How climate change will make atmospheric rivers even worse
In recent weeks, a slew of storms has slammed California, bringing torrential rains and deadly flooding. Storms are typical in the winter, including those associated with atmospheric rivers, or long and wide plumes of water vapor flowing from the tropics. But as Earth warms, climate scientists warn these atmospheric river events may be amplified, bringing even more destruction. 1 / 2 In other words, the recent events could be just a modest preview of whats to come in warmer years ahead. The impact of these storms is a paradox . Atmospheric rivers generally provide precipitation critical to a regions water cycle. These massive rivers, which sometimes carry 15 times the water volume of the Mississippi River, deliver half of the western United States total precipitation in less than 15 total days. But too much rain in a short amount of time can have devastating effects on communities. Atmospheric rivers account for nearly 90 percent of Californias flood damage. Infrastructure has been destroyed and more than a dozen people have been killed by the storms of the past two weeks. Calif. storm triggers more floods, tornado warnings as death toll climbs To help water managers and local authorities better prepare for future events, researchers are studying how a warmer world will influence the precipitation, intensity, location and societal effects of atmospheric rivers. In some aspects, climate change is already having detrimental effects on the systems. Here are four ways increased temperatures affect atmospheric rivers in the western United States. Perhaps one of the most well-understood aspects of climate change is its effect on rain. In a warmer atmosphere, evaporation rates increase and transform more liquid water molecules to a vapor state in the air. In fact, the atmosphere can hold about 7 percent more water for every 1-degree Celsius (1.8-degree Fahrenheit) increase. This moisture-laden air can drop heavier amounts of rainfall at one time, increasing the intensity of rain events. Earths atmosphere has warmed about 1.3 degrees Celsius (2.3 degrees Fahrenheit) since pre-industrial times, and researchers have already observed heavier rain in storms , hurricanes , daily precipitation and now in atmospheric rivers. In a recent study , researchers found climate change increased the amount of rainfall from two atmospheric rivers in February 2017, which notably damaged Oroville Dam, Californias second-largest dam, and prompted the evacuation of 188,000 people. The atmospheric rivers produced about 11 and 15 percent more rain because of warming brought on by fossil fuel burning. If the same events were to take place in an even warmer world as projected by the end of the 21st century the researchers found rainfall quantities would have been 20 and 60 percent higher. Scientists will need to conduct a similar in-depth analysis of the current atmospheric activity to determine how human-caused climate change has played a part, but researchers say they wouldnt be surprised to see heightened rainfall because of climate change. Other studies modeling future atmospheric rivers show rainfall will undoubtedly increase up to 40 percent more in a warming world. Were more than likely going to see more rain associated with atmospheric rivers and more precipitation in general, said Allison Michaelis, an atmospheric researcher at Northern Illinois University and lead author of the recent study. She added that some atmospheric rivers may be more susceptible to the environmental changes associated with climate change and show larger changes than others. Atmospheric rivers can be a bane or boon to local communities. Smaller events can help replenish water reserves, but larger or persistent systems can cause millions to billions of dollars in flood damage. Studies show that in a warmer world, atmospheric rivers will trend more harmful than beneficial. According to recent research , the number of atmospheric rivers that are mostly or primarily hazardous to water resource management will increase in intensity, occur in closer succession and drop more rainfall in a season. In fact, researchers estimate that atmospheric rivers in the western United States will become 15 percent larger in size and last six hours longer for every 1-degree Celsius (or 1.8-degree Fahrenheit) spike. These more intense atmospheric rivers will also become more frequent. In a world without human-induced climate change such as during the pre-industrial era, intense atmospheric rivers occur about 2 percent of the time in a season. At 3 degrees of warming, they could make up more than 8 percent of all atmospheric rivers in a season. The study authors found that in a situation where the world hits 3 degrees Celsius of warming, more intense events could raise flood damage costs from $1 billion to about $3 billion by the end of the century. Thats a subtle but important uptick in those [most intense] events along the coast of the western U.S., said Alan Rhoades, lead author of the study and research scientist at the Lawrence Berkeley National Laboratory. We found that translates to roughly $1 billion in potential flood insurance damage for every Celsius degree of warming. Rhoades said there is still time to keep warming below 3 degrees Celsius. If Earths temperature stays between 1.5 to 2 degrees Celsius of warming, the most intense atmospheric rivers could occur 4 to 5 percent of the time instead of 8. Atmospheric rivers play a critical role in supplying mountain snowpack, which serves as an important source of freshwater as it melts in the spring and summer. Some research shows the weather systems provide about a quarter of the snowpack in the Sierra Nevada. Yet as temperatures rise, snowfall is decreasing. Snow may vanish for years at a time in Mountain West with climate warming Studies have already shown atmospheric rivers are delivering less snow in the northern Sierra Nevada, instead falling as rain because of Earths excess warmth. Some of the rain can run off into rivers and cause flooding. In some cases, the rain can also land on top of snow, accelerating melt. In February 2017, rain from atmospheric rivers fell atop snow, leading to runoff that damaged the main and emergency spillways of the Oroville Dam . The real risk here is Californias whole infrastructure was built with what we understood of the snowpack, meaning a natural reservoir through the whole winter, said Anna Wilson, who studies atmospheric rivers at Scripps Institution of Oceanography. Now were getting into situations where were getting more rain throughout the year. Sometimes they have rain on top of snow. Researchers project such rain events will multiply across the western United States. Rain-on-snow events are expected to increase at higher elevations, raising the flood risk most notably in the Sierra Nevada, Colorado River headwaters and the Canadian Rocky Mountains. Wilson and other scientists are working to improve forecasts of atmospheric rivers, which could better inform water managers on impending heavy rain events. In response, she said water managers could open reservoirs beforehand, helping to prevent flooding during an atmospheric river event. A megaflood in California could drop 100 inches of rain, scientists warn Climate change may also alter where atmospheric rivers touch down, although researchers are still investigating how. Atmospheric rivers are steered by the low-level jet stream, said Christine Shields, a climate scientist at the National Center for Atmospheric Research. A warmer climate, however, is expected to push the jet stream toward the equator. Shields and her colleagues found that such a shift would bring more atmospheric river events and higher rainfall rates to Southern California in the winter. Although other scientists, like Wilson, said theres not yet a consensus on how or if atmospheric rivers will change in latitude. Shields said its still an open question if climate change has already changed the landfall location of atmospheric rivers along the West Coast.