Tuesday, August 16, 2022

SCIENCE ADVANCES . CLIMATOLOGY Climate change is increasing the risk of a California megaflood



SCIENCE ADVANCES | RESEARCH ARTICLE

CLIMATOLOGY Climate change is increasing the risk of a California megaflood 

Xingying Huang1 *† and Daniel L. Swain2,3,4 *† 

Despite the recent prevalence of severe drought, California faces a broadly underappreciated risk of severe floods. Here, we investigate the physical characteristics of “plausible worst case scenario” extreme storm sequences capable of giving rise to “megaflood” conditions using a combination of climate model data and high-resolution weather modeling. 

Using the data from the Community Earth System Model Large Ensemble, we find that climate change has already doubled the likelihood of an event capable of producing catastrophic flooding, but larger future increases are likely due to continued warming. We further find that runoff in the future extreme storm scenario is 200 to 400% greater than historical values in the Sierra Nevada because of increased precipitation rates and decreased snow fraction. These findings have direct implications for flood and emergency management, as well as broader implications for hazard mitigation and climate adaptation activities.


Copyright © 2022 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S.Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC)

INTRODUCTION 

California is a region more accustomed to water scarcity than overabundance in the modern era. Between 2012 and 2021, California experienced two historically severe droughts—at least one of which was likely the most intense in the past millennium (1, 2)—resulting in widespread agricultural, ecological, and wildfire-related impacts (3, 4) and ongoing drought-focused public policy conversations. 

Yet, historical and paleoclimate evidence shows that California is also a region subject to episodic pluvials that substantially exceed any in the meteorological instrumental era (5)—potentially leading to underestimation of the risks associated with extreme (but infrequent) floods. Observed extreme precipitation and severe subregional flood events during the 20th century—including those in 1969, 1986, and 1997—hint at this latent potential, but despite their substantial societal impacts, none have rivaled (from a geophysical perspective) the benchmark “Great Flood of 1861–1862” (henceforth, GF1862). 

This event, which was characterized by weeks-long sequences of winter storms, produced widespread catastrophic flooding across virtually all of California’s lowlands—transforming the interior Sacramento and San Joaquin valleys into a temporary but vast inland sea nearly 300 miles in length (6) and inundating much of the now densely populated coastal plain in present-day Los Angeles and Orange counties (7). 

Recent estimates suggest that floods equal to or greater in magnitude to those in 1862 occur five to seven times per millennium [i.e., a 1.0 to 0.5% annual likelihood or 100- to 200-year recurrence interval (RI)] (5, 8). 

The extraordinary impacts resulting from GF1862 provided motivation for a 2010 California statewide disaster scenario—known as “ARkStorm” (ARkStorm 1.0)—led by the U.S. Geological Survey in conjunction with a large, interdisciplinary team (9). The meteorological scenario underpinning the ARkStorm 1.0 exercise involved the synthetic concatenation of two nonconsecutive extreme storm events from the 20th century (10). Subsequent analysis suggested



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