Golden State aflame
Drought conditions in California over the past four years – the warmest and driest period in its recorded history – have elevated the wildfire hazard to extreme levels. The dry conditions fuelled several large wildfires in the state during this period, but all occurred in remote, sparsely populated areas with little human habitation or property exposure. Unfortunately, this pattern would change in September 2015, when two large conflagrations – the Valley Fire and the Butte Fire – broke out near populated areas of northern California. By the time they were extinguished, the fires had become two of the most damaging on record in the state.
No end to the rain deficit
Drought conditions in California continued to worsen during the first half of 2015. Los Angeles only received about 100 mm of rainfall over this period, about 170mm below normal. Further north, the cities of Sacramento and Fresno, in the heavily agricultural central valley, saw half-year rainfall deficits of 170 mm and 120 mm respectively. And after four years of similarly below-average rainfall across the state, some indices of drought intensity indicate that the current drought in California was the worst since the 1840s. One of the most dire effects of the continuing drought is its impact on the Sierra Nevada snow pack, the source of most of the state’s fresh water during the dry summer season. The water content of the snow pack was decimated by another year of drought, dropping to just 5% of its normal amount, eclipsing the previous record low of 25% set in 2014. The lack of snow pack, combined with significant depletion of available ground and surface water, led to the first state-wide water restrictions in California’s history.
Droughts increase wildfire hazard
Large wildland fires typically require two meteorological ingredients: dry conditions and high winds. Years of extreme drought conditions in California created exceptionally hot and dry conditions, turning brush, chaparral and forests across the state into tinder boxes. The second ingredient, high winds, can arise from several different meteorological sources. Most large historical wildfires in California have been associated with so-called “Santa Ana” wind events. These occur when high pressure over the western United States causes dry, easterly winds from inland deserts to be funnelled through mountain passes, causing an increase in velocity. Furthermore, air temperatures rise considerably as Santa Ana winds descend towards the coast due to adiabatic heating, resulting in hot, dry conditions and the potential for wind speeds in excess of 140 km/h. Although Santa Ana wind events are more commonly associated with wildfires in southern California, similar downsloping wind events can occur in northern California as well. However, Santa Ana wind conditions were not present during the Valley and Butte fires. Instead, the fires took advantage of both terrain and copious amounts of fuel to create their own wind. As wildfires heat the air around them, the air expands and begins to rise, creating a localised area of low pressure. The low pressure forces air to be sucked in, providing more oxygen for the fire to grow. This, combined with very low humidity and large amounts of dry fuel available, caused the fires to grow quickly, lowering the pressure further and ultimately generating winds that exceeded gale force. Terrain can help exacerbate this phenomenon as well, as fires tend to race quickly uphill, and hills can funnel winds into a narrow area, increasing velocities.
Large fires in northern California
The larger of the two fires, the Valley Fire, was ignited on 12 September north of the Napa Valley winemaking region. The fire rapidly spread out of control, and grew to 40 km2 in size in less than six hours and 200 km2 by the following day. Over 10,000 residents of the county were ordered to evacuate the rapidly growing fire area. But several small towns in the path of the fire were largely destroyed by the advancing flames. By the time the Valley Fire was contained on 6 October, over 1,900 structures had been destroyed, including approximately 1,300 homes and 70 businesses, making the Valley Fire the third most destructive wildfire in California history, in terms of total structures burned. The Butte Fire, which burned in the foothills of the Sierra Nevada mountains east of Sacramento, began three days earlier on 9 September. Similar to the Valley Fire, the conflagration grew rapidly, covering an area of 60 km2 in just a few hours and 130 km2 by the next day. The local terrain hampered firefighting efforts against the blaze, and for a period of time the town of San Andreas, the county seat of Calaveras county, was evacuated due to the fire threat. Although the town of San Andreas remained unscathed, the Butte Fire destroyed 475 homes and 343 outbuildings before being contained on 1 October.
Insurance impacts and underwriting lessons
It is estimated that the Valley and Butte fires collectively caused US$ 1.8bn in overall losses, of which US$ 1.2bn was insured, with the Valley Fire making up about 80% of the above totals. The fires were the most damaging in California since the 2007 Witch Fire in San Diego (US$ 1.5bn insured loss, all values in 2015 dollars), and the worst in northern California since the 1991 Oakland Hills firestorm in 1991 (US$ 3bn insured loss). As with most wildfires, the majority of insured losses from the Valley and Butte fires were from residential buildings and automobiles. Although there can be exceptions, large commercial losses due to wildfire tend to be limited, as the majority of large commercial properties are located in urbanised areas, not along the wildland interface. Some exceptions to this rule are small, “main street” type businesses, “big box” retail stores that follow residential development into wilderness areas, and vacation resorts in forested regions. Major wildfires that cause significant amounts of property damage are much less common in northern California than in southern California. Several factors drive this difference in wildfire frequency. Northern California, in particular the densely populated San Francisco Bay region, typically receives more precipitation than Los Angeles or San Diego, reducing the hazard. Nor does northern California see as many Santa Ana wind events as southern parts of the state. From a socio-economic perspective, rugged terrain around San Francisco bay limits developed areas to narrow strips along its perimeter. Comparatively, the terrain in coastal southern California is more conducive to suburban and exurban sprawl, and has more areas of dense exposures near the wildland-urban interface. Due to the rapid speed and size of major wildfires, losses tend to be binary in nature: either a building survives with only minor damage or is completely destroyed. This occurs because the fires are fought with limited resources on rugged terrain that makes firefighting difficult. Many locations become impossible to reach or protect during the fire, and ultimately decisions must be made to triage the situation and prevent the fire from spreading to additional populated areas. But even within the burned area, not all buildings are destroyed. Many structures survive intact, but typically have some level of smoke damage.
A continued dry future for California
A strong El Niño event brought above-average precipitation to California during the winter of 2015–2016, bringing short-term relief to the drought and fire-stricken state. However, it is unclear whether the precipitation will completely alleviate drought conditions or only reduce the drought’s severity. Furthermore, excessive amounts of rainfall in a short period of time may act to destabilise hillsides recently affected by the fires, increasing the landslide and mudslide hazard within the burn area. Looking further into the future, insured losses from large wildfires in the American West are expected to continue to increase in frequency and severity. This increase is primarily being driven by the continued construction of new homes and businesses within the wildland-urban interface and increasing values of both real and personal property. Federal and state budgets for fighting wildfires also tend to be underfunded, limiting the ability of firefighters to protect as much property as possible. However, environmental conditions are also contributing to the increased wildfire risk. Warming temperatures, in part due to anthropogenic influences, are extending the wildfire season, as well as causing earlier snowmelts that reduce the amount of groundwater and soil moisture available to plants and increase the amount of fuels during peak wildfire season. The heat and recent drought conditions have also stressed trees, making them more vulnerable to disease and insects, such as the invasive pine bark beetle that has killed off over an estimated 12.5 million trees in the state. And more wildfires are likely in the future as California’s climate continues to become drier over time.