What are atmospheric rivers?


Dr Karl Kruszelnicki


Dr Karl Kruszelnicki

Dr Karl is a prolific broadcaster, author and Julius Sumner Miller fellow in the School of Physics at the University of Sydney. His latest book, Vital Science is published by Pan MacMillan. Follow him on Twitter at @DoctorKarl
By Dr Karl Kruszelnicki 10 October 2022
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Although they’ve been around for millennia, atmospheric rivers were only discovered by humans during the past 25 years.

In February 2022, one dumped cubic kilometres of water onto the city of Brisbane. An atmospheric river is a narrow, fast-flowing stream of moist air. It can be many thousands of kilometres long, and a few hundred wide. It’s a giant and invisible conveyor belt of water in the sky, moving above and across the planet.

At any given moment, there are about a dozen of these atmospheric phenomena across the globe – most of them over water. But, unlike a land-based river, they are not fixed in location. Instead, they continually form, fade, reform and evolve. So they come and go.

Atmospheric rivers are essential to the water cycle. They shift 90 per cent of the air’s water vapour, but cover less than 10 per cent of the planet. A big atmospheric river can move a quarter of a million tonnes of water each second past a given point. If one gets really big, it can be disastrous. 

In 1862 an atmospheric river turned Central California into a temporary inland sea, 500km long and 30km wide. Not only did thousands of people die, so did one-quarter of the 800,000 head of cattle in California at the time.

Sacramento, the state capital, was flooded with more than three metres of muddy water and took six months to completely dry out. By then, California was bankrupt. And just for a little extra ecological impact, the water in San Francisco Bay turned from salt water to fresh water.

The atmospheric river that hit Australia’s east coast in March 2021 caused several fatalities, forced the
evacuation of more than 24,000 people, and cost the Australian economy about $652 million.

Greenhouse gases generated by human activity are now capturing an extra 600,000 Hiroshima atom bombs’ worth of heat each day. This means the amount of both heat and moisture carried by atmospheric rivers is much higher than in the recent past.

A quarter of a century ago we didn’t know atmospheric rivers existed, much less that climate change would make them worse. Who knows what other surprises climate change has for us?