In the atmosphere, long, narrow, whip-like plumes carry water vapor from the tropics to the mid latitudes. These plumes are shaped by variability in and geographic differences in air pressure. Stretching longer than 1,000 miles, these rivers in the sky called atmospheric rivers can transport massive water vapor amounts, sometimes as much as 15 times the water flow at the mouth of the Mississippi River. Upon landfall, they dump rain and snow, and can cause severe flooding and overwhelm reservoirs. But less intense atmospheric river events are beneficial to drought-stricken regions.
Take the coastal western U.S., for example, where atmospheric rivers account for nearly 50 percent of the region’s annual precipitation in a span of less than 15 days. Now, a new Berkeley Lab study published in Weather and Climate Extremes finds that global warming will deliver bigger and wetter atmospheric river storms to the Pacific Coast, with a subtle but important uptick in the fraction that are deemed hazardous to water resource management. This increase in hazardous atmospheric river storms could translate to billions of dollars in flood damages, costing, on average, an additional billion dollars for every degree Celsius rise in global temperature.
“This has implications for water management in the future,” says Alan Rhoades, a hydroclimate research scientist in Berkeley Lab’s Climate and Ecosystem Sciences Division and lead author of the study, “as water managers balance flood control and reservoir storage for water supply.”
As air warms, its capacity to “hold” water vapor increases. For every degree Celsius spike in air temperature, scientists predict an approximate seven percent increase in air’s capacity to “hold” water. That means with global warming, there’s potential for future atmospheric rivers to produce more rain upon landfall.
Between 1987 and 2017, just 13 atmospheric river events spanning 65 days were responsible for more than $1 billion in flood damages in the western U.S. To assess the characteristics of future atmospheric rivers, Rhoades and his collaborators used computer models to simulate different global warming scenarios and estimated the frequency and intensity of atmospheric river events that may occur along coastal western U.S. They found that with an increase of three degrees Celsius in global temperatures from the 2006-2015 baseline, the number of landfalling atmospheric rivers that reach at least a Category 1 event increased from 19 each year to nearly 24. With every degree spike, the researchers estimated that the atmospheric rivers become 15 percent larger in size and last six hours longer.
That meant an increase from 2 to nearly 8 percent in the proportion of hazardous category 4 and 5 atmospheric river events for a three-degree Celsius warming scenario, while the beneficial category 1 and 2 atmospheric river events dipped from 91 to 78 percent. Flood damages are closely tied to these hazardous events, as they incur an order of magnitude higher cost than the category 1 and 2 atmospheric rivers. For a three-degree Celsius warming, the researchers estimate flood damage costs will increase, on average, from nearly $1 billion to $3.25 billion each year.
“We’ve shown that there is a lot at stake as the world warms,” Rhoades says. “If we don’t start reducing emissions pretty dramatically over the next 10 years, subtle increases in category 4 or 5 atmospheric rivers could occur and this translates to a lot of flood damage potential and billions of dollars.”