Extreme rain heads for California’s burn scars, raising the risk of mudslides – this is what cascading climate disasters look like
Two powerful storm systems known as are heading for northern California and Oregon, a region in the midst of an historic drought.
While the storms will bring much-needed water to a parched region and should significantly lower the wildfire risk, they also bring dangerous new flood and mudslide risks, particularly in areas .
Wildfires strip away vegetation and leave the soil less able to absorb water. A downpour on these vulnerable landscapes can quickly erode the ground as fast-moving water carries debris and mud with it.
The National Weather Service has from Oct. 21-26 in several burned areas, including the site of the nearly 1-million-acre in the Sierra Nevada.
I study cascading hazards like this, in which . Studies show climate change is raising the risk of multiple compound disasters, and it’s clear that communities and government agencies aren’t prepared.
Big-time precipitation is expected for the over the next 10 days. An atmospheric river will dump inches of rain and feet of snow across , , and as numerous storm systems roll in off the Pacific...
— Christopher Nunley, Ph.D. (@chrisnunley)
When storms hit burn scars
California has experienced this kind of cascading disaster before.
In early 2017, following years of drought, the region had a wet winter that fueled dense growth of vegetation and shrubs. An unusually warm and dry spring and summer followed, and it dried out the vegetation, turning it into fuel ready to burn. That fall, extreme Santa Ana and Diablo winds – known for their sustained low humidity – created the .
The began near Santa Barbara in December 2017 and burned over 280,000 acres. The following January, extreme rainfall hit the region, including the burn scar left by the fire, and caused the deadliest in California’s history. More than 400 homes were destroyed in about , and 23 people died.
These kinds of cascading events aren’t unique to California. Australia’s Millennium Drought (1997-2009) also ended with that inundated urban areas and breached . A study linked some of the levee and dike failures to earlier drought conditions, such as cracks forming because of exposure to heat and dryness.
Individually, they might not have been disasters
When multiple hazards, such as droughts, heat waves, wildfires and extreme rainfall, interact, human disasters often result.
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 , 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
have shown that compound events with both drought and heat waves have in recent years. 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 , which increases the surface temperature and eventually the air temperature in the area. Data shows temperatures during droughts are of the United States, including the Southwest – a pattern that is expected to .
Numerous studies have also shown that the . And wildfires can trigger other cascading hazards, turning otherwise unexceptional events into .
At the same time, extreme rainfall events are . 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 . Soot and ash 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 .
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 .
Recent studies and disasters show a strong need to strategies of droughts and flood. Focusing on one hazard by one agency can potentially have for another hazard. For example, 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.
