Focus on Compound Events

Allison R. Crimmins; Deepti Singh

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Focus on Compound Events

Climate change is increasing the chances of multiple climate hazards occurring simultaneously or consecutively across the US and its territories. Such interactions between multiple hazards across space or time, known as compound events, exacerbate the societal and ecosystem impacts of individual hazards and hinder the ability of communities, particularly frontline communities, to respond and cope. Therefore, infrastructure design, planning, governance, and disaster preparedness for compound events are critical for building resilient systems.

What Are Compound Events?

Compound events result from the occurrence of multiple climate drivers or hazards either in an individual location or across multiple locations that, when combined, have greater impacts than isolated hazards on ecosystems, water resources, public health, energy infrastructure, transportation, food systems, and interconnected societal networks, often straining disaster response.¹^,²^,³ Compound events can also result from the intersection of climate hazards with other environmental hazards like pollution, non-climate hazards such as wars and pandemics, or socioeconomic stressors like poverty and lack of adequate housing that disproportionately impact overburdened communities, thereby deepening existing societal inequities (e.g., KMs 5.2, 18.2, 20.1, 23.1, 27.1, 29.2). Compound events are broadly categorized as:

Multivariate: co-occurring hazards in a location, such as simultaneous precipitation deficits and extreme heat that contributed to the severe Pan-Caribbean 2013–2016 drought⁴
Temporally compounding: successive hazards in a location, such as destructive wildfires in 2017 followed by heavy rainfall on burned landscapes in January 2018 that resulted in mudslides and debris flows, damaging ecosystems and infrastructure (KMs 3.5, 5.2, 6.1, 27.2)
Spatially compounding: similar or disparate hazards occurring simultaneously or within a short time window in multiple locations that are connected by physical processes or complex human and natural systems, such as simultaneous megafires across multiple western states and record back-to-back Atlantic hurricanes in 2020 that caused unprecedented demand on federal emergency response resources (Figure F1.1)⁵
Preconditioned: extreme events superimposed on long-term trends, such as higher sea levels, heavier precipitation, and/or changing storm seasonality causing more frequent and severe coastal flooding (KMs 4.2, 9.1, 30.1),⁶^,⁷ like during Hurricane Florence (2018) in the Southeast (KM 22.1),⁸ Typhoon Surigae (2021) in Palau, and Typhoon Merbok (2022) in Alaska (KM 29.1)⁹
Complex events: non-climatic stressors that exacerbate climate hazards, such as COVID-19, which exacerbated climate-driven food, water, and livelihood insecurities facing Tribes, Indigenous Peoples, and other frontline communities (KMs 5.2, 16.1; Focus on COVID-19 and Climate Change)

Art × Climate

Andrea Ruedy Trimble
Under Pressure
(2023, ink and watercolor on paper)

Artist’s statement: “Under Pressure” represents the stress that a changing climate is placing on our built environment. Heat, depicted with warm colors, descends upon the city, increasing in intensity. The blue of the rising flood waters meets the heat, resulting in compounding consequences. This piece also represents the pressure that we need to put on ourselves to respond. The intersection of natural elements - water, light, and trees - emerging from human-made infrastructure elements, signifies hope that we can quickly act to reduce emissions and transition towards more nature-based, renewable solutions to improve quality of life for all.

View the full Art × Climate gallery.

Artworks and artists’ statements are not official Assessment products.

Recent Events

Compound events have resulted in multiple recent disasters across several US states. The following examples illustrate their cascading societal impacts (Figure F1.1):

Heat, drought, and wildfires: A series of compound events between 2020 and 2021 stressed communities and ecosystems across the western US and caused economic damages exceeding $38.5 billion (in 2022 dollars; KMs 27.2, 28.1, 28.2, 28.4).¹⁰ In 2020, co-occurring heat and drought caused concurrent destructive fires across California, Oregon, and Washington¹¹ that resulted in infrastructure and property damage and human fatalities, threatened access to energy and water supplies, and strained firefighting resources.¹⁰ Millions of residents were exposed to harmful pollutants in wildfire smoke, affecting public health and worsening COVID-19 related mortality.¹²^,¹³^,¹⁴ Drought persisted into 2021 and amplified the record-breaking Northwest heatwave,¹⁵ killing over 229 people in the US. Co-occurring heat, drought, low streamflow, and low tides in 2021 triggered toxic algal blooms and mass die-offs of shellfish and low survival of salmon, species important to Indigenous communities and the West Coast economy (KM 27.2; Figure 10.2).¹⁶ West Coast crab fishery revenue losses were exacerbated by management actions implemented during earlier marine heatwaves.¹⁷
Compound flooding: Back-to-back storms affected the Northeast in 2021, resulting in 55 deaths and more than $21.4 billion (in 2022 dollars) in damages (KM 21.1). On August 22, Hurricane Henri brought intense rainfall to the Northeast (7 inches in New York City, including 2 inches in one hour in Central Park) that caused $749 million (in 2022 dollars) in damages despite mild winds. On August 29, Hurricane Ida, which made landfall as a Category 4 in Louisiana, moved northeast, and delivered record rainfall during September 1–2 to already-saturated Northeast soils, causing catastrophic flooding. This temporally compounding event was about 30 times more deadly and more damaging than Hurricane Henri alone, straining local governance and emergency management systems.¹⁸

Will Compound Events Increase with Climate Change?

Compound events are expected to become more frequent with continued climate change (e.g., KMs 2.2, 9.1). The increasing frequency and severity of climate hazards such as extreme heat, heavy precipitation, and severe storms are projected to increase the chances of 1) a sequence of hazards occurring within a short time span and 2) simultaneous independent events in a location or multiple locations. For instance, increasing swings from dry-to-wet extremes in western states and Pacific Islands will increase the chances of intense rains on parched or recently burned landscapes, increasing risks of postfire flash flooding, debris flow, and contaminated drinking water supplies (KMs 4.2, 30.1).¹⁹^,²⁰^,²¹ Climate change is also expected to alter the physical drivers of compound events. For instance, more frequent extreme La Niñas²² would simultaneously elevate the risk of western US droughts and back-to-back severe Atlantic hurricanes, increasing the chances of compounding disasters similar to the 2020 season (Figure F1.1).²³^,²⁴ Changes in weather patterns such as more frequent atmospheric high-pressure systems could increase the risk of co-occurring heat, drought, and marine heatwaves.²⁵^,²⁶

How Can We Adapt?

Low-income communities, communities of color, Tribes and Indigenous Peoples experience high exposure and vulnerability to climate hazards due to their proximity to hazard-prone areas, infrastructure deficits, limited disaster-management resources, and governance challenges, which are legacies of colonialism, redlining, and other discriminatory policies (KMs 4.2, 16.2, 18.2, 20.1).²⁷ Consequently, these communities could face higher risks through complex event outcomes, which can magnify existing disproportionate health risks (KM 15.2). Transformative, socially just adaptation approaches (KM 31.3), investment in emergency preparedness, and governance structures that account for the inequitable distribution of climate impacts can avoid further exacerbating such existing social disparities (KMs 12.4, 20.3, 31.2).²⁸^,²⁹^,³⁰ Incorporating compound event risks in infrastructure design standards and regulations, updating aging infrastructure, and planning at the community-level can improve climate resilience and protect against risks like displacement and gentrification.²⁸^,³¹^,³² Despite the availability of tools to evaluate infrastructure alternatives (e.g., Helgeson et al. 2020³³), communities with limited adaptation resources face significant challenges in making such investments.

Resource allocation toward solutions that address multiple community resilience objectives can address some of these challenges.³⁴^,³⁵ For example, blue-green infrastructure—use of green-areas and water bodies in urban planning—and tidal marsh restoration can sequester greenhouse gas emissions and protect against floods while also providing ecosystem services like reducing heat and air pollution, creating recreational spaces, and advancing environmental justice in urban environments (KM 12.3).³⁶ Enhanced monitoring of adaptation actions, scenario planning activities (e.g., Gerlak et al. 2021³⁷), and sharing best practices among stakeholders can alleviate planning challenges and improve management of the growing risk of compound events.

Timeline, icons, satellite image, and text illustrate compound events. At top, a timeline titled temporal compounding of events in 2020 and 2021 shows months from August to December 2020 and from June to December 2021. West Coast events are described at the top. Beginning in August 2020, the West Coast saw severe drought, extreme heat, and widespread lightning, causing multiple wildfires that covered large portions of the western US with harmful air pollutants from heat and smoke. In September and October, strong offshore winds and drought facilitated the growth of multiple wildfires in Oregon and Washington. Starting in June 2021, drought on the West Coast led to poor streamflow conditions and harmful algal blooms and exacerbated a record-breaking heatwave that killed more than 1,400 people. At the same time, heat and low tides killed shellfish and impacted other marine species. Persistent drought contributed to another destructive wildfire season in July through September of 2021, which was followed in October by an atmospheric river that caused flooding and mudslides in burned and drought-affected areas. On the East Coast, starting in August 2020 a series of record-setting storms in the Atlantic (named Isaias, Laura, Sally, Teddy, Delta, Zeta, and Eta) delivered strong winds, heavy rainfall, and storm surge that led to widespread flooding, power outages, and infrastructure damage. In summer and fall of 2021, the storms Elsa, Henri, Nicholas, and Ida delivered more of the same, with Ida hitting communities affected by 2020 hurricanes. Nationwide, starting in 2020 the COVID-19 pandemic limited access to, and the response capacity of, hospitals, cooling centers, and evacuation centers. At bottom, a satellite image of North America illustrates spatially compounding climate-related disasters on September 15, 2020. On the West Coast, large wildfires in Oregon, Washington, and California damaged homes and infrastructure. Wildfire smoke in western states caused weeks of bad air quality across the West. In the East, Hurricane Sally, the 18th named Atlantic storm of 2020, was about to make landfall. It brought widespread flooding, infrastructure damage, and power outages to Louisiana, Alabama, and Florida.

Compound events have amplifying impacts on ecosystems and human communities and affect their capacity to respond.

Figure F1.1. (a) The timeline shows temporally compounding events in 2020–2021 on the West and East Coasts and their cascading impacts on communities and ecosystems. (b) The satellite image shows simultaneous disasters—multiple wildfires in the US West and Hurricane Sally in the Southeast. The orange and red colors show wildfire smoke traveling across the US. Figure credit: Washington State University Vancouver. See figure metadata for additional contributors. Satellite image credit: Joshua Stevens, NASA Earth Observatory, using GEOS-5 data courtesy of NASA GSFC and VIIRS data courtesy of NASA EOSDIS/LANCE and GIBS/Worldview and the Suomi National Polar-orbiting Partnership.

Authors

Federal Coordinating Lead Author: Allison R. Crimmins, US Global Change Research Program
Chapter Lead Author: Deepti Singh, Washington State University Vancouver

Contributors

Technical Contributors: Justin M. Pflug, University of Maryland, College Park, Earth System Science Interdisciplinary Center; Patrick L. Barnard, US Geological Survey; Jennifer F. Helgeson, National Institute of Standards and Technology; Andrew Hoell, NOAA Physical Sciences Laboratory; Fayola H. Jacobs, University of Minnesota; Michael G. Jacox, NOAA Southwest Fisheries Science Center and NOAA Physical Sciences Laboratory; Alessandra Jerolleman, Jacksonville State University; Michael F. Wehner, Lawrence Berkeley National Laboratory
Review Editor: Alton P. Williams, University of California, Los Angeles
USGCRP Coordinator: Christopher W. Avery, US Global Change Research Program / ICF

Recommended Citation

Singh, D., A.R. Crimmins, J.M. Pflug, P.L. Barnard, J.F. Helgeson, A. Hoell, F.H. Jacobs, M.G. Jacox, A. Jerolleman, and M.F. Wehner, 2023: Focus on compound events. In: Fifth National Climate Assessment. Crimmins, A.R., C.W. Avery, D.R. Easterling, K.E. Kunkel, B.C. Stewart, and T.K. Maycock, Eds. U.S. Global Change Research Program, Washington, DC, USA. https://doi.org/10.7930/NCA5.2023.F1

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Description of Evidence Base and Research Gaps

There is broad agreement across the physical and social sciences and engineering communities that compound events are a growing threat to communities, sectors, emergency management, insurance companies, and interconnected societal systems. Recent studies have developed frameworks for studying compound events and quantified future changes in risks in several types of compound events such as hot and dry or hot and humid extremes, compound coastal and fluvial flooding, drought and marine heatwaves, marine heatwaves and ocean acidity extremes, and wildfires followed by heavy rainfall. Identification of various types of compound events has grown in recent years. Compound events are rare, and thus the short observational record for many climate variables limits the ability to quantify historical changes, characterize present-day probabilities, and evaluate the ability of climate models to simulate them. There are also gaps in the scientific understanding of the range of physical processes that lead to various types of compound events affecting many regions, communities, and sectors. Literature on their societal impacts is even more limited and challenging to quantify because compound events are still relatively rare and result from a complex set of factors. Together, these result in uncertainties and low confidence in estimates of projected changes in their risks.

Compound events span a wide variety of physical phenomena, societal impacts, and different research communities with different needs, requirements, and impacts. There is a diversity of definitions of compound events, and much of the literature consists of case studies. Compound events of multiple weather and climate variables are often treated by combining those variables into a single metric. For instance, the literature contains multiple formulations of the combined effect of high temperatures, humidity (or aridity), and winds (or stagnancy) on human health and fire risk. While such univariate formulations are more amenable to standard analysis techniques, the richness of the multivariate space can be lost. Advanced multivariate extreme statistical analysis tools have not seen widespread adoption by the scientific community. The recent development of large climate model ensembles combined with event identification analysis tools offers the opportunity to increase our understanding of the physical processes that lead to compound events and to evaluate their historical and future risks under different warming levels.

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Likelihood

Virtually Certain	Very Likely	Likely	As Likely as Not	Unlikely	Very Unikely	Exceptionally Unlikely
99%–100%	90%–100%	66%–100%	33%–66%	0%–33%	0%–10%	0%–1%

Confidence Level

Very High	High	Medium	Low
Strong evidence (established theory, multiple sources, well-documented and accepted methods, etc.) High consensus	Moderate evidence (several sources, some consistency, methods vary and/or documentation limited, etc.) Medium consensus	Suggestive evidence (a few sources, limited consistency, methods emerging, etc.) Competing schools of thought	Inconclusive evidence (limited sources, extrapolations, inconsistent findings, poor documentation and/or methods not tested, etc.) Disagreement or lack of opinions among experts