Atmospheric river flowing from Asia to the Pacific Northwest of America.
Composite photo by NASA taken in October 2017. Source: Wikipedia
In A River Runs Through It, two brothers put on their waders and fish the gorgeous Blackfoot River. The Blackfoot rolls west to merge with the Clark Fork at Missoula, later draining into the vast Columbia River system. The Columbia is known as the largest system in the Pacific Northwest (PNW), but that is incorrect. At least one river system is even larger: the atmospheric river.
Atmospheric rivers (AR), elongated tendrils of highly concentrated moisture in the atmosphere, are often thousands of kilometers long and a few hundred wide. They tend to move moist air from the tropics or subtropics northward toward the pole. They play a critical role in the water cycle and, though some are severe, the majority over time have been helpful rather than the reverse.
The total moisture load of these rivers makes them, according to the U.S. Geological Survey (USGS), the largest fresh-water rivers on Earth – though hairsplitters might ask whether ‘on earth’ is proper nomenclature for rivers that flow in the troposphere.
Boise River fly fishing. Photo by Henry Fraczek / Unsplash
When ARs reach land, especially in a mountainous region, the moisture “is pushed upwards, causing much of the water vapor to condense and fall to the ground as rain or snow, creating an atmospheric-river driven storm,” and dumping many times the daily discharge of the Mississippi River on whatever lies below. Often what lies below are California and the Pacific Northwest.
Such phenomena aren’t new: there are usually several moving around the planet all the time, occurring regularly in North America, Australia, and Europe. Recently (2019) one was reported in the Middle East, “potentially resulting from the [sic] climate change.”
ARs are responsible for much of the precipitation in the PNW and numerous other regions, and when they are absent, droughts often result. They are normal, even necessary to the hydrology, climatology, and ecology of many regions.
Category rating chart for atmospheric rivers.
Their impacts are a matter of severity and, starting in 2019, ARs have been ranked on a scale like hurricanes and tornadoes, from Category 1, the mildest, to the nightmarish Cat 5. Up to now, the trick has been to dodge the most destructive while benefiting from Cat 1 and Cat 2s, but here’s the rub: “the warmer the air is, the more water vapour it can carry.” Spoiler alert: the atmosphere has been warming, and… well, you get the idea.
Per a 2018 study by NASA’s famed Jet Propulsion Laboratory, given the global warming trend, ARs “will become about 10% less frequent by the end of this century, but about 25% longer and wider… That will lead to nearly double the frequency of the most intense atmospheric river storms.” Some areas are expected to be hurt worse than others, researchers at Yale wrote last October. “Europe, for instance, is expected to experience atmospheric river-driven, extreme rain more than 100% more often than over the last century. That means much more frequent strong storms with the potential to harm life and property.”
Keeping the Category Chart in mind, while there may be slightly fewer ARs overall – including fewer ‘primarily beneficial’ Cat 1 and Cat 2 storms – there will likely be a doubling of the Cat 3 – 5 ARs, which tilt way over into the ‘hazardous’ category.
Snowpack on Mt. Deception, Olympic range, Pacific Northwest. Photo by Eggbones, 2019.
Source: Wikimedia Commons.
However, though all assessments appear to agree that ARs will become more destructive, not all models agree there will be fewer. “We expect North Atlantic ARs to become stronger and more numerous by the end of this century,” asserts Carbon Brief, especially under the current high-emissions trend scenario that currently obtains. There may be an increase in the number of days under an AR of between 200%-300% by century’s end.
As we pointed out in our Sustainable Brands article Fool Me Twice: Accepting the ‘Now Normal,’ other impacts of climate change can make ARs even more destructive. It happened last year in the PNW, when a massive AR followed hard on the heels of droughts and wildfires. Events in a ‘heat, fire, drought, flood’ sequence generally lead to unpleasant compound effects, such as landslides, and climate change makes this cascade more and more likely.
Photo by Ryan Arnst / Unsplash
Speaking of which, an ‘extreme’ Atmospheric River drenched the PNW last year, derailing trains, melting snowpack, drowning Vancouver, and closing its critical port. Another hit Washington State just a few weeks ago. The world has a warming atmosphere, and rivers run through it. So long as global warming continues, we may all need to put on our waders, but they may not be enough.