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LETTER
Co-occurring climate events and environmental justice in
California, 2018–2019
Brittany Shea1,∗, Gabriella Y Meltzer2, Benjamin B Steiger1, Robbie M Parks1, Vivian Do1,
Heather McBrien1, Nina Flores1, Milo Gordon1, Elizabeth M Blake3and Joan A Casey1,3,4
1Department of Environmental Health Sciences, Columbia University Mailman School of Public Health, New York, NY, United States
of America
2The Collaborative for Women’s Environmental Health, Department of Obstetrics and Gynecology, Columbia University Irving
Medical Center, New York, NY, United States of America
3Department of Environmental and Occupational Health, University of Washington School of Public Health, Seattle, WA, United
States of America
4Department of Epidemiology, University of Washington School of Public Health, Seattle, WA, United States of America
∗Author to whom any correspondence should be addressed.
E-mail: bes2161@cumc.columbia.edu
Keywords: climate change, disasters, social vulnerability, temperature, power outages, wildfires
Supplementary material for this article is available online
Abstract
Climate change will increase the frequency of extreme weather events. This means climate-driven
events like wildfires and power outages will likely co-occur more often, potentially magnifying
their health risks. We characterized three types of climate-driven events—anomalously warm
temperatures, wildfire burn zone disasters, and long power outages—in 58 California counties
during 2018–2019. We defined county-day anomalously warm temperatures when daily average
temperatures exceeded 24 ◦C and the 85th percentile of the long-term county average. We defined
county-day wildfire burn zone disasters when an active wildfire burn zone intersected a county,
burned 1+structures, killed a civilian, or received a Federal Emergency Management Agency Fire
Management Declaration, and overlapped with a community. For a subset of the 38 counties
(66%), long power outage county days were identified using PowerOutage.us data when an outage
affected >0.5% of county customers for 8+h. Co-occurring events were when 2+of these events
occurred on the same county day. Using the CDC/ATSDR Social Vulnerability Index (SVI), we
determined whether co-occurring events disproportionately affected vulnerable populations.
Nearly every county (97%) experienced at least one day of anomalously warm temperatures, 57%
had at least one wildfire burn zone disaster day, and 63% (24/38 counties with available data) had
at least one long power outage day. The most common co-occurring events (anomalously warm
temperatures and wildfire burn zone disasters) impacted 24 (41%) counties for 144 total
county-days. We did not find a clear connection between co-occurring events and social
vulnerability. We observed an inverse correlation between co-occurring wildfire burn zone
disasters and long power outage days with SVI, and a positive correlation between co-occurring
anomalously warm and long power outage days with SVI. This analysis can inform regional
resource allocation and other state-wide planning and policy objectives to reduce the adverse
effects of climate-driven events.
1. Background
Warm temperatures, wildfires, and power outages have increased in frequency, severity, and geographic
range due to climate-change-induced rising temperatures and prolonged drought conditions [1–5]. Of all US
states, California has been most affected by wildfires: 76% of structures destroyed by wildfires and 20% of the
total area burned by wildfires from 1990 to 2020 occurred in California [6,7]. Extreme heat is increasingly
© 2025 The Author(s). Published by IOP Publishing Ltd
Environ. Res.: Health 3(2025) 021001
affecting the health of California residents [8]. In addition, the number of weather-related power outages is
rising across the US, and from 2000 to 2023 California had the third most (n=145) weather-related power
outages of any state [9,10]. Furthermore, these events disproportionately impact persistently marginalized
communities, including those of color and low-income [11].
Co-occurring wildfires, power outages, and heat pose serious health risks due to the potential
inaccessibility/inoperability of cooling mechanisms and air filters powered by electricity. Anomalous heat
[12,13], wildfire burn zones [14,15], and power outages [4,16,17] individually harm health, and
co-occurring events may have synergistic negative effects on health [18]. Despite these potential risks, no
studies have characterized these three potentially co-occurring events. Evaluating the frequency and
distribution of climate-driven events can inform interventions to protect against their adverse health effects.
Considering differences in co-occurrence among vulnerable populations is imperative to inform
interventions and target resources effectively [11,19]. Vulnerable individuals experiencing a climate-driven
event may experience a higher exposure or have a worse response. Wildfires have broad public health
consequences, and vulnerable populations are particularly at risk of experiencing negative health outcomes
[20–22]. For example, older adults, young children, and pregnant persons are more at risk due to
physiological differences, and individuals of low socioeconomic status are more at risk due to fewer assets to
adapt to wildfires when they occur. These same groups are also more vulnerable to heat’s adverse health
impacts, as are people of color, due to disproportionate exposure and less access to resources resulting from
historical racism [23,24]. Finally, communities that are more socioeconomically vulnerable may experience
longer power outages, and may also have a stronger response due to worse housing quality or underlying
conditions [4,25,26].
Here, we evaluate three interconnected climate-driven events: anomalously warm temperatures, wildfire
burn zone disasters, and long power outages (8+h). First, we identify California counties exposed to one,
two, or three climate-driven events on the same day during 2018–2019. Next, we compare the social
vulnerability of counties exposed to varying levels of the three climate-driven events.
2. Methods
During 2018–2019 in California, we examined three county-daily-level climate-driven events: anomalously
warm temperatures, wildfire burn zone disasters, and long power outages. We also assessed whether exposure
to these events differed by level of social vulnerability. We focused on the years 2018–2019 because these were
the years for which data were available.
A county day was defined as anomalously warm if the daily mean ambient temperature on that given day
exceeded the 85th percentile of the weekly long-term county average for the day (1981–2010) [27,28], and
exceeded an absolute threshold of 24 ◦C. Since we are capturing anomalous temperatures that have health
relevance and are an interplay between absolute and relative thresholds, we used ⩾85th percentile to have a
percentile relative extreme, and this measure has largely behavioral implications [5]. Of the days that were
≥85th percentile, the median temperature was 17.3 ◦C (IQR: 12.7, 23.6). We added a double condition with
an absolute cutoff of >24 ◦C because we wanted to have an absolute extreme that ensured we captured truly
warm temperatures as the study covers the entire year. Absolute temperatures have largely physiological
implications and there are limits of adaptation physiologically [12].
We identified wildfire burn zone disasters using data from the California Department of Forestry and
Fire Protection, and the Federal Emergency Management Agency (FEMA). Wildfire burn zone disasters were
those that either burned 1+structures, killed a civilian, or received a FEMA Fire Management Declaration,
and that overlapped with a community (population density of ≥250 people km−2) [29,30]. We defined a
county-day as exposed to a wildfire burn zone disaster when a wildfire was burning (i.e. the day fell between
ignition and containment date), and the wildfire burn zone boundary overlapped spatially with the county
perimeter.
To identify county-days with long power outages, we used PowerOutage.us data and identified outages
that lasted for 8+h while affecting >0.5% of county customers, as in prior work [4,31]. Reliable power
outage data was available for 38/58 (65.5%) of counties, and all events involving power outage data are only
defined for this subset of counties due to limited data availability. Power outage data was considered reliable
if county utility providers’ application programming interfaces report ⩾50% of the time and if reported
customers covered ⩾50% of total county customers [4].
We defined climate-driven events as co-occurring when they took place in the same county on the same
day. We counted the total number of single and co-occurring climate-driven events on each county-day from
2018 to 2019.
We defined county-level social vulnerability using the California-specific 2018 Centers for Disease
Control and Prevention/Agency for Toxic Substances and Disease Registry Social Vulnerability Index (SVI),
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Environ. Res.: Health 3(2025) 021001
Figure 1. County-days of exposure to climate-driven events in California, 2018–2019. Percentages calculated out of total
county-days (n=42,340) from 2018 to 2019. AW, anomalously warm; PO, power outage; WBZD, wildfire burn zone disaster.
Events are not mutually exclusive.
which used American Community Survey 2014–2018 data for county estimates of: individuals living below
the poverty line, unemployment, income, individuals without a high school diploma, persons aged 65 or
older, persons aged 17 or younger, civilians with a disability, single-parent households, minority status,
persons who speak English ‘less than well,’ households in multi-unit housing, mobile homes, crowded
households, households with no vehicle available, and persons in group quarters [32]. The SVI assigns a
number between 0 (least vulnerable) and 1 (most vulnerable) to each county to compare relative social
vulnerability concerning community preparedness for and recovery from disasters [32]. We plotted the
relationship between the total number of climate-driven events and SVI and estimated the Spearman
correlation coefficient among affected counties.
3. Results
This analysis found that 56/58 California counties (97%) experienced at least one anomalously warm day in
2018–2019, totaling 2004 county-days or 4.73% of all county-days (figure 1). Wildfire burn zone disasters
were also prevalent, experienced by 57% (n=33) of counties on 1131 county-days. Long power outages
lasting 8+h affected 63% (24/38 counties with available data) of counties for a total of 597 county-days. SVI
values ranged from 0 (no vulnerability, Sierra County) to 1 (highest vulnerability, Imperial County), with a
median of 0.5.
The most common co-occurring event was anomalously warm temperature and wildfire burn zone
disaster days, experienced by 24 (41%) California counties for a total of 144 county-days. 13.2% (5/38
counties with available data) of counties experienced co-occurring anomalously warm temperature and long
power outage days (29 county-days in total), and 18.4% (7/38 counties with available data) counties
experienced co-occurring wildfire burn zone disaster and long power outage days (10 county-days in total)
during the study period. Sometimes, a county experienced more than one wildfire burn zone disaster on the
same day; on 63 d, two different wildfire burn zone disasters occurred in the same county (including in Los
Angeles, Riverside, Shasta, and Ventura counties).
When examining the distribution of total co-occurring events and the variety of co-occurring event
types, four counties exhibited the highest counts in both categories: Mariposa, Shasta, Sonoma, and Ventura
Counties (figure 2). Only Mariposa County experienced all three climate-driven events on the same day (10
August 2018, during the Ferguson Fire). Mariposa County also had the greatest number of co-occurring
events, with 22 co-occurring anomalously warm temperature and wildfire burn zone disaster days during the
study period (supplementary table 1).
Among the top quarter most vulnerable counties in California (SVI >.75), there were 4 county-days
with co-occurring anomalously warm temperatures and wildfire burn zone disasters, 3 county-days with
co-occurring anomalously warm temperatures and long power outages, and 1 county-day with co-occurring
wildfire burn zone disasters and long power outages. Conversely, among the bottom quarter of least
vulnerable counties, there was 1 county-day with all three events (anomalously warm temperature, wildfire
burn zone disaster, and long power outage), in addition to 27 county-days with co-occurring anomalously
warm temperatures and wildfire burn zone disasters, 2 county-days with co-occurring anomalously warm
temperatures and long power outages, and 18 county-days with co-occurring wildfire burn zone disasters
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Environ. Res.: Health 3(2025) 021001
Figure 2. Distribution of daily co-occurring climate-driven events by California county, 2018–2019. AW, anomalously warm; PO,
power outage; WBZD, wildfire burn zone disaster.
and long power outages (supplementary table 2). Among counties lacking power outage data, the average
SVI was 0.573.
We plotted the relationship between county-level number of co-occurring and individual disaster days
and SVI (figure 3). Among the 24 counties experiencing co-occurring anomalously warm and wildfire burn
zone disaster days, we observed a very weak positive correlation between the number of co-occurring disaster
days with SVI (Spearman ρ=0.051), indicating little to no relationship between the two variables. For
4
Environ. Res.: Health 3(2025) 021001
Figure 3. Climate-driven events and the Social Vulnerability Index (SVI). The x-axis is county SVI. The y-axis is number of days.
The dots are counties. The dashed line represents the best linear fit. Panels are labeled with Spearman ρ.
co-occurring wildfire burn zone disasters and long power outage days, we observed a strong inverse
correlation with SVI (Spearman ρ=−0.927), suggesting that a greater number of co-occurring wildfire
burn zone disasters and long power outage days were associated with less county-level vulnerability. For
co-occurring anomalously warm and long power outage days, we observed a strong positive correlation with
SVI (Spearman ρ=0.949), suggesting that a greater number of co-occurring anomalously warm and long
power outage days were associated with more county-level vulnerability. These findings are only suggestive
since our sample size was very small, with 7 counties (29 county-days) and 5 counties (10 county-days),
respectively. We also observed a positive correlation between anomalously warm days and county SVI
(Spearman ρ=0.499), meaning that more anomalously warm days were associated with higher county-level
vulnerability. We did not see a similar relationship between other individual events and SVI (figure 3).
4. Discussion
In this analysis, we examined single and co-occurring climate-driven events at the county-level from 2018 to
2019 in California. We found nearly every (97%) California county experienced anomalously warm
temperatures, with 2004 county-days total over the two-year period. The most common co-occurring events
were anomalously warm temperatures and wildfire burn zone disasters, experienced by 41% of counties on
0.3% of study county-days. We observed an inverse correlation between co-occurring wildfire burn zone
disasters and long power outage days and SVI, a positive correlation between co-occurring anomalously
warm and long power outage days and SVI, and a positive correlation between anomalously warm days and
SVI. We did not observe a relationship between other co-occurring or individual events and SVI.
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Environ. Res.: Health 3(2025) 021001
Prior studies have examined co-occurring events and issues related to environmental justice in
California. Masri et al evaluated warm temperatures and high fine particulate matter concentration
compound risk days in California from 2018 to 2020 and found census tracts with more compound risk days
were more socially disadvantaged using the American Community Survey five-year averaged data from 2018
[19]. Rosenthal et al looked at the co-occurrence of extreme heat and wildfire smoke in California in 2020
and found that 68% of the state experienced days with both extreme heat and extreme smoke at some point
during the year [33]. We did not observe a correlation between co-occurring anomalously warm and wildfire
burn zone disaster days with SVI, but we did find that counties with more anomalously warm days alone
tended to have higher SVI scores. These differences may have arisen because our study was conducted at a
different spatial scale than prior studies (e.g. counties versus census tracts) and because we assessed
co-occurring anomalously warm and wildfire burn zone disaster days, rather than wildfire smoke days. Since
air pollution from wildfire smoke can travel long distances, a greater number of counties would have had
wildfire smoke days than had wildfire burn zone disaster days [2].
We identified 184 county-days with some type of co-occurring climate-driven events in California
between 2018 and 2019. Single events threaten health, but co-occurrences can have multiplicative effects on
adverse health outcomes. Chen et al examined compound extreme heat and wildfire smoke days and
cardiorespiratory hospitalizations in California from 2006 to 2019 and observed a greater risk of
hospitalizations from exposure to both hazards simultaneously compared to exposure to single hazards.
Health effects of compound hazards were stronger among more socially disadvantaged communities [18].
Climate-driven events in communities that are more socially vulnerable are at greater risk of experiencing
negative health outcomes. This study found that disadvantaged areas experienced more co-occurring
anomalously warm and long power outage days, suggesting these communities could have less resilient power
systems that cannot withstand the pressures of extreme heat. We also found that anomalously warm days
occurred more often in areas with higher social vulnerability where individuals are more at risk of suffering
from heat. For example, individuals of low socioeconomic status could live in crowded households or
multi-unit housing that trap heat or lack air conditioning. Given the prevalence and potential health impacts,
public health risk communication is required surrounding co-occurring exposures. Coker et al surveyed
public health agency communications from 2013 to 2023 in Canada and the United States on co-exposure to
wildfire smoke and extreme heat and found that out of 15 online resources, only 2 were from California [34].
Our study had limitations. It was conducted at the county-level, possibly missing within-county
heterogeneity in the three climate-driven events. Further, reliable power outage data was only available from
2018 to 2019 for 38/58 (65.5%) California counties. Therefore, we likely underestimated the number of
co-occurring events. Our analysis did not consider wildfire smoke exposure; therefore, our analysis also
underestimates the number of people impacted by wildfires, an area for potential future research.
Our findings regarding co-occurring climate-driven events direct attention to which counties recently
experienced such events in California. The findings can inform regional resource allocation and other
state-wide planning and policy objectives to reduce adverse effects.
Data availability statement
The data that support the findings of this study are openly available at the following URL/DOI: https://
github.com/brittshea/ca_compound_climate [35].
Acknowledgements
Brittany Shea was supported by the National Institute of Environmental Health Sciences T32ES007322. Joan
A Casey was supported by the National Institute on Aging R01AG071024. The funders had no role in study
design, data collection, analysis, decision to publish, or preparation of the manuscript.
Funding
NIEHS T32 ES007322; NIH R01AG071024.
ORCID iDs
Brittany Shea https://orcid.org/0000-0002-2545-3634
Gabriella Y Meltzer https://orcid.org/0000-0002-6885-9730
Benjamin B Steiger https://orcid.org/0000-0001-8324-9562
Nina Flores https://orcid.org/0000-0002-2723-1728
6
Environ. Res.: Health 3(2025) 021001
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