NEW YORK (BLOOMBERG) - The extreme weather that's hammered California with runaway wildfires and hit Louisiana with its most powerful hurricane in 160 years may be about to get even worse.
La Nina - a phenomenon that occurs when the surface of the Pacific Ocean cools - has officially formed, the US Climate Prediction Centre said on Thursday (Sept 10).
It triggers an atmospheric chain reaction that stands to roil weather around the globe, often turning the western US into a tinder box, fuelling more powerful hurricanes in the Atlantic and flooding parts of Australia and South America.
"It can create a ripple effect over North America," said Ms Michelle L'Heureux, a forecaster with the Climate Prediction Centre in College Park, Maryland.
The effects are already evident.
Rising temperatures and an extreme mega-drought across the US West are fuelling fires from Washington to Arizona.
California is having its worst fire season on record, torching an unprecedented 1.01 million hectares.
And in the Atlantic, a record number tropical storms have formed by September, including Hurricane Laura, which killed more than a dozen people across the Caribbean and the US last month.
The first half of 2020 was already quite hot - just 0.05 deg C lower than the record set in 2016, according to the National Centres for Environmental Information in Asheville, North Carolina.
The odds are high 2020 will end up in the top five warmest years ever.
A La Nina doesn't just mean more heat. It also raises the chances for a colder winter across the northern US and increases the prospects of flooding rains in northern Australia, Indonesia and north-east Brazil.
California's rainy season typically starts by early winter, and can counter the fire-spreading Santa Ana winds that are starting now.
But if La Nina gets in the way, it could have dire consequences for the state, where fires have already charred more than 1 million hectares, the most on record, and 80 per cent of the land is abnormally dry.
In the Atlantic, having La Nina develop in the last three months of hurricane season could mean more storms of greater power.
This year's 17 named storms make for the quickest that tally has been reached in data going back to 1851, well above the annual average of 12.
The Climate Prediction Centre forecast in August that there would be 19 to 25 Atlantic storms this year, based in part on the potential for La Nina to appear.
