What Is An Atmospheric River – And Why Are They Causing Changes In The Weather?

What’s the biggest river in the world? No, it’s not the Amazon, nor the Nile – it’s an atmospheric river, flowing a few kilometers above your head.

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That’s not a joke. Atmospheric rivers are typically thousands of kilometers long and hundreds wide, and they contain, on average, twice the amount of water as the Amazon. The only difference is that it’s water vapor, rather than liquid – plus the little fact that they move through the sky, not the land.

They’re a vital part of the climate cycle – and, for a long time now, they’ve also been moving away from the places that used to benefit from them, shifting noticeably polewards instead.

So uh… should we be worried? What’s going on?

Rivers in the sky

If you’ve ever looked at the water cycle – the evaporation of the oceans and subsequent precipitation on the mountains – and thought “okay, but how did it get there, those mountains are hundreds of miles away from the ocean”, then buckle up: the answer is, mostly, “atmospheric rivers”. 

“Atmospheric rivers are like highways of moisture in the sky,” explained Zhe Li, then a researcher in climate and atmospheric dynamics at the University of California, Santa Barbara, back in 2024. With their gigantic size, they’re the largest transport mechanisms of freshwater on Earth, taking it from the tropical oceans and releasing it as rain or snow in more polar regions. 

As the bringer of sometimes overwhelming amounts of water, these rivers are both a vital and a potentially catastrophic phenomenon. “We need [atmospheric rivers] – without them in the West we have droughts,” Anna Wilson, an atmospheric scientist and extreme weather expert from the Scripps Institution of Oceanography at the University of California, San Diego, told BBC Future in 2024.

Indeed, up to two-thirds of the droughts that do hit the West Coast are solved when an atmospheric river hits land – they’re known as “drought busters” for that reason – and up to half of California’s rain and snow would be absent if not for these skybound waterways. Evidently, they are useful meteorological events.

But when an especially large one makes land – or a confluence of many, such as those that regularly batter the Golden State – the results can be devastating. 

“For example, […] a category 3 or 4 event could produce losses between $10,000 to over $1 billion across the state,” explained Katy Serafin, an assistant professor in the University of Florida’s Department of Geography, in 2024. “A category 5 event could produce losses between about $5 million to over $1 billion.”

Tracing the course

Both the lifesaving and life-threatening results of atmospheric rivers mean the same thing: we need more information on them. We need to know the basic facts: “Where is it going to make landfall? How strong will it be? How long will it last?” explained Wilson. “And we continue to get better at [answering] that.”

The questions may be simple, but answering them sure isn’t. “You can't see [atmospheric rivers], actually, because it's water vapor,” Wilson pointed out – and “they're really close to the surface” too, hiding under cloud cover. “It’s really hard for the satellites to sort of see through that, to what's going on at the near-surface,” she said.

Their tropical origins add to the difficulty of tracking them. For obvious reasons, it’s harder to observe meteorological events in the middle of the ocean – and during the thousands of kilometers of travel to where we can more readily monitor them, they can go through any number of changes in speed, intensity, or interactions with other atmospheric rivers. Each of these will affect their impact, explained Qian Cao, a hydrologist at the Scripps Institution of Oceanography – but thanks to the stochastic, nonlinear nature of climate dynamics, exactly how they affect it is virtually impossible to answer in any timeframe past the most immediate.

“If we can forecast or predict these atmospheric rivers better, if we can predict them more accurately, with longer lead times, then we have more time to make operational decisions, for example, whether we want to release water or save water in the reservoirs,” Cao told the BBC. But right now, she added, “researchers are working very hard to improve forecasts beyond week two.”

A changing flow

With increasing awareness of the lifesaving – or destructive – potential of atmospheric rivers, it’s no wonder that researchers are turning up new results all the time. And, a few months ago, one paper revealed a trend that surprised even the scientists who found it: atmospheric rivers are on the move.

“Atmospheric rivers are shifting poleward in both hemispheres,” said Zhe Li, who in October last year coauthored a paper with his doctoral supervisor Qinghua Ding describing the discovery. It’s a change that will “[bring] heavy rain and storms to higher latitudes,” he explained, “which could reshape precipitation patterns globally.”

It is, overall, only a six-to-10 degree shift so far – but on a body as large as the Earth, that corresponds to hundreds or even thousands of kilometers. Subtropical areas have already seen the effects of this shift, with a lack of expected atmospheric rivers increasing the chance of a drought by up to 90 percent in vulnerable regions – and in places that aren’t expecting to host the rivers’ output, it has a similarly high impact on the risk of floods.

No surprise, then, that meteorologists are taking note. “Understanding these changes will help us make better predictions about future rainfall patterns and water availability,” said Li.

But here’s the weird part: it doesn’t seem to be our fault. At least, not entirely.

A natural cycle?

Facts are facts, no matter how angry our comment section gets: climate change is real, it’s devastating, and it’s our fault. But, in this one particular part of this one particular topic, our hands are clean.

That’s what Li and Ding’s work seems to suggest, at any rate. Once the pair noticed this poleward shift, they ran climate models to suss out what might be causing it – and they were surprised to find that anthropogenic climate change had a much smaller impact than they expected.

“Based on that, we hypothesize that this observed shift is not totally due to anthropogenic forcing,” Ding said.

“We think it’s part of a natural cycle, rather than an anthropogenically driven change.”

Still, the picture more generally is one in which we hold most of the blame. The movement away from the equator isn’t the only thing affecting how impactful atmospheric rivers will be – and human-driven climate change is already bringing a lot of trouble to those in the rivers’ paths.

“Warmer air can hold more moisture,” explained Cao in a 2024 article for The Conversation. “As global temperatures rise in the future, we can expect more intense atmospheric rivers, leading to an increase in heavy and extreme precipitation events.”

“My research also shows that more atmospheric rivers are likely to occur concurrently during already wet conditions,” she added. “So, the chance of extreme flooding also increases.” 

Add to that increasing unpredictability in timing and intensity, and it’s clear that research into atmospheric rivers is going to become extremely important, extremely quickly.

Lucky for us, the people doing it seem to enjoy their work.

“It's a really awesome feeling as a scientist to work on something that is so immediately applicable,” Wilson said. “This is making an impact right now for people on the ground.”