By Amir AghaKouchak, University of California, Irvine
Two powerful storm systems known as Atmospheric Rivers are heading into northern California and Oregon, a region plagued by historic drought.
While storms will bring much needed water to a parched region and are expected to greatly reduce the risk of forest fires, they also bring dangerous new risks of flooding and mudslides, especially in areas that are receding. recover from forest fires.
Forest fires tear away vegetation and leave the soil less able to absorb water. A downpour over these vulnerable landscapes can quickly erode the soil as the rapidly moving water carries debris and mud with it.
The national meteorological service has warned of ash and debris flows October 21-26 in several burned areas, including the site of the nearly one million acre Dixie fire in the Sierra Nevada.
I study cascading risks like this, in which consecutive events lead to human catastrophes. Studies show that climate change increases the risk of multiple disasters and it is clear that communities and government agencies are unprepared.
When storms strike the scars of burns
California has already experienced this kind of cascading disaster.
In early 2017, after years of drought, the region experienced a wet winter that fueled dense growth of vegetation and shrubs. An unusually hot and dry spring and summer followed, and it dried out the vegetation, turning it into ready-to-burn fuel. That fall, the extreme winds from Santa Ana and Diablo – known for their sustained low humidity – created ideal conditions for wildfires.
The Thomas Fire began near Santa Barbara in December 2017 and burned more than 280,000 acres. The following January, extreme rainfall hit the area, including the scar left by the fire, and caused the deadliest mudslide and debris in California history. More than 400 homes were destroyed in about two hours and 23 people died.
These types of cascading events are not unique to California. Australia’s millennial drought (1997-2009) also ended with devastating floods that inundated urban areas and shattered dikes. One study has linked some of the levees and levees failures to previous drought conditions, such as the formation of cracks due to exposure to heat and drought.
Individually they may not have been disasters
When multiple hazards, such as droughts, heat waves, wildfires and extreme rainfall, interact, it often results in human disasters.
Individual drivers may not be very extreme on their own, but combined they can turn fatal. These types of events are generally referred to as compound events – for example, a drought and a heat wave that strike at the same time. Their combined impact may be more difficult to predict. A cascading event involves successive compound events, such as forest fires followed by downpours and mudslides.
The video shows how quickly a mudslide invades a city.
Although the drivers and physical mechanisms behind compound and cascading events are not fully understood, they are often linked to large-scale circulation patterns like El NiÃ±o-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 frequent in a warming world, it will be increasingly essential to be able to prepare for and deal with multiple dangers.
Climate change intensifies the risk
Several research studies have shown that events composed of both drought and heat waves have become more severe and more frequent in recent years. One study attributed the increased risk of these hot-dry events in California to man-made global warming and predicted that the increased risk of hot-dry conditions will continue into the future.
Earth-atmosphere interactions are an important physical process responsible for increasing dryness and compound heat. Evaporation from the soil cools the soil surface, in the same way that the human body cools itself through perspiration. During droughts, the lack of moisture limits evaporation from the soil, 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 trend that is expected to continue into the future.
Numerous studies have also shown that droughts and heat waves increase the likelihood of forest fires. And forest fires can set off other cascading dangers, turning otherwise non-exceptional events into human disasters.
At the same time, extreme precipitation events are expected to intensify in a warming climate. A warmer atmosphere can hold more moisture, resulting in wetter storms. This means there will likely be more burnt acres exposed to potentially extreme precipitation in a warmer world.
The cascading dangers are not limited to rain on the scorched areas. Deposits of soot and ash on the snowpack can increase snowmelt, change the timing of runoff and cause snow flooding. Fires don’t only increase in size and severity, they also occur at higher elevations and well above the snow line.
It is also important to recognize that human activities and local infrastructure can also affect extreme events. Urbanization and deforestation, for example, can intensify flooding and exacerbate mudslides or debris and their impacts.
Managing multiple disasters and climate change
Despite the high risk when extreme rainfall and drought interact, most research in this area focuses on one or the other. Different government agencies oversee the monitoring, warning and management of floods and droughts, even though both are extremes of the same hydrologic cycle.
Recent studies and disasters show a strong need to integrate drought and flood risk management and reduction strategies. Focusing on one hazard by one agency can potentially have unintended consequences for another hazard. For example, maximizing reservoir storage when drought is expected can increase the risk of flooding.
As a society, we cannot prevent the cascading dangers from occurring. But we can better prepare for plausible cascading risks in a changing climate.