Understanding the risks of compound climate events and cascading risks

Abstract

Increases in the frequency, intensity, and duration of extreme weather and climate events, such as heatwaves, droughts, fire weather, and flooding, are increasing the likelihood of compound and cascading events. The associated impacts are harming human health and well-being, ecosystems, livelihoods, and communities today, with larger impacts projected with additional climate change unless there are timely and significant investments in adaptation and mitigation. At this moment, risk management is not keeping pace with increasing risks. Investments in transdisciplinary research and implementation, accounting for barriers and capacities, is urgently needed.

Introduction

Human activities have warmed the climate at a rate that is unprecedented in at least 2000 years (IPCC, 2021). This warming is increasing average temperatures, precipitation, and other variables such as sea level rise; and is increasing the frequency, intensity, and duration of extreme weather and climate events in every world region (IPCC, 2021). A human fingerprint is increasingly being identified in individual extreme heatwaves, heavy precipitation, droughts, and tropical cyclones, with some events virtually impossible without climate change (e.g. World Weather Attribution). In essence, the climate system is releasing the additional energy humans are adding from burning of fossil fuels and deforestation. As these events become even more frequent, they are increasing the probability of compound events, including increases in the frequency of concurrent heatwaves and droughts, fire weather, and compound flooding. All have been highly visible in the media over the past few months.

For clarity, it is helpful to differentiate impacts from risks. Impacts are observed consequences of changing weather patterns for human and natural systems. A wide range of observing systems collect data to document the magnitude and pattern of impacts, from the numbers of heat-related deaths to changes in the geographic range of species, although there are very limited data from most low resource settings. Risks are potential future impacts; risks are the probability of a change in a system (from an exposure to a hazard) multiplied by its consequences (IPCC, 2012). Whether a risk from a climate-related hazard materializes as an impact depends on the exposure of the human or natural systems to the hazard, the underlying vulnerability to that hazard, and the capacity to prepare for and manage exposure to the hazard.

Risk varies across temporal and spatial scales

While everyone is exposed to a changing climate, the consequences vary considerably between populations and regions. This is because risk is determined by more than the presence of a hazard; it varies because of differential exposures and/or differential impacts.

Exposure varies across local to national geographic scales, and from population to population. For example, the temperature experienced during a heatwave will differ by neighborhood, with poor and marginalized areas of a city generally hotter than surrounding areas because of fewer trees and blue spaces, and because of more intense urban heat islands from building materials and other factors (Jay et al., 2021). For another example, similar strength tropical cyclones/hurricanes or heavy precipitation events can have different consequences depending on the extent of exposure that is determined by, for example, urban infrastructure or the presence of mangroves.

Differential impacts arise from greater susceptibility of some population groups. For example, there are multiple groups at greater risk during a heatwave, inter alia older adults, individuals with chronic medical conditions or who take certain prescription drugs, pregnant women, infants, and outdoor workers (Ebi et al., 2021). Understanding these susceptibilities can inform effective targeted interventions to protect more vulnerable groups. Impacts also are determined by the timeliness and effectiveness of interventions within the context of achieving the Sustainable Development Goals.

Compounding hazards

For most of human history, extreme events were rare occurrences, with sufficient time between events for recovery, with the goal of restoring the community or ecosystem to its original state. Floods, wildfires, and cyclones now routinely affect the same region over shorter time periods, reducing the time available for response and recovery—and raising questions about the wisdom of trying to restore to the original and now more vulnerable state versus building communities and ecosystems that are more resilient to future weather patterns.

Among many critical research gaps is understanding the extent to which compound and sequential extreme weather and climate events alter vulnerability to future extremes. Typhoons and floods can destroy critical infrastructure that can take years to rebuild. Drought and heatwaves can increase the likelihood of wildfires, which then can destroy infrastructure and ecosystems that increase vulnerability to future extremes. For example, in 2023, Vanuatu was hit by two category four tropical cyclones in 24 h, followed by a category five tropical cyclone a few months later. A year later, life was “still hard” (RNZ 4 March 2024).

Less extreme events can be as devastating in regions with high vulnerability. Population growth and more people moving into harm's way compound the impacts of extreme events (IPCC, 2012).

Unfortunately, there is limited understanding of the magnitude and pattern of impacts of compounding events, in part because data from surveillance and monitoring systems are generally designed to record impacts from individual events. The impacts from a heatwave are captured separately from the numbers adversely affected by the smoke from wildfires that were exacerbated by the high temperatures. Better linkage is needed across surveillance and monitoring systems to provide a more comprehensive picture of the impacts to human and natural systems.

Projections indicate future changes in extreme events could be dramatic, with changes past mid-century depending on the extent of reduction of greenhouse gas emissions. An extreme temperature event that occurred once in 50 years during the period 1850–1900 is now 4.8 times more likely (IPCC, 2021). At 1.5 °C above preindustrial temperatures, the same extreme event is projected to be 8.6 times more likely and 2 °C hotter. At 2 °C above preindustrial temperatures, the same event is projected to be 13.9 times more likely and 2.7 °C hotter. These increased likelihoods also increase the probability of compound events.

In the US, the National Oceanographic and Atmospheric Administration (NOAA) tracks annual billion-dollar disasters. In 2023, there were 28 weather and climate disasters costing about USD 93 billion in insured costs; the previous record was 22 in 2020 (Smith, 2024). 2023 was the fourth consecutive year in which 18 or more separate billion-dollar disaster events impacted the US. The average annual number of events over 1980–2023 was 8.5 events; the annual average for the past five years was 20.4 events. These events exclude heatwaves because they rarely result in insured losses; including them would have increased the numbers of events particularly because of the high temperatures during 2023, including the hottest July and August ever recorded. It would be useful to have monitoring of extreme compound events.

Cascading risks

Climate change is a threat multiplier, interacting with societal vulnerabilities to exacerbate current or create new risks that then can compound and cascade through societies, such as extreme temperatures resulting in droughts that lead to crop failures and undernutrition, which then increase vulnerability to infectious diseases (Semenza et al., 2022). Decreased worker productivity because of high temperatures can then affect livelihoods and higher food prices can affect mental health and economic productivity. Similarly, floods, storms, and tropical cyclones can have economy-wide consequences, including creating breeding grounds for mosquitoes that then lead to disease outbreaks, such as dengue fever.

Warming temperatures are increasing the geographic range and seasonality of the mosquito genus (Aedes) that can carry dengue fever, zika virus, yellow fever, and chikungunya, which can increase the intensity of transmission (Lancet, 2024). Urbanization furthers these trends. Outbreaks can overwhelm healthcare services, reducing their ability to address other population health issues, from vaccination of children to providing medication for hypertension and other chronic disease. Outbreaks also can impact economies. For example, 2024 is the worst year on record for dengue cases, with over 10 million reported from 176 countries, with the Americas accounting for most, more than 24,000 severe cases, and over 6500 deaths (Lancet, 2024). Reported cases, which significantly underestimate the total number of cases, increased tenfold over the past 20 years. A detection and attribution study in Central and South America and East Asia concluded that climate change over 1995–2014 increased the incidence of dengue by 18% (12–25%), with projections suggesting future warming could increase dengue incidence by 40–57% by mid-century (Childs et al., 2024). Romanello et al. (2023) estimated that rising temperatures alone increased the global transmission potential of dengue by more than 42% between the 1950s and 2010s. A comprehensive, global-scale synthesis of the economic costs of dengue over 45 years estimated the minimum cumulative reported cost estimate was USD 94.7 billion (2022 dollars). This suggests a 14-fold increase in costs, with an average annual expenditure of USD 3.1 billion, with a maximum of USD 20.3 billion (Roiz et al., 2024). Health system preparedness is lagging the consequences, resulting in preventable suffering.

Risk management is not keeping pace with the increasing risks

Unfortunately, efforts to increase climate resilience are not keeping pace with the level of risks, resulting in increasing impacts (IPCC, 2022). As detailed in international and national adaptation assessments, there are many possible feasible and effective interventions to reduce regional and sectoral risks (e.g. IPCC, 2022). A key approach is conducting impact, vulnerability, and adaptation assessments to inform developing local or national adaptation plans. Working with stakeholders, adaptation processes can identify multi-sectoral solutions to manage climate risks while reducing social inequities. Their implementation depends upon the capacity and effectiveness of decision-making.

A valuable tool for managing compounding and cascading risks is conducting stress testing. Stress tests are desk-based scenario exercises that focus on events and conditions outside the range of historic experience that could result in significant harm to humans or nature (Ebi et al., 2018). Stakeholders from relevant agencies and organizations, working with impacted communities, use a stress test to identify urgent and immediate changes in policies and programs to begin reducing vulnerabilities to further extreme changes in weather patterns. Limited experience with stress tests in health and other sectors highlight opportunities for increasing resilience (Berry et al., 2024).

Path forward

The near future will be characterized by increasing compounding hazards and cascading risks. Coordinated transdisciplinary research and implementation are urgently needed to identify effective and efficient interventions tailored to local risks, accounting for barriers and capacities. As with mitigation, the best time to have ramped up investment was decades ago. The second best time is now.