- The impacts of compound event can be greater than the sum effects of individual events.
- New data and modelling highlight the challenges associated with compound events. Some effects of compound events are cumulative and may lead to new equilibrium states of the impacted systems.
- Risk-management planning should account for compound events, especially through integrated assessments to reveal natural or human-induced linkages among seemingly disparate risk factors.
Compound events are defined as events that occur when a combination of drivers and/or hazards contribute to environmental or societal risks (Figure 6). These phenomena span scales and interaction types, including phenomena that enable others, those involving multiple variables and those that occur in sequence and across different spaces. Examples include the heavy rain, extreme wind and storm surge from Hurricane Sandy in 2012, and the sequential storms in California’s winter 2023 where metres of snow entrapped some residents for weeks.
Physical science research on compound events mostly began with atmospheric and bivariate events, such as drought–heatwave interactions. Researchers have since adapted the concept to other domains, including terrestrial ecosystems, the ocean and inter-domain linkages. An array of methodological approaches, such as large ensembles and extreme event attribution, are confirming the relevance of compound events to a wide range of potential areas of impact. For example, compound events pose critical risks to food security and ecosystem services, complicate disaster risk management, interfere with adaptation strategies and affect human migration patterns.
Crops are particularly sensitive to the simultaneous occurrence of extreme hot and dry conditions. Variability, such as an early spring followed by a late frost, can also harm crops. Given that a large proportion of crops are grown in just a few breadbasket regions, low yields in the same harvest in more than one region could threaten global food security.
Ecosystems are threatened by compounding impacts. Plant recovery usually lags after extreme events, which in turn increases vulnerability to another (compound) event, as well as, among other consequences, limiting vegetation’s capacity to act as a carbon sink (see Insight 4). The interaction of effects from separate events, such as cyclones and fires, can alter equilibrium ecological states. Compound ocean events, such as marine heatwaves, changes in oxygen concentration, ocean acidity and/or net primary production, can impact marine ecosystems at individual, population and community levels. Compound events across the land–ocean continuum, such as severe droughts in South America and marine heatwaves in the South Atlantic in 2013/14 led to water shortages in Brazil and impacted food supply globally.
“Compound event thinking” improves early warning, emergency response, infrastructure management, long-term planning and capacity-building. So far, however, few adaptation efforts sufficiently consider compound events. This is often due to a lack of knowledge about the physical system that forms compound events, the difficulty in translating existing knowledge into action as well as the fact that most adaptation and early warning systems are structured around single hazards.
Identifying and quantifying compound events will require understanding how discrete climate hazards interact with and intensify each other. Improved models and statistical methods are revealing that the impacts from compound events are likely to exacerbate each other, in part because of longer recovery timescales. This interconnectedness implies the need for cooperation at the scales of compound event impacts, which are often larger and longer than what the existing decision-making frameworks encompass. Local preconditions appear to shape compound event impacts, whether societal (e.g. migration, poverty, conflict) or environmental (e.g. scarce resources, overfishing, denuded landscape), making those context features of crucial importance to assess and incorporate.
Extreme factors can amplify each other
The last few years have seen exceptional climatic and extreme weather events far outside the previous local historical range, with severe socio-ecological impacts. Events are being connected to combinations of antecedent and/or simultaneous drivers that only together were able to achieve the observed conditions. For example, researchers now interpret the heatwaves in western North America in June 2021 as the integrated outcome of multi-scale processes including atmospheric ridging, low soil moisture and latent heating from upwind precipitation. Even when occurring in a single region, these exceptional events can be compounded by the heightening of multiple types of impacts simultaneously: e.g. simultaneous heat stress, wildfire risk and air pollution, or heat-drought and heat-flood linkages.
The same can also happen at sea. Some of the devastating impacts of the North-East Pacific 2013–2015 marine heatwave and accompanying extreme high sea surface temperature – including extreme mortality and reproductive failure of sea birds, mass stranding of whales and sea lions, and shifts in species composition towards warm-water species – were amplified by co-occurring extreme events including ocean acidity, low oxygen, and crop failures in several major agricultural regions simultaneously leading to price shocks and food shortages. Increasingly, these types of ocean-based events are also co-occurring with land events, multiplying the impacts.
Implications & Recommendations
- Adaptation planning and risk management at any scale should incorporate an assessment of the likelihood and consequences of discrete events becoming compound events. To this end, the perspectives of people who have previously been affected by natural hazards should inform the development of targeted intervention points and anticipatory action, including early warning systems.
- Anticipatory action and risk management of compound events should not only encompass but transcend the geographical and temporal scales at which these events often occur. For example, through development of international and intersectoral climate finance mechanisms or cooperation agreements for sharing supplies and personnel, ideally among regions or sectors with a comparatively small risk of simultaneous disaster.
- Emergency preparedness and risk-management planning should always account for local preconditions. It should recognise that global attention to and the distribution of data on compound events is not representative, as well as be aware of potential avoidance among decision-makers of discussing future compound or multi-hazard events and associated response mechanisms (as was the case of the flood planning for New York in the aftermath of Hurricane Sandy).
- Allocation of adaptation investments should reflect the geographically unequal distribution of the impacts of compound events. For example, investment cost/benefit analyses based on single events in isolation (e.g. a severe storm affecting a province) should also consider additional emergent risks in case of multiple such events in close proximity. Relative to single events, the impacts of compound events tend to more heavily interact with underlying infrastructure and socio-economic systems, creating a special need for well-coordinated anticipatory and response actions (ensuring, for example, that flood risk reduction efforts in one area do not increase flood risk in a neighbouring area).