- The most important rainstorms for the west coast of the Americas are changing in a big way.

They're moving towards the poles and not just by a little bit, they've already moved 400 to 700 miles.

These are atmospheric rivers and they provide the US West coast with up to half of its water.

If this shift continues, it could be a tipping point for areas already experiencing extreme drought.

And the key to understanding why this is happening and if it will continue, has been called the most important unanswered question in climate science.

It starts with a cold spot in the Pacific known as the cold tongue, and it's inexplicably getting colder as the rest of the ocean warms.

- Over the last four years, the western side of the Pacific, the warm part has warm alongside the planet one degree, and the eastern side has cooled.

So the cold tongue has cooled half of a degree Celsius.

- That's a 1.5 degree difference, which is enormous.

And so are the implications for our planet's weather and climate.

And most mysterious of all, it directly contradicts our climate models, which means we can't really understand the full scope of its impacts.

So what's going on here?

Is it a random fluke or part of some larger still undiscovered natural cycle, or is it something we are doing?

First, let's start with what we do know.

Atmospheric rivers have been in the news a lot recently, but they're not new.

Remember, they bring the west coast of the US up to 50% of its annual rainfall, which means that any changes would have significant impacts.

So when the study came out showing that they've moved away from the equator and towards the polls by about six to 10 degrees of latitude over the last 40 years, it turned some heads.

So what's behind this enormous shift?

These atmospheric rivers are affected by something called the walker circulation, and this is an enormous conveyor belt of wind that spans the entire tropical Pacific.

It's powered by the temperature difference between the Eastern and western Pacific.

The Eastern Pacific is naturally cooler than the western side due to the upwelling of cold deep ocean water.

These colder temperatures in the east lead to higher air pressure because cold air is denser and heavier.

Meanwhile, on the western side of the Pacific, known as the warm pool, the pressure is lower.

This pressure difference drives air to flow from east to west, - And those are the trade winds and the blow along the surface.

When they get over Indonesia, the Philippines, that air rises and it's humid, produces rainfall, and then that air has to come back - After reaching the top of the troposphere, this rising air flows eastward at high altitudes before sinking back down over the cooler Eastern Pacific, completing the loop of the walker circulation.

And it was this circulation that was our first clue that something weird was going on in the Pacific Ocean.

- And actually this is how we detected these changes, because before we thought that the walker circulation should weaken as the planet warms up.

- But what they saw was the opposite.

The walker circulation was getting stronger.

And this unexpected shift acts like the first domino in a cascade of global effects on weather.

Remember, the trade winds are the surface part of the walker circulation, and as they strengthen, they push more warm surface water towards the Western Pacific supercharging the system.

- So once that outside of the Pacific becomes Rainier, the western side, you have air ascending up and that air you can imagine as as a hammer on the atmosphere.

And that creates these teleconnections, which are stationary waves in the atmosphere that propagate away from the equator.

And that pattern produces a shift in the jet stream to the north.

And that pushes winter storms that bring rainfall to the US west coast.

- And these winter storms are the atmospheric rivers that Julie studies, - It's like the river, but up in the air stretching like S of kilometers.

These atmospheric rivers can move moisture from the tropics region to some higher latitude regions.

- So what happens when you move all of that moisture away from the tropics and towards the poles?

First, some of these atmospheric rivers are reaching all the way up into the arctic, dumping warm rain on snow and ice causing more melting.

And the extra moisture that atmospheric rivers carry also directly warms the atmosphere itself.

- The is the water vapor, right?

It's a very powerful greenhouse gas.

So that means the the, the air become more humid in Arctic, right?

And that means the greenhouse effect becomes stronger.

There can further the warming process there.

- The Arctic is already warming faster than the rest of the planet because sea ice is melting and more warm rain and less sea ice means more warm sun hitting dark ocean water, which absorbs more heat than the white reflective ice.

If you've been watching this channel for a while, that's not a new concept, but it is a big deal because even warmer arctic temperatures thaw permafrost, which releases the greenhouse gas methane increasing global warming, A warmer arctic also speeds up the loss of Greenland's ice sheet, which raises sea levels.

And finally, it makes our planet's largest heat transfer.

The amoc more likely to slow or collapse closer to the equator.

The impacts are more immediate.

California is home to 39 million people and it's the nation's largest agricultural generating 11% of all US farm cells in 2022.

The state relies on atmospheric rivers for around half of its water.

As we've covered in previous episodes.

The American southwest is in the midst of a mega drought, the driest in 1200 years.

And there's no end in sight.

This doesn't mean atmospheric rivers have stopped hitting California and the US southwest.

In fact, a series of atmospheric rivers in 2022 and 2023 brought significant flooding while providing a much needed break from the drought.

However, these crucial weather systems are shifting northward and California is receiving fewer of them over time.

If this trend continues, it could drastically reduce the state's already fragile water supply.

The same is true in the Colorado Basin.

- If the jet stream moves a little bit to the north then, and you are a farmer in Northern California, this could have a big effect and it's probably more disruptive than warming itself.

- And we're seeing a mirror image of these impacts in the southern hemisphere as well.

Meaning the impacts of the walker circulation are truly global, which is why the question we posed at the beginning is so important.

Is this just a temporary blip in our climate system or is this a long-term shift?

And to answer it, scientists have to crack the mystery of the Pacific cold tongue.

That's because all of this, the strengthening of the trade winds supercharging, the walker circulation and atmospheric rivers moving north is all caused by the increasing difference in temperatures between the Eastern and Western Pacific Ocean.

But none of this is supposed to be happening.

Climate models predict that the cold tongue should actually be warming - Those models that we use to predict the future.

And they're so successful at predicting many features of the climate system that are changing.

They don't predict cooling in the tropical Pacific.

They actually predict that part of the world.

The golden should be warming faster than the rest of the world, not cooling.

- So when scientists first observed the cooling of the cold tongue, they were very confused.

- We thought it was just a fluke.

By luck, the Pacific can fluctuate a lot naturally.

We know this happens on year to year dime scales as what we call a linear emia.

- But instead of it being a yearly trend, this shift has occurred over 40 years.

- So are the models wrong?

Are models missing some, some physical process, or is this trend just a natural fluctuation and eventually it's going to go away?

And answering this question is, is very important because if the trend continues driven by human emissions and, and if it's a pattern associated with global warming, then we'll continue to drive these really disruptive effects.

And that's why I think this is one of the most important unanswered questions in in climate science.

- So what should we make of all this?

This mysterious patch of cold water in the Pacific defying both global warming and our best climate models and the resulting chain reaction reshaping weather patterns all over the globe?

Is it natural?

Is it anthropogenic?

This question seems like a microcosm of climate change in general as climate scientists continue to gain a better understanding of how our planet works and how our actions as humans affected - So many people around the world are trying to figure out what's missing, that they cannot produce this pattern.

But we're close to having a, a complete answer that points into a much bigger human contribution.

- They're not quite there yet, but Pedro says they might be able to crack the code in as little as a year if they're lucky.

So we'll be sure to update you if they do.