B.C. flood shows how climate change could fuel atmospheric river storms

Atmospheric rivers of the kind that flooded British Columbia and renched California in recent weeks will become larger -- and possibly more destructive -- because of climate change, scientists said.

Columns in the atmosphere hundreds of miles long carry water vapour over oceans from the tropics to more temperate regions in amounts more than double the flow of the Amazon River, according to the American Meteorological Society.

These "rivers in the sky" are relatively common, with about 11 present on Earth at any time, according to NASA.

But warming air and seas around the globe causes conditions that scientists said will make them hold more moisture, causing extreme precipitation when they make landfall, often on the west coasts of North America, South America and Western Europe.

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Because of climate change, atmospheric rivers are projected to become slightly less frequent, but more intense, according to a 2018 study led by researchers from NASA's Jet Propulsion Laboratory.

"There may be fewer, but they are going to be lasting longer, and more intense," Vicky Espinoza, an author of the NASA study who is now a graduate student at the University of California Merced, said.

Atmospheric rivers will become about 10% less frequent by the end of this century, but about 25% longer and wider, the study found. That will lead to nearly double the frequency of the most intense atmospheric river storms.

Weak atmospheric river systems provide rain and snow that are needed for freshwater supplies. Stronger storms, however, can lead to flooding and extreme winds that disrupt travel, cause mudslides and damage property.

One such event this month dumped a month's worth of rain in two days, prompting fatal floods and landslides in British Columbia, devastating communities and severing access to Canada's largest port.

They are not always catastrophic. One such storm last month in drought-stricken California triggered mudslides, toppled utility poles and blocked roadways, but also helped replenish depleted reservoirs and reduced the risk of wildfires by saturating the state's parched vegetation.

"Because everything was so dry in California, there were a lot of benefits to that storm," said Julie Kalansky, deputy director for operations at the Center for Western Weather and Water Extremes at the Scripps Institution of Oceanography.

The atmospheric river that hit California last month was accompanied by a so-called "bomb cyclone," a weather hazard that holds more precipitation and produces stronger winds.

"It's like the river supercharges the cyclone's engine," said Carl Schreck, a researcher with North Carolina State University's Cooperative Institute for Satellite Earth System Studies.

But as the events in Canada have shown, when heavy precipitation falls on fire-ravaged areas the results can be devastating.

Wildfires, like the ones that hit British Columbia this summer, destroy tree canopies that catch rain and roots that suck up moisture and cling to soil. The fires leave a thick layer of water-repellent ash along forest floors.

"Not only are we having more of these so-called atmospheric rivers, but we also have the added secondary effect of heat and drought... which then amplify the effects of heavy rainstorms,” said Matthias Jakob, a geoscientist with Canada's BCG Engineering.