Becoming Arizona: How climate change is transforming California thru fire

When climatologists predicted that Sacramento would have Phoenix’s weather by 2100, and Portland would have Sacramento’s, they didn’t explain the ecological implications nor the process. Yet it’s apparent that an awful lot of trees need to disappear for the Sierra to look like the rock, grass, and cacti that make up Camelback Mountain in Phoenix.

Camelback Mountain near Phoenix

A new “new normal” every year

This ecological transformation, the likes of which would normally take a thousand years even during a rapid warming event, is happening, driven by rapid climate change. All those trees are flying away in the form of ashes and smoke.

The process, in human and ecological terms, is brutal. Californians experience a new “new normal” each year, each one stunning in its own right. In 2017 we were shocked when 6,000 homes burned in Santa Rosa, killing dozens as people fled in their bathrobes. Despite decades of fires in suburban California, there had never been anything of that magnitude. Before the year was out, the Thomas fire became the largest in state history as it burned thru Christmas and New Year. The next summer, the Carr fire stunned us with an EF-3 firenado that generated 140 mph winds. A few months later, the past was eclipsed when the entire town of Paradise burned, killing 85 people. That may be the largest climate-induced mass mortality event in history.  


After a reprieve in 2019, we arrive at 2020, where acreage burned has exceeded two million and three million for the first time. We keep having to adjust our vertical axes to make room for each new year. Five fires burning at the same time in 2020 qualified for the top 20 largest fires in the history of the state. Three of those, still burning as a write, are first, second, and fourth on the list.

California under smoke, September 9, 2020.

Each year has its macabre highlights. This year, over 300 people were rescued by military helicopters, many at night high in the Sierra. For the first time ever, all 18 national forests were completely closed to the public. The National Weather Service had to create a firenado warning. A dystopian pall of smoke created hazardous air from California to Canada for weeks, forcing people into their homes with all windows shut. And my hometown, Woodland Hills, hit 121 degrees, the highest temperature ever recorded in Los Angeles County.  

In 2019, the media reported that Oregon firefighters make an annual trek to California to provide mutual aid. In 2020, that changed. A quarter of the west slope of the Cascades from Portland to Medford appears to be on fire. One out of eight Oregonians are evacuating. The media is filled with horrific stories of grandmothers and teenagers burned alive while the father asks a badly burned woman along a roadside if he’s seen his wife. “I am your wife,” she responds.

Eugene, Oregon on the morning of September 8, 2020.

The process

We have heard for years that, with longer and hotter summers and declining snowpack, fire season has grown by months. In 2006, Westerling predicted such an increase in fires that the forests of the western US would become net carbon emitters. The US Forest Service now plans for fire year-round.

A series of academic analyses lays out the factors and processes of Arizonification. Decreased summer rains, as well as warmer winter and spring temperatures, are creating dry and stressed trees. But that’s not all. Summers that have become 1.4C (2.5F) warmer have led to an exponential increase in atmospheric vapor pressure deficit (VPD). It’s getting drier and, more importantly, vegetation is getting drier. This leads to big fires. Williams et al (2019) noted, “The ability of dry fuels to promote large fires is nonlinear, which has allowed warming to become increasingly impactful.” The Camp Fire, which destroyed the town of Paradise, occurred during some of the lowest vegetation moisture ever recorded. Add to that hot dry winds and vulnerable PG&E transmission lines, and the Paradise disaster looks predictable.

Northern California, being at western North America’s southern edge of the low elevation temperate forests, is especially at risk. As documented in the Verdugo Mountains near Los Angeles, high fire frequency converts forest and chapparal to weeds and rocks. That southern edge is pushing north. Forests are migrating north; so are deserts. (So are bird populations.)

To summarize, slightly warming temperatures, even in winter and spring, and less summer rain lead to an exponential increase in dry vegetation, which leads to an exponential increase in large fires, which leads a conversion of habitat from forest and chaparral to the grass and rock-dominated landscapes of arid desert mountain ranges. Sacramento becomes Phoenix. The Sierra and Coast Ranges become Camelback Mountain.

The future

Nearly the entire east side of the northern Coast Ranges have burned since 2018. Much of the southern Sierra forests died during the recent drought; most of those have yet to burn.

Arizona State University fire historian Prof. Stephen Pyne calls this a new epoch, the Pyrocene. “The contours of such an epoch,” he writes, “are already becoming visible through the smoke. If you doubt it, just ask California.”

Abatzoglou and Williams (2016) conclude, “anthropogenic climate change has emerged as a driver of increased forest fire activity and should continue to do so while fuels are not limiting.” Williams et al repeat this, “Given the exponential response of California burned area to aridity, the influence of anthropogenic warming on wildfire activity over the next few decades will likely be larger than the observed influence thus far where fuel abundance is not limiting.”

In layman’s terms, it’s going to get worse until there’s nothing left to burn.

The annual area burned in California has increased fivefold from 1972 to 2018 (Williams et al 2019). Several individual fires in 2020 exceed the average from 1987-2005. The point shown here for 2020 is still increasing.

Academic papers

Here is a partial list of recent research on the increase of fires in California and the western US.

Abatzoglou and Williams (2016). Impact of anthropogenic climate change on wildfire across western US forests. PNAS 113 (42) 11770-11775.

Goss et al (2020). Climate change is increasing the likelihood of extreme autumn wildfire conditions across California. Environmental Research Letters 15(9).

Haidinger and Keeley (1993). Role of hire fire frequency in destruction of mixed chaparral. Madrono 40(3): 141-147.

Holden et al (2018). Decreasing fire season precipitation increased recent western US forest wildfire activity. PNAS 115 (36) E8349-E8357.

Kitzberger et al (2017). Direct and indirect climate controls predict heterogeneous early-mid 21st century wildfire burned area across western and boreal North America. PLOS One.

Lareau et al (2018). The Carr Fire Vortex: A Case of Pyrotornadogenesis? Geophysical Research Letters 45(23).

Seager et al (2014). Climatology, variability and trends in United States 2 vapor pressure deficit, an important fire-related 3 meteorological quantity.

Swain (2020). Increasingly extreme autumn wildfire conditions in California due to climate change. Weather West Blog (related to Goss et al 2020 above).

Syphard et al (2019). The relative influence of climate and housing development on current and projected future fire patterns and structure loss across three California landscapes. Global Environmental Change 56: 41-55.

Williams et al (2019). Observed Impacts of Anthropogenic Climate Change on Wildfire in California. Earth’s Future 7(8): 892-910

Westerling et al (2006). Warming and Earlier Spring Increase Western U.S. Forest Wildfire Activity. Science 313(5789): 940-943.