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Atmospheric rivers

16 August 2022
They’ve been around for millennia, but we’ve only recently discovered them.
An “atmospheric river” is a narrow, fast-flowing stream of moist air - many thousands of kilometres long, a few hundred kilometres wide, and usually traveling faster than 45 kph.

It’s a giant, yet invisible, conveyor belt of water in the sky, moving above and across the planet. A big atmospheric river can move a quarter of a million tonnes of water each second past a given point.

At any time, there are about a dozen of these atmospheric rivers on the globe – mostly over water. They are not fixed in location (like a river), but instead, continually form, fade, reform and evolve.

Beginning Christmas Eve in 1861, an atmospheric river hit Central California, after travelling from the other side of the Pacific Ocean. It dumped huge quantities of rain for the next 43 days, virtually unabated. The result was catastrophic.

California’s Central Valley turned into a temporary inland sea - 500 kilometres long and 30 kilometres wide. Not only did thousands of people die, so did one quarter of all the cattle in California – about 800,000 of them.

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

In early 2017, “some parts of California received nearly twice as much rain in a single deluge as normally fell in the preceding five months.”

Atmospheric river

Water vapor imagery of the eastern Pacific Ocean from the GOES 11 satellite, showing a large atmospheric river aimed across California. Image from United States Naval Research Laboratory, Monterey.

A few decades ago, atmospheric rivers hit West Antarctica, and collapsed two massive ice shelves.

In late March 2021, an atmospheric river lasted for 10 days while hitting the east coast of Australia. This created very unusual rainfall that was both prolonged and very heavy, which led to extensive flooding. The entire coastline of New South Wales suffered under more than 200 mm of rain – but some locations recorded more than 400 mm of rain.

This tremendous deluge killed two people, forced the evacuation of another 24,000, and cost the Australian economy about A$652 million.

Almost a year later, in February 2022, another huge atmospheric river dumped a year’s worth of rain in just a few days, onto south-east Queensland and Northern New South Wales. It caused widespread flooding from the Sunshine Coast to Sydney.

Greater Brisbane got about 80% of its annual rainfall in just three days – 8 cubic kilometres of water. Before this event, Brisbane had just eight days in their entire records of rainfall above 200 millimetres. But in February 2022, there were three such days in a row. Thousands of people had to evacuate their homes. More than 22 people died, and over 20,000 homes and businesses were flooded in Queensland.

Positive feedback loops related to Climate Change make atmospheric rivers so much worse – thanks to positive feedback loops. A 1Co rise in air temperature leads to a 7% increase in water carrying capacity of the air, which leads to a 100% increase in rainfall.

A while ago we didn’t even know that atmospheric rivers existed, much less that Climate Change would make them worse. Who knows what other surprises Climate Change has in store for us?

Julius Sumner Miller Fellow

Dr Karl Kruszelnicki
Dr Karl Kruszelnicki
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