California’s fourth, and most recent, climate assessment report reflects years of increasingly frequent and threatening natural disasters that have plagued the nation’s most populous state. In coming decades, they’re expected to become even more severe.

The state in its 2007 forecast had envisioned a tough 10 years ahead. That was the year the Witch Fire in San Diego County, which raged from October 21 to November 6, consumed nearly 200,000 acres, destroying 1,650 structures and killing one person. I remember that one distinctly: My wife and I had just returned from our honeymoon in Hawaii, and we evacuated our suburban neighborhood as thick brown clouds of smoke darkened the skies above.

The Zaca Fire in Santa Barbara County, which burned from that previous July 4 to September 4, was actually the biggest wildfire of 2007 at more than 240,000 acres. It consumed mostly rural land and destroyed just one building. At the time, it was the second largest fire in California history, behind San Diego County’s Cedar Fire in 2003, on which I reported as science writer with The San Diego Union-Tribune.

State’s common companions: wildfire and drought

Wildfire and drought are common companions in California. The consequences of living in an arid landscape, which has grown drier as global temperatures increase, have piled up year after year over the past decade. Bigger wildfires, especially in 2017 and 2018; record droughts, especially the one we had from 2012 to 2016; and whiplash weather, switching from heat waves to deluges, all point to a chaotic future for the state and also for the West Coast.

That’s the basic message from the recently released California’s Fourth Climate Change Assessment.

“This year has been kind of a harbinger of potential problems to come,” Dan Cayan, a climate researcher at the Scripps Institution of Oceanography in La Jolla, CA and editor-in-chief of the new report, told the Los Angeles Times. “The number of extremes that we’ve seen is consistent with what model projections are pointing to, and they’re giving us an example of what we need to prepare for.”

California today is home to about 40 million people, concentrated mostly in the San Francisco Bay Area and Southern California. And the state continues to grow: back in 2007, there were about 36.5 million people; by 2055, California’s population is projected to reach 50 million. More people of course only makes more difficult, and further complicates, the challenges facing the state in addressing climate concerns.

The Fourth Assessment’s latest temperature projections, meanwhile, paint a frightening picture. Compared with historical averages – that is, average temperatures during the first half of the 20th century, from 1901 to 1960 – average annual temperatures across California could rise between 2.5 and 2.7 degrees Fahrenheit early this century (2006-2039); between 4.4 and 5.8 degrees F in mid-century between 2040 and 2069; and between 5.6 and 8.8 degrees F by late century, between 2070 and 2100.

That translates into the following average temperatures:

2006-2039: 72.6 to 72.8 degrees F
2040-2069: 74.5 to 75.9 degrees F
2070-2100: 75.5 to 78.9 degrees F

Remember, these are average temperatures statewide.

The range depends on what scenarios of greenhouse gas emissions are used: The Fourth Assessment uses two “Representative Concentration Pathways,” or RCPs. The higher of the two, RCP 8.5, represents a higher emissions scenario that assumes a concentration of 900 parts per million by 2100. The lower of the two, RCP 4.5, assumes 550 ppm by century’s end. The current concentration of CO2 in the global atmosphere is about 406 ppm.

Coming soon … Less snow, more hot days, higher sea level

California’s Fourth Assessment doesn’t just discuss abstractions. It offers concrete examples of what the state faces. Many areas, the authors write, will experience exponential increases in the number of extreme heat days by the end of this century. In Fresno, in the state’s agricultural Central Valley, only four days annually exceeded 106.6 degrees F between 1961 and 2005. Between 2050 and 2099, that number is projected to increase to 26 per year, if GHG emissions are reduced at a moderate rate. Between 2050 and 2099, the number is projected to jump to 43 per year, if GHG emissions continue to rise at current rates.

The Fourth Assessment details other projected changes. Among them:

• By 2050, the state’s average water supply from snowpack in the Sierra Nevada is projected to decline by two-thirds, relative to historic levels. Without continued emissions reductions globally, water from Sierra snowpack could decline to less than one third of historic levels by 2100.
• Shasta and Oroville reservoirs, the largest in the state, are projected to have about one-third less water stored annually by 2100 if current water management practices don’t evolve.
• Sea-level rise is “virtually certain” to increase by another six inches – the amount that much of California experienced over the past century. Melting in Greenland and Antarctica will be the biggest determinants. There is a “slim” possibility, the report says, that sea levels could rise by more than nine feet by 2100.
• In a future with a two-meter rise in sea levels (6.6 feet), a 100-year storm in Southern California could impact 250,000 people and lead to damages of $89 billion in property and building damage.
• Under one model in the Fourth Assessment, large wildfires spanning more than 25,000 acres could become 50 percent more frequent by 2100. The state could experience a 77 percent increase in mean and up to a 178 percent increase in maximum area burned by wildfires by 2050 (compared with 1961-1990).
• Under current emissions levels, between 45 and 56 percent of natural vegetation in California could become “climatically stressed” by 2100. Already in the Sierra Nevada, scientists estimate that more than 100 million trees have died since the 2012-2016 drought.
• As heat waves become more common, mortality risk for people older than 65 could increase by ten times by the 2090s, compared with current levels.

‘Miles to go’ … but ‘time not on our side’

As Governor Jerry Brown, now serving his final term, in September announced his state’s commitment to 100 percent clean electricity production by 2045, he acknowledged that California’s fate will clearly depend on what happens far outside its boundaries.

“California is committed to doing whatever is necessary to meet the existential threat of climate change … but have no illusions,” Brown said, “California and the rest of the world have miles to go before we achieve zero-carbon emissions.”

In August, as wildfire smoke blanketed the San Francisco Bay area and much of the state, San Jose Mercury News reader Barbara Fukumoto wrote a short Letter to the Editor wondering what the future might hold.

“Fundamental change is urgently needed,” she wrote, concluding that “Time is not on our side.”