Like payphones, typewriters and VCRs, a wildfire “season” is a thing of the past.
Outside of the historic July-October season, we’ve seen wildfires ignite and burn deeper in November, start earlier in the spring, and in the case of last winter, we’ve seen The Colorado Marshall Fire burning in December and the Colorado fire in California burning in January.
The consequences of this are evident when looking at trends in burned areas across California (Fig. 1). There is no forest fire season. Wildfires are with us all year round now. But is it normal? And will forest fires over the seasons get worse?
Spring forest fires
Climate change has accelerated the arrival of spring, with major consequences for forest fires (Fig. 2).
Several studies have connected the spring time in the extent of wildfire damage, showing that earlier snowmelt and higher spring and summer temperatures are correlated with greater area burned by wildfires. These changes give vegetation and soils more time to dry out, increasing their flammability.
Autumn forest fires
In autumn, higher temperatures and delayed onset of rain mean fuels stay drier and more flammable longer into the fall. Together, these conditions have increased the likelihood of extreme late-season wildfires.
Forest fires in winter
Until 2017, winter wildfires in California didn’t burn much area. Between 1950 and 2020, an average of 9,923 acres burned each year in December, January and February (Fig. 4).
In 2017, however, a record number of fires burned during the winter months. The largest of these, the Thomas Fire in Ventura and Santa Barbara counties, has burned over 200,000 acres in Southern California.
The scale and timing of these large wildfires in a winter month were unprecedented, but the conditions and mechanisms that led to these forest firessuch as high temperatures and late onset of rains, illustrate how climate change can increase the likelihood of extreme events.
What fuels forest fires all year round?
Climate change has disrupted historical trends and created conditions for year-round wildfires. The forests are about 50% drierand the number of high fire danger days have increased and are expected to double by the end of the century. This is laid on top hazardous forest conditions that stem from a legacy of fire suppression and forest management that increased the amount of vegetation available to burn.
A cycle of weather impacts throughout the year can escalate and create hazardous conditions, such as we saw before 2017 unprecedented winter fires (Fig. 4).
The winter before those fires, intense rains triggered a spurt of vegetative growth in California, which meant that fine fuels like grasses and shrubs quickly produced biomass. This rainy period was followed by an exceptionally hot summer in 2017, which was at that time the hottest on record in California (25°C).
The high temperatures allowed the previous winter’s vegetation to dry out, creating a landscape ready to burn. An abnormally warm autumn and the later than usual arrival of the autumn rains further exacerbated the risk of forest fires by allowing dry vegetation to remain flammable until December. These conditions, combined with downslope winds (called Diablo winds in Northern California and Santa Ana winds in Southern California), led to the massive spike in scorched areas seen during the winter of 2017.
While the simultaneous occurrence of these factors led to unprecedented winter fires in 2017, climate change projections suggest that each individual contributing factor is likely to occur more frequently in the future.
Preparing for the wildfires of the future
As humans continue to emit greenhouse gases, global temperatures will continue to rise. hotter summers are likely to continue for California and the state remains in record-breaking drought conditions, with the majority of the state experiencing severe drought.
A study 2020 led by the Union of Concerned Scientists predicted that weather whiplash, or swings between very wet and very dry years, is likely to increase across California. Further away, a 2021 study showed that the onset of the rainy season in California is increasingly delayed and that these rains occur over a more concentrated period. Together, these studies illustrate how climate change has increased the threat of wildfires throughout the year.
In light of this threat, adapting and preparing for wildfires can help protect individuals and communities. Federal and state agencies can increase the area treated with risk reduction activities such as prescribed burns, cultural burns and thinning. Programs like FireWise USA® provide advice on protecting homes and neighborhoods. Individuals can prepare for smoke impacts by installing air filters, monitoring air quality and stocking protective masks.
Wildfire seasons may be a thing of the past, but learning to live with wildfires is definitely part of our future.
Data note: Forest fire perimeter dataset (fire perimeters to 2020) accessed from California Fire and Resource Assessment Program (FRAP).The fires were sorted according to their date of ignition or discovery. We focused on California due to data availability and quality, particularly for fires under 300 acres.
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