A single overlying factor is blamed for contributing to catastrophes along the Pacific Coast, from floods in California to deadly wildfires in Chile – hotter ocean temperatures.

But whether it's El Niño or climate change that's turning up the heat, or some combination of the two, is still a subject of some debate.

Some scientists say they see evidence of both in the week’s disastrous weather. Other scientists say it’s too soon to say for sure whether climate change is contributing to more intense moisture in the atmospheric rivers pummeling the Pacific Coast this winter and last. 

Either way it's clear ocean temperatures offshore are significantly warmer than normal. And warmer oceans and warmer air allow the atmosphere to hold more moisture, almost 4% more moisture for every degree of warming.

In California this winter, for the second winter in a row, atmospheric rivers are parading across the Pacific Coast dumping huge volumes of water. The corridors of moisture, which can be 200-300 miles wide, can transport huge volumes of moisture across hundreds of miles. The West has always depended on atmospheric rivers for much of the annual rainfall, but there’s a fine line between too much and not enough.  

Torrential rain and high winds caused widespread destruction in the state on Sunday and Monday, with up to 10 inches or more of rain in some locations and wind gusts up to 102 mph that downed massive trees and power lines. Three people died. 

The rain in Los Angeles over Sunday and Monday – 7.03 inches – was the third wettest two-day rainfall in the city dating back to 1877.  And it was almost half the city’s normal seasonal rainfall. Thanks to earlier rain events, the city is already at 98% of its average 14.25-inch total rainfall for the water year that ends Sept. 30.

Scientists expect these “rivers in the sky” to become wetter and more intense in the future, but what’s less clear is exactly how much today’s warmer water and air temperatures can be attributed to natural climate variability, such as El Niño or La Niña, and how much to the changing climate. Scientists do not yet agree on this point.

What’s happening with the warm oceans?

Ocean temperatures have been warming globally for years, and the last three decades have been the warmest on records that go back more than 100 years, said Alexander Gershunov, a research meteorologist at Scripps Institution of Oceanography at the University of California San Diego. 

“And it appears to be accelerating,” Gershunov said. 

Federal scientists say the oceans are absorbing much of the excess heat energy trapped by greenhouse gas emissions and that the rising temperatures at the surface and deeper below are symptoms of a warming world.

Global average sea surface temperatures between the latitudes of 60 South and 60 North have been hovering at 21.1 degrees Celsius this week, about 70 degrees Fahrenheit, said Boyin Huang, an oceanographer at the National Centers for Environmental Information. That temperature matches a record set in April and August last year. 

The warmer water temperatures are to be expected, because ocean average temperatures are usually high after a major El Niño, Huang said. The same thing happened during the strong El Niño in 2015-2016. 

El Niño weakens trade winds along the equator in the Pacific Ocean off the South American coast. That pushes warmer water eastward across a vast swath of the tropical Pacific, which explains why global temperature records are typically reached during El Niño years, Huang said. 

The El Niño also can move warmer water northward along the Pacific Coast of the United States. Gershunov said temperatures off the California coast are roughly 3.5 degrees warmer than normal. 

Is climate change making atmospheric rivers wetter?  

Climate change is expected to increase the moisture that atmospheric rivers carry and the rain and snow that falls out of atmospheric rivers, particularly on mountain slopes, Gershunov said. 

To evaporate more water, you need warmer sea surface temperatures, stronger winds and/or lower relative humidity, he said. “That’s the primary reason why we expect atmospheric rivers to become wetter in a warming climate.”

The Intergovernmental Panel on Climate Change and other studies have concluded precipitation in atmospheric rivers would increase in the future. However, scientists don't have a consensus about whether it’s actually happening already. 

An increasing trend in atmospheric river precipitation isn’t yet seen in observations, Gershunov said, but models suggest it will become detectable. Whether that will be in a few years or a few decades isn’t known, he said. “It could be that the storms we’re seeing now are the harbingers of that change.”

However, a group of scientists with the international consortium ClimaMeter used models to complete a rapid study to see if the Feb. 1 rain event could be attributed to climate change. 

Consortium scientists concluded that over the last two decades, floods affecting coastal California from events like the Feb. 1 rains have been up to 15% wetter. 

The events are wetter and more windy along the Pacific Coast than they were before, said Tommaso Alberti, at the National Institute of Geophysics and Volcanology in Italy. The consortium attributed most of that increase to human-driven climate change, with a modest role played by natural climate variability. 

'Rivers in the sky':What exactly is an atmospheric river?

“Our findings emphasize a changing climatic landscape for California, attributing the extreme characteristics of the California floods to the dual forces of human-driven climate change and a subtle influence of natural climate variability,” said Davide Faranda, a researcher in climate physics at the French National Center for Scientific Research. 

Heat waves rage in South America

Meanwhile, intense heat waves have been occurring across much of South America, including Chile, Argentina, Paraguay and Colombia.

On Jan. 31, the temperature in Santiago, Chile’s capital city, hit 99.1 degrees, the third highest temperature in 112 years of reporting, the World Meteorological Association reported. Drought, low humidity and strong winds also are blamed for creating conditions that contributed to the catastrophic fires occurring across the continent.

At least 130 people have died in Chile, with hundreds more missing in the Valparaiso region, including the coastal town of Viña del Mar, the WMO said.

“While El Niño is a natural phenomenon, it’s taking place amidst worsening climate change,” said Bárbara Tapia Cortés with WMO’s Regional Office for the Americas. “Recent El Niño events, including this one, are being created from an ocean that is already warmer.”

“It is worth remembering that we are emerging from 2023, which was the hottest year ever recorded,” Tapia Cortes said. “It is likely that the warming effect of the current El Niño episode will intensify the heat even more during 2024. This will cause more extreme weather events that ruin lives and destroy livelihoods.”