Extreme Rain Heads for California’s Burn Scars, Raising the Risk of Mudslides – This Is What Cascading Climate Disasters Look Like

The individual drivers might not be very extreme on their own, but combined they can become lethal. These types of events are broadly referred to as compound events – for example, a drought and heat wave hitting at the same time. Their combined impact can be harder to forecast. A cascading event involves compound events in succession, like wildfires followed by downpours and mudslides.

While the drivers and physical mechanisms behind compound and cascading events are not fully understood, they are often linked to large-scale circulation patterns like the El Niño-Southern Oscillation (ENSO). Meanwhile, lack of preparedness and high degrees of vulnerability at the local level can also increase the impacts of multiple connected events.

With compound and cascading events likely to become more common in a warming world, being able to prepare for and manage multiple hazards will be increasingly essential.

Climate Change Intensifies the Risk
Several research studies have shown that compound events with both drought and heat waves have become more severe and frequent in recent years. One study attributed the increase in the risk of these dry-warm events in California to human-caused global warming and projected that the increased risk of dry-warm conditions will continue in the future.

An important physical process responsible for increases in compound drought and heat is land-atmosphere interactions. Evaporation from soil cools down the land surface, similar to how the human body cools down by sweating. During droughts, the lack of moisture limits soil evaporation, which increases the surface temperature and eventually the air temperature in the area. Data shows temperatures during droughts are rising in many parts of the United States, including the Southwest – a pattern that is expected to continue in the future.

Numerous studies have also shown that droughts and heat waves increase the likelihood of wildfires. And wildfires can trigger other cascading hazards, turning otherwise unexceptional events into human disasters.

At the same time, extreme rainfall events are expected to intensify in a warming climate. A warmer atmosphere can hold more moisture, leading to wetter storms. This means there will likely be more burned acres exposed to potentially extreme rainfall events in a warmer world.

Cascading hazards are not limited to rain over burned areas. Soot and ash deposits on snowpack can increase snowmelt, change the timing of runoff and cause snow-driven flooding. Fires are not only increasing in size and severity, they are also occurring at higher elevations and well above the snow line.

It’s also important to recognize that human activities and local infrastructure can also affect extreme events. Urbanization and deforestation, for example, can intensify flooding and worsen mud or debris flow events and their impacts.

Managing Multiple Disasters and Climate Change
Despite the high risk when extreme rainfall and droughts interact, most research in this area focuses on only one or the other. Different government agencies oversee flood and drought monitoring, warning and management, even though both are extremes of the same hydrological cycle.

Recent studies and disasters show a strong need to integrate management and risk reduction strategies of droughts and flood. Focusing on one hazard by one agency can potentially have unintended consequences for another hazard. For example, maximizing reservoir storage when expecting a drought can increase the flood risk.

As a society, we cannot prevent cascading hazards from happening. But we can become better prepared for plausible cascading hazards in a changing climate.

Amir AghaKouchak is Professor of Civil & Environmental Engineering and Earth System Science, University of California, Irvine.