We’ve all seen the news stories of the “seemingly never-ending parade” of storm systems battering California over the last month. ‘Atmospheric rivers’ have deluged the state with flooding and extreme winds, claiming at least 20 lives, forcing thousands to evacuate, and causing at least $5B in damages.
We talked with Prof. Paul Loikith from Portland State University's department of Geology to better understand what atmospheric rivers are and how climate change might impact them.
The simplest way to define an atmospheric river, Loikith said, is “a narrow and long area in the atmosphere where there is a lot of moisture moving fast.” They require strong winds and lots of water vapor, and are not uncommon in Portland (about 14% of the time during the winter there is an atmospheric river over us).
We asked about the impact of climate change on rainfall. “Globally speaking, the basic property of the atmosphere is that warmer air can hold more moisture, air has to be lifted for precipitation to occur. As the atmosphere warms, more precipitation can fall because the air is holding more water,” Loikith explained. We are seeing an increase worldwide in heavy precipitation due to climate change, but it is a dynamic process; in the western US, in particular, the relationship between a warmer atmosphere and heavier precipitation is complicated.
Thankfully, researchers - including Loikith - are exploring this relationship worldwide and locally, and producing data that will help to identify trends, improve the accuracy of weather predictions, and prepare for future changes. Loikith's focus is on the climatology and meteorology of atmospheric rivers across the United States.
In California’s current situation, Loikith admits, drawing a direct connection to climate change is hard. The state has a history of episodic periods of atmospheric river events, and Californians past and present have relied on these storms to end droughts and replenish their rivers and reservoirs. This winter, while severe, is not outside the range of what has historically happened across the state, making it difficult to attribute this winter’s rain to a changing climate. While global warming does lead to higher amounts of water vapor in the atmosphere, it is difficult to make a direct connection between that effect and this winter’s rains in California. The increase in devastating wildfires also influences the impact of these enormous rainfall events, as devegetation has left mountain slopes more susceptible to landslides. Unfortunately, the confluence of multiple climatic events means that this rainfall will lead to an increase in forest and understory growth, which will provide more fuel for forest fires during the summer once this moisture is gone.
The Portland area has experienced severe rainfall events too, most recently in December 2015, when we received 17 inches of rain throughout the month. And of course, there was the catastrophic flood of 1996. When asked if there is anything Oregonians should do to prepare for such events in the future, Loikith advises paying attention to weather forecasts, as they are the best they have ever been, and to be aware of potential hazards such as landslides, river flooding, or infrastructure damage due to heavy rainfall.
The most important step we can take is to continue to research this phenomenon to better understand its causes and potential future impacts.