Yes, climate change influences atmospheric rivers

Too often, we’re asking the wrong question about atmospheric rivers and climate change.

It’s understandable as Californians are bracing for the next system, watching footage of the Pajaro River levee break, wondering what fails next.

It’s not just Californians, either. Our atmospheric rivers don’t always stop at the Sierras. They can plow across the rest of the country, dumping more rain and snow when conditions are right, like they were earlier this week in the Northeast.

This year’s storms have seemed worse than many people remember, and the data back that up: This winter we’ve set new records for snowpack, rainfall intensity, snowfall. Weather is erratic and memories can be skewed, but clearly something is different. It’s natural to ask, “Is climate change causing all this?”

But the right question is, “How is climate change affecting all this?”

The answer is more straightforward than you might expect, but I want to start with some background.

I’m a Californian and a climatologist. Atmospheric rivers are nothing new here. Their scientific definition can sound technical, but they’re basically long, narrow bands of moisture that flow through the atmosphere, and they play a vital role in the health of a host of Western ecosystems. At lower elevations they replenish depleted soils, and at higher elevations they build up snowpack, which essentially stores water for delivery into the summer and fall. In some years, atmospheric rivers can account for 30 percent to 50 percent of total precipitation in the West.

Atmospheric rivers existed before carbon emissions began trapping heat and warming our planet, but their impact is different now: Simply put, climate change is making them worse.

First, their precipitation is more intense because climate change has warmed our atmosphere, which allows it to hold — and then release — more water. 

For every Fahrenheit degree of warming, the atmosphere can store roughly 4 percent more moisture. Because carbon emissions have warmed our planet by nearly 2 degrees F, our atmosphere can store roughly 7 percent more moisture. Statistically, when it rains in a warmer world, it pours.

Second, increasingly in a warmer world, it rains more than it snows. That’s what we saw recently, when warmer storms brought rain, even at higher elevations. (That also made the fallen snow heavier, collapsing roofs around Lake Tahoe.)

Rain-instead-of-snow is already more common as winter temperatures trend higher across the country. When temperatures are low enough for snow, as they were across the Great Lakes earlier this winter, and across New England this week, it can snow a lot. Again, that’s the influence of climate change, making more moisture available to boost precipitation — which has been mostly rain in the East this year. Unlike the West, the story of 2023 in most eastern cities has been record warmth and barely any snow.

The first big problem with warmer atmospheric river systems in the West is flood risk, and we’re already seeing evacuations and massive losses directly caused by rain, with the potential for more coming early next week. That could place more stress on levees and dams.

Snowmelt from a storm that dumps rain at higher elevations would compound the risk, potentially overwhelming flood control structures that weren’t built for this much water. Our flood protections — as well as critical infrastructure, city planning and building codes — were designed for a cooler climate, with less intense rainfall and less violent shifting between extremes.

Catastrophic flood risk isn’t limited to a single storm, either. The last snowpack measurements of the year typically happen on April 1, because it’s near the annual peak. In many places, melting may have already started with this week’s storms, and a shift toward warmer, wetter weather may mean that the record-setting 2023 snowpack has already reached its peak depth. The transition to spring temperatures — which have also trended higher as the climate warms — would only accelerate the melting, sending immense volumes of water downhill for weeks.

The influence of climate change makes another wild swing more likely. Higher temperatures have been depleting the snowpack earlier in the season, reducing the water available to natural systems — as well as farms and cities — during the dry months. This year’s snowfall doesn’t mean we’ll buck that trend. Instead, after a fast thaw and devastating flood season, all the stored water in this historic year could be gone in a flash, leaving more drought in its wake — and then fire.

People logically assume that wet years mean lower fire risk, but when damp conditions supercharge spring blooms, and then give way to a hot, dry summer, the opposite can be true. New vegetation can turn into an abundance of fuel for wildfires.

As with atmospheric rivers, if someone asked whether climate change causes wildfires, they’d miss the point. Climate change influences all of these events. It makes extreme weather more intense and erratic. It amps up an array of risks. It makes disastrous outcomes more likely.

When it comes to these events, we need to ask questions that lead us to a better understanding of what to prepare for, and how to protect ourselves in a changing climate.

Kaitlyn Trudeau is a data analyst for Climate Central’s Climate Matters program. Her research includes analyzing atmospheric influences on western wildfire risk. She has a B.A. in physical geography from California State University, Sacramento and a M.S. in geography from the University of Nevada, Reno, where her research focused on climate change in the Arctic region.