California at a heightened risk of megafloods, says NCAR

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According to a new study from the National Center for Atmospheric Research (NCAR), California is facing a heightened risk of megafloods, despite its current period of historic drought.

Although California is currently contending with a historic drought, the study indicates that climate change is sharply increasing the risk of a megaflood that could submerge large swaths of the state and displace millions of residents. The study found that the likelihood of such a calamitous flood has already doubled because of climate change. According to these scientists, if society were to follow a worst-case climate scenario of greenhouse gas emissions, this century could bring the state a catastrophic megaflood approximately every 30-35 years, instead of just once in two centuries.

The research indicates that the extreme rainfall required to create a megaflood would likely be associated with a series of atmospheric rivers – long and narrow regions in the atmosphere that are a critical source of cool-season moisture on the West Coast. Previous research has shown that climate change is altering the intensity and path of atmospheric rivers. They found that the state not only faces the threat of more intense winter precipitation but also disproportionately greater volumes of runoff. That is partly because more precipitation will fall as rain instead of snow, thereby immediately rushing down slopes instead of slowly melting into the soil.

Daniel Swain, a co-lead author of the study, said, “Although California has recently experienced historically severe drought and the broader Southwest is facing an accelerating water scarcity crisis, it’s important to remember that this is still a region susceptible to rare but potentially severe floods. It may seem paradoxical that climate change is increasing the risks associated with both droughts and floods in a place like California, but that’s exactly what the scientific evidence suggests.”

The study emphasizes that the state can take steps to mitigate the impacts of a megaflood, through such strategies as floodplain restoration and levee setbacks, greater flexibility in reservoir operations, and revised evacuation and contingency plans in the case of a flood. It also finds that the probability of a catastrophic flood might be predictable because the risk is much higher in El Niño years. Although the scientists focused on California, Xingying Huang, the co-lead author of the study and a scientist at NCAR, said the findings could hint at greater risks for other regions along the West Coast and elsewhere that receive substantial precipitation from atmospheric rivers.

California has faced several severe floods in recent decades, including the so-called New Year’s Day flood of 1997, which damaged more than 20,000 homes and businesses, forced the evacuations of more than 120,000 residents, and left nine people dead. The last such megaflood, caused by weeks of winter storms in 1861-62, transformed the Sacramento and San Joaquin valleys into a temporary inland sea nearly 300 miles in length and up to 30ft deep while inundating much of the coastal plain in what is now the densely populated Los Angeles and Orange counties. That year’s deluge of water also killed 1% of the population and resulted in devastating economic losses.

However, if such an event were to occur now, estimates by the US Geological Survey suggest that it would displace millions of people, shut down critical transportation corridors and ultimately lead to nearly US$1tn in economic losses. To examine how climate change may affect such floods, Huang and Swain coupled two computer models – the NCAR-based Community Earth System Model, which simulates global climate, and the NCAR-based Weather and Research Forecasting Model, which simulates regional weather events at high resolutions. They simulated multi-week storms similar to those that triggered the 1861-62 flood, comparing the resulting impacts in a climate that existed from 1996 to 2005 with a hypothetical warmer climate from 2071-2080 that would occur if society continued to release greenhouse gasses into the atmosphere at a high rate (a scenario known as RCP 8.5). They performed the simulations on the Cheyenne supercomputer at the NCAR-Wyoming Supercomputing Center.

In both scenarios, the storms unleashed torrential amounts of precipitation across widespread regions, totaling about 20-40in in the 1996-2005 period and up to about 55in in the future scenario. Parts of the Sierra Nevada faced even greater deluges: 60-70in in the 1996-2005 period and, in the future, as much as 100in. The peak runoff in the future scenario was up to five times that of 1996-2005, which was the result of greater precipitation rates and of rain, rather than snow, falling at all but the very highest elevations. In addition to steep hillsides, runoff was high in major urban areas with impervious, paved surfaces, raising the specter of urban flooding.

The research showed that every degree of warming increases the risk. Already, the annual likelihood of a megaflood is about twice as high as 1920, when the climate was cooler. By 2060, on a high emissions trajectory, the likelihood of a megaflood occurring in any particular year will be roughly four to seven times more than a century ago. Even if the international community were to meet the goals of the Paris Agreement, which aims to hold warming to no more than about 2°C, the risk of a California megaflood would be substantially greater than it is currently.

The study, published in Science Advances, was funded by the Yuba Water Agency and the California Department of Water Resources, as well as by the US National Science Foundation, which is NCAR’s sponsor. It represents the first phase of a broader project, known as ARkStorm 2.0 project, that involves multiple organizations – including the University of California, US Geological Survey, California Department of Water Resources, Desert Research Institute and NCAR – to better understand and prepare for the risks of catastrophic floods in a warming climate.

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