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Article | WTW Research Network Newsletter

On co-occurring and cascading hazards

By Daniel Bannister | March 20, 2023

How will climate change alter the (inter)dependency between individual and interrelated hazards?
Environmental Risks|Property Risk and Insurance Solutions|Willis Research Network
Climate Risk and Resilience|Insurer Solutions|Risk Culture

Under the background of ongoing global climate change, we traditionally think of how the risk, vulnerability, and exposure of a society changes to individual hazards: it is well known that extreme events (like maximum day-time temperatures, rainfall, and wildfires) are projected to increase in frequency and in magnitude, and it is relatively easy to model and account for such future trends in risk management. But what happens when natural hazards and disasters co-occur (events which occur simultaneously) or cascade (events which lead to, or are a consequence of, another)? How will climate change alter the (inter)dependency between individual and interrelated hazards? And since pricing physical climate risks is already uniquely challenging, how can we better integrate co-occurring and cascading hazards into existing and future disaster risk management and decision-making frameworks?

Nature’s complexity

Traditional risk assessment frameworks frequently use statistical methods and techniques to identify and isolate historical trends in the trigger, magnitude, or the frequency of an individual hazard, often from (patchy and discontinuous) observational datasets. While this captures the risk one hazard at a time, it does not adequately capture the risk associated with co-occurring or cascading hazards.

For instance, alone, an individual but prolonged drought or heat wave can trigger significant socioeconomic effects. However, droughts and heat waves can also trigger and intensify wildfires, which in themselves further trigger other cascading hazards. The complex, interrelated nature of extreme events has the potential to turn otherwise moderate events into disasters (AghaKouchak et al., 2018).

Landslides: a cascading hazard

Landslides are a major hazard that have the potential to cost millions of dollars in damage, and cause thousands of fatalities, across the world every year. While the primary trigger for a landslide is usually heavy rainfall or seismic activity, they are also frequently a consequence of two or more consecutive hazards, such as extreme rainfall over a burned area (Li et al., 2022, see Figure 1).

Schematic diagram depicting events leading to a landslide: drought, followed by wildfires, followed by intense precipitation.
Schematic diagram depicting events leading to a landslide: drought, followed by wildfires, followed by intense precipitation.

Credit: Tierney Acott/Institute of Sustainability and Energy at Northwestern

A recent example of this is the mudslides in Southern California in September 2022. The remnants of Tropical Storm Kay brought heavy rainfall to a region that had been scorched by a wildfire two years prior, leading to a mudslide which destroyed and damaged buildings, infrastructure, and vehicles. Since climate change has the potential to increase the likelihood of both wildfires and heavy rainfall events, how can we more accurately capture the co-occurrence in the trigger, magnitude, and frequency of both these hazards to better constrain immediate and future landslide risk? An added complexity to this question is also how individual but consecutive wildfires and individual but consecutive storms will also change in the future: are consecutive wildfires correlated and does one wildfire precondition the environment for subsequent wildfires, and how does climate change and natural climate variability influence storm clustering?

Such examples emphasise how the non-linear (spatial and temporal) dependency and causal sequences between superficially individual extreme events makes them challenging not only to study but also to model in existing disaster risk management frameworks. This leaves society ill-prepared for co-occurring and cascading hazards.

Spotlight on co-occurring and cascading hazard research

To close the gap between risk management and business decision making frameworks and processes with the risks posed by complex natural hazards, we need to improve our fundamental understanding and modelling of the interrelated nature, characteristics, and trigger mechanisms of co-occurring and cascading hazards. In 2022, the WRN has supported numerous academics, across the atmospheric and geophysical hazard space, considering various angles of co-occurring and cascading hazards. These include:

  • James Carruthers, Dr Selma Guerreiro, & Professor Hayley Fowler (Newcastle University): “High resolution climate modelling of extreme weather hazards and application to the analysis of risks relating to climate change” – James’ PhD project will involve designing relevant metrics that capture the frequency and severity of storm and flooding hazards from high-resolution climate model outputs for global-scale diagnostic risk and impact assessment
  • Dr Hannah Bloomfield (University of Bristol): “Compound wind and flood risk over Europe” – Hannah has been exploring the current and future joint risk between heavy rainfall and strong wind events when they co-occur across Europe, with her journal article published earlier this year (Bloomfield et al., 2022)
  • Dr Matt Priestley, Professor David Stephenson, & Professor Adam Scaife (University of Exeter): “Climatic drivers of extreme European windstorms” – The research at Exeter University will look at how European windstorm activity can be predicted on a seasonal to decadal timescale, and how climate change and natural variability is affecting the number, timing, and frequency of European winter storms
  • Dr John Hillier (University of Loughborough): “ROBUST - Enabling better management of UK multi-hazard risk” – ROBUST builds on Hillier et al. (2015), which identified that the most damaging flooding and extreme wind events in the UK tend to occur in the same season but not necessarily during the same storm. ROBUST will further contribute to providing a better understanding of multi-hazard risk and how this may change with climate change to quantify likely future losses and impacts

In 2023, we will continue to coordinate with our existing and new academic partners (including the National University of Singapore), as well as with our clients, to develop event sets of co-occurring and cascading hazards and to define the rigorous techniques required to quantify and mitigate them.

The WRN also supports projects on interconnecting risks and how physical hazards impact critical infrastructure and organisational systems. For instance, a project with Mitiga Solutions (“Volcanic Ash Risk Transfer in Aviation (VOLARISK)”) aims on providing a global, high-resolution probabilistic view of volcanic ash risk for the aviation industry to understand how this individual hazard has the potential to disrupt infrastructure (e.g., aircraft maintenance and safety), organisational (e.g., supply and procurement chains), and technological (e.g., flight operations) systems. Such work can provide a better understanding of how the co-occurrence or cascading nature of two or more hazards also intersects the human and socioeconomic systems.

Thinking ahead

Climate scientists understand that individual extreme events are increasing (in terms of their magnitude and frequency) due to climate change. While the risk posed by individual hazards will increase in a warmer world, so too will the associated risks of co-occurring and cascading hazards. Therefore, it is apparent that there is a clear gap in research, and in the application into disaster risk management, in how one extreme event may alter all other interrelated hazards (whether that is via co-occurring and cascading, simultaneous or asynchronous, occurrence) and how this further disrupts socioeconomic systems. The research the WRN supports on the nature and dependencies of these events enables the community to identify appropriate datasets, methodologies, and technical approaches to analyse, simulate, and estimate the risk, vulnerability, and exposure of a society for accurate and decisive risk management and insurance purposes.


AghaKouchak, A., Huning, L.S., Chiang, F., Sadegh, M., Vahedifard, F., Mazdiyasni, O., Moftakhari, H. and Mallakpour, I., 2018. How do natural hazards cascade to cause disasters?.

Bloomfield, H., Hillier, J., Griffin, A., Kay, A., Shaffrey, L., Pianosi, F., James, R., Kumar, D., Champion, A. and Bates, P., 2022. Co-Occurring Wintertime Flooding and Extreme Wind Over Europe, from Daily to Seasonal Timescales. Available at SSRN 4174051.

Hillier, J.K., Macdonald, N., Leckebusch, G.C. and Stavrinides, A., 2015. Interactions between apparently ‘primary’ weather-driven hazards and their cost. Environmental Research Letters, 10(10), p.104003.

Li, C., Handwerger, A.L., Wang, J., Yu, W., Li, X., Finnegan, N.J., Xie, Y., Buscarnera, G. and Horton, D.E., 2022. Augmentation of WRF-Hydro to simulate overland-flow-and streamflow-generated debris flow susceptibility in burn scars. Natural Hazards and Earth System Sciences, 22(7), pp.2317-2345.


Weather & Climate Risks Research Lead
WTW Research Network, WTW
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