The Pacific Northwest is still reeling after a series of devastating floods and landslides spurred by torrential rains last week.

Floodwaters across western Washington state and British Columbia swamped homes, swept away vehicles, destroyed roads and highways, and cut off railroads. At least four people were killed by mudslides and nearly 20,000 were displaced by the floods.

Canadian officials called the deluge a “once-in-500-year” event. The storms broke rainfall records across the region, in some places dropping more than 6 inches of rain in just 24 hours.

Historically speaking, it’s a rare and exceptional event for the Pacific Northwest. But these kinds of floods could grow more common in the future.

The storms that caused last week’s flooding were brought by a weather system known as an “atmospheric river”—a narrow band of moisture streaming through the air. Atmospheric rivers can stretch long distances across the globe, and they’re known to dump heavy rain or snow when they make landfall.

These are natural events, which happen all the time around the world. But in British Columbia, they seem to be happening more frequently. The region has seen an uptick in the frequency of landfalling atmospheric rivers since the 1940s, according to one recent study.

At the same time, research suggests that atmospheric rivers all over the world likely will grow bigger and more intense as the climate warms. That could mean more severe rainfall when they hit land.

Just last month, a study in the journal Nature Climate Change found that atmospheric rivers haven't changed much over the last century—but they’re likely to get stronger in the future.

In theory, rising global temperatures should cause atmospheric rivers to become more severe. But the effects of global warming have been partly offset by the influence of air pollution, which has the opposite effect.

That won’t last forever. Global pollution clean-up efforts, beginning in the latter half of the 20th century, have caused air pollution levels to fall across parts of the globe, particularly Europe and North America. At the same time, the influence of global warming is growing stronger.

As a result, the study projects that atmospheric rivers will intensify in the coming decades.

Other research has come to similar conclusions.

2020 study, which reviewed the existing literature on the subject, anticipates that atmospheric rivers will hold more moisture as the world warms. In theory, that means they could drive more intense rainfall events wherever they land (although, the study notes, various other factors also can affect their outcomes).

Another study, from 2018, also projected that atmospheric rivers would generally get bigger and more intense as the climate warms.

In some places, extra rainfall may seem like a good thing. The parched western United States, for instance, is in the grips of a yearslong “megadrought,” exacerbated by climate change. Concerns over dwindling water resources and increasingly severe wildfires are at an all-time high.

Atmospheric rivers are a major supply of precipitation to the West Coast—and at first glance, it may seem as though heavier rainfall would be a boon to the region. But that’s not necessarily the case. The damage caused by extreme rainfall events sometimes can cancel out their benefits.

Several recent studies have highlighted these risks.

2020 study found that, under an extreme warming scenario, precipitation from atmospheric rivers could shift from mostly beneficial to mostly hazardous.

The study assumes more warming than is likely to happen between now and the end of the century (although the more pessimistic scenario is technically possible). Still, it notes that extreme rainfall poses a variety of threats in the western United States. It can wipe out valuable mountain snowpack—an important source of freshwater as it melts in the spring and summer, and it increases the odds of dangerous floods.

2019 study points out that the majority of floods on the West Coast already are driven by atmospheric rivers. The study also concludes that more intense atmospheric rivers could erode mountain snowpack and increase the odds of floods and landslides.

Another study, examining 40 years of data on flood insurance claims, finds that atmospheric rivers are the biggest drivers of flood damages across the western United States, and these damages increase with the severity of the atmospheric river.

Compound climate threats

Intensifying atmospheric rivers would add another threat to an increasingly volatile region.

Western North America is already facing a variety of climate-related pressures. Much of the western United States is struggling with long-lasting drought, which studies have linked to climate change. Mountain snowpack is shrinking in some regions, heightening concerns about water shortages. Wildfires are growing more severe, a trend that scientists have linked to persistent drought and rising temperatures.

The past summer also raised new concerns about deadly heat in formerly temperate parts of the continent. A heat wave earlier this year sent temperatures skyrocketing across the Pacific Northwest, shattering records in some cases. Seattle broke 108 degrees Fahrenheit. Portland hit 116 F. And the small village of Lytton, in British Columbia, reached a jaw-dropping 121 degrees.

Scientists later determined that the heat wave would have been “virtually impossible” without the influence of climate change (Climatewire, July 8).

Prolonged drought isn’t necessarily inconsistent with more intense rainfall events. Climate models generally suggest that dry periods will get longer and more severe as the climate warms, but they may be punctuated by heavier precipitation events.

The combination of all these threats, working together, can make their outcomes worse.

Heat and drought can prime the landscape for wildfires. Severe fires can destroy vegetation and damage the soil, making it more difficult for the landscape to absorb excess water. Heavy rainfall events, when they do occur, may become more likely to cause floods and landslides.

That’s likely what happened in British Columbia last week.

In a recent essay published in The Conversation, scientist John Clague of Simon Fraser University noted that many of the places drenched by rain last week were scorched by wildfires earlier this summer. Soils often become “hydrophobic” after they’ve been burned—they repel water.

“It is possible that runoff from these burned grounds was greater and more rapid because of the hydrophobicity of the soil,” Clague wrote.

Other experts have highlighted the compound risks of multiple climate threats.

“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,” geoscientist Matthias Jakob told Reuters.

Reprinted from E&E News with permission from POLITICO, LLC. Copyright 2021. E&E News provides essential news for energy and environment professionals.