Torrential rains forecast to hit California on Wednesday and again over the weekend are poised to be the third and fourth heaviest storms to march in less than two weeks, raising the possibility of more misery in a season that has already brought flooding, debris flows and power outages to parts of the state.
Over the weekend, rescuers scoured rural areas of Sacramento County looking for people trapped in homes or cars. Levees failed near the Cosumnes River, flooding a road.
Winter rain and snow typically provide much of the water used throughout the year in California, which has suffered several years of severe drought. But when these storms, known as atmospheric rivers, are particularly severe or move quickly, they can do more harm than good, delivering too much water, too fast, for state reservoirs and emergency services to handle.
So far, this winter’s storms have largely been in line with past ones, except in their relentless pace, said Michael Anderson, California state climatologist. “This is where we’re getting hit this year: We’re seeing a lot of big storms pretty quickly.”
What is an atmospheric river?
These storms get their name from their long, narrow shape and the prodigious amount of water they carry.
They form when winds over the Pacific pull a filament of moisture from the band of warm, humid air over the tropics and funnel it toward the west coast. When this ribbon of moisture hits the Sierra Nevada and other mountains, it is forced upward, cooling it and turning its water into immense amounts of rain and snow.
Climate scientists also distinguish atmospheric rivers from other types of storms by the amount of water vapor they carry. These amounts form the basis of a five-point scale used to classify atmospheric rivers from “weak” to “exceptional.”
Is climate change making them more extreme?
As humans continue to burn fossil fuels and warm the atmosphere, warmer air can hold more moisture. This means storms in many places, including California, are more likely to be extremely wet and intense. Scientists are also studying whether global warming could be changing the way winds carry moisture through the atmosphere, which could influence how many atmospheric rivers flow through California each year and how long they last. However, they have not yet reached firm conclusions on these questions.
“What is predominantly happening is that in a warmer atmosphere, there is exponentially greater potential for it to hold water vapor,” said Daniel L. Swain, a UCLA climatologist. “And that has a really profound influence on things.”
How common are they?
Atmospheric rivers have a major influence on California’s climate and water supply. They cause the heaviest rains in the state and fuel the biggest floods. They run their cycles of dry and wet, famine and feast. But they also cause a large part of the state’s levee breaches and debris flows.
An atmospheric river can be enough to inundate homes, bring down power lines, and wash away hillsides and roads. But when several come ashore in a matter of days or weeks, as appears to be happening this week, the potential damage is multiplied.
Soils already saturated with rainwater might not be able to absorb more, leading to flooding and landslides. Rivers and streams already swollen after a storm could overflow their banks. In the high mountains, rain could fall on the snow, melting it and causing water to cascade down to communities below. Emergency services could be stretched to breaking point.
When big storms come one after the other, it’s also harder for the infrastructure to channel all that water into the ground or into reservoirs where it can be held in reserve for dry summers.
“It’s really helpful if the storms are kind enough to space them a week or two apart so we have time for the water to flow through the system,” said Jeanine Jones, an official with the California Department of Water Resources.
An accumulation of wet weather caused catastrophic flooding in California and the Pacific Northwest in the winter of 1861-62, when floods washed away homes and farms and turned valleys into large lakes. As global warming continues, scientists say the risk of a repeat of such floods is increasing.
Are they happening more frequently?
In a study published last year, Swain and Xingying Huang of the National Center for Atmospheric Research in Boulder, Colorado, estimated that California today has about a 1 in 50 chance each year of experiencing a storm of comparable intensity to that of last year. 1861-62. Climate change has already doubled those odds compared to a century ago, they estimated.
It’s not yet clear how global warming might be affecting the likelihood that atmospheric rivers will crash into California in rapid-fire clumps. Another study last year found that in nearly 4 out of 5 years between 1981 and 2019, half or more of all atmospheric rivers that affected the state were part of a “family” of atmospheric rivers or a swift parade of storms.
Still, the warmer atmosphere’s increased ability to hold moisture is reason enough for California officials to prepare for more catastrophic rainfall events today and in the future, Swain said.
“Even if that were the only thing going on,” he said, “it would act to animate, if you will, whatever kind of atmospheric river is going on, whether it’s families of atmospheric rivers or one-offs.”
This article originally appeared in The New York Times.
