On a Tuesday morning in July 2023, three things happened simultaneously in Phoenix: temperatures hit 118°F, parts of the electrical grid failed, and a flash flood warning was issued for the Salt River basin. Emergency managers had plans for heat events. They had plans for flooding. They had no plan for both at once.
This is the compound risk problem, and it is one of the most important — and least understood — challenges in urban resilience planning.
The Compound Risk Gap
Most municipal hazard mitigation plans address risks in isolation. A flood section, a wildfire section, a heat section. But climate change is increasing the probability that multiple hazards will interact, either simultaneously or in rapid sequence.
Risk scores on a 0–100 scale, 100 = highest risk. Source: Urban Insight Group Climate Vulnerability Database.
The radar chart above shows the compounding nature of risk: not one hazard is declining, and most are accelerating faster than existing infrastructure was designed to handle.
Why Compound Risks Are Worse Than the Sum of Parts
When hazards interact, the impacts are often multiplicative rather than additive. Consider:
Heat + Power Outages. High temperatures increase electricity demand for cooling. Sustained heat events can overwhelm grid capacity, leading to outages precisely when cooling is most needed. The people most at risk — elderly residents, those with medical conditions, low-income households without backup cooling — are also least able to self-protect during an outage.
Wildfire + Air Quality + Respiratory Illness. Smoke from wildfires hundreds of miles away has degraded air quality in major cities for days at a time. This is a new form of risk that most urban planners have not incorporated into their frameworks.
Flooding + Wastewater. Combined sewer overflow events — where stormwater overwhelms sewage capacity during heavy rain — are already common in many older cities. Climate change is making these more frequent and more severe, creating public health risks that compound flooding damage.
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FEMA's National Flood Insurance Program rates are based on historical flood frequency data. These rates do not reflect the accelerating pace of climate change and systematically understate risk in rapidly warming regions.
The Equity Dimension
Compound risks are not distributed equally. Research consistently shows that lower-income neighborhoods and communities of color face higher exposure to multiple overlapping hazards and have lower adaptive capacity.
Average number of climate hazards rated 'high risk' per census tract. Urban Insight Group analysis, 2025.
The pattern is not coincidental. Low-income neighborhoods were often sited in flood-prone or industrially contaminated areas. They have older housing stock less able to withstand extreme weather. They have less tree canopy to mitigate urban heat island effects. They have less political power to secure protective infrastructure investment.
A Framework for Compound Risk Planning
Over the past five years, our team has developed and refined a compound risk planning framework that has been applied in seven municipalities. The core steps are:
1. Multi-Hazard Exposure Mapping
Rather than mapping each hazard separately, we create composite vulnerability indexes that overlay multiple risks at the census tract level. This reveals priority areas where multiple hazards converge — what we call "risk hotspots."
2. Scenario Stress Testing
We develop 3–5 compound risk scenarios — specific combinations of hazards, timing, and intensity — and test the community's infrastructure and social systems against each. This reveals which systems are most fragile and where investment is most needed.
3. Adaptive Capacity Assessment
Adaptive capacity is not just about infrastructure. It includes social networks, economic resources, institutional capacity, and health. We assess all of these dimensions to identify where investment in social resilience is as important as physical infrastructure.
4. No-Regrets Investment Strategies
Given deep uncertainty about how climate risks will evolve, we prioritize investments that provide resilience value across a wide range of scenarios — what we call "no-regrets" strategies. Green infrastructure, for example, reduces urban heat and manages stormwater and improves air quality and provides community amenity.
"The communities that will fare best are not necessarily the wealthiest or the most technically sophisticated — they are the most adaptive."
Moving from Assessment to Action
The biggest challenge in resilience planning is not identifying risks — it is closing the gap between assessment and implementation. Too many resilience plans sit on shelves.
The strategies that work:
- Embed resilience in capital planning. Every major infrastructure investment decision should be evaluated through a climate lens.
- Create dedicated resilience funding streams. Without dedicated funding, resilience investments compete with and typically lose to routine maintenance.
- Build cross-departmental capacity. Resilience cannot be owned by a single department. It requires coordination among public works, planning, emergency management, public health, and utilities.
- Prioritize frontline communities. An equity-first approach is both morally correct and practically smart — high-risk, low-capacity neighborhoods are where resilience investment creates the most value.
Elena Vasquez is Director of Environmental Planning at Urban Insight Group and leads the firm's climate resilience practice.