What are atmospheric rivers and why are they increasing?

A man jogs with a dog near the rain swollen Los Angeles River as a historic atmospheric river storm inundates Los Angeles, California, on February 6, 2024. A powerful storm lashing California has left at least three people dead and caused devastating mudslides and flooding, after dumping months' worth of rain in a single day. (Photo by DAVID MCNEW / AFP) (Photo by DAVID MCNEW/AFP via Getty Images)
A storm caused by an atmospheric river dumped months' worth of rain onto Los Angeles. (Getty Images)

Huge rivers of air that carry vast amounts of water through the atmosphere for thousands of miles shape our planet and are forecast to double this century.

When these 'atmospheric rivers' make landfall, they can dump huge amounts of rain and snow, and a new study has warned that they could increase thanks to climate change, possibly causing floods around the world.

The study, published in JGR Atmospheres, found that rising surface temperatures will continue to raise the moisture content of the air, leading to a rise in atmospheric rivers around the world. If the world continues to use fossil fuels heavily, by 2099 atmospheric rivers could rise by 113% in summers and 84% in winter. If the world cuts greenhouse gas emissions, atmospheric rivers will still increase 46% in summer and 34% in winter.

What are atmospheric rivers?

Atmospheric rivers are long, narrow bands of concentrated water vapour flowing through the atmosphere.

Hundreds of miles wide, they can be thousands of miles long and typically carry moisture from the equator towards the poles.

LOS ANGELES, CALIFORNIA - FEBRUARY 06: A view of vehicles were buried in mudslides at Beverly Crest neighborhood, as atmospheric river storms hit Los Angeles, California, United States on February 6, 2024. (Photo by Tayfun Coskun/Anadolu via Getty Images)
Vehicles are buried after atmospheric river storms hit Los Angeles on 6 February. (Getty Images)

Atmospheric rivers are behind 20% to 30% of the annual rain and snow in parts of Europe.

When a 'river in the sky' meets a barrier, such as a mountain range, it can produce extreme levels of rainfall or snowfall. Researchers have previously warned that they could become more common over Europe as the world warms.

The term was first coined by the American Meteorological Society in 2017, but the effects of atmospheric rivers have been visible for decades.

What would the changes mean?

Climate change is predicted to alter the timing and distribution of atmospheric rivers, potentially redistributing the global supply of water.

For regions unaccustomed to receiving heavy rain and snow, these shifts could be disruptive.

Sudden increases in precipitation can overwhelm infrastructure, leading to damaging flooding.

The potential impacts "should not be underestimated", the researchers wrote.

Where will be most affected?

The effects will be felt around the world, the researchers warn, with some areas worse affected.

The north Indian Ocean will see the most substantial increase, with atmospheric rivers doubling or perhaps even tripling in frequency.

Greenland will also see a pronounced rise, with the interval between atmospheric rivers shrinking from an average of 59 days to between 30 and 41 days, depending on how fossil fuel consumption progresses.

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