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
