more poleward,
so northward in the northern hemisphere,
and southward in 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
and with 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.
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Okay, let's get back to the idea
of places getting wetter and drier.
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,
in 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 Basin's melting snow pack
couldn't recharge crucial reservoirs for farming.
- So in California, right now, you know,
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 that climate change
also appears to be increasing the risk of a megaflood.
- 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 megafloods
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 tumbling 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 onshore
as a massive rainstorm.
Back to back, these storms dump more water
than most west coast residents can imagine,
especially in parched California.
- [William] These are the famous floods,
where you have, like, 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,
it's 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,
'cause 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 paleoclimate 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 timescales,
but with great regularity
on geologic and earth history timescales.
So these are the kinds of events
that happen every 100 or 200 years.
- The next megaflood now has a name, ARkStorm.
Because of this strong paleoclimate 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.
- 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 megaflood 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 drier 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 floodplains,
rather than building ever-higher flood walls,
'cause we're never gonna be able
to contain all of the water in a megaflood,
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.
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