top of page
Bougainvillea lines the winding roads across San Diego, including near where Hanna grew up. (Hanna Webster for The Xylom)

This story features Beeline Reader for enhanced readability. Click to turn the feature on or off. Learn more about this technology here.

Heat Waves, Wildfires, and Downpours: San Diego is Changing and Climate Change is One Culprit

Fuchsia bougainvillea and scarlet trumpet vines drip over the fences on the winding road toward the house where I grew up.

Reaching the crest of the hill, the lush valley of San Diego's North County stretches toward a silver strip of the horizon—the ocean, gleaming, at what looks like the edge of the world. Eucalyptus trees shed their ashy scythes onto the rough blacktop, hot to the touch. Growing up, our side yard was home to avocado and orange trees, generous with their bounty and thriving in near-tropical temperatures that rarely dipped below 50 degrees Fahrenheit (10 degrees Celsius). In the summer, a pitcher of fresh-squeezed orange juice often sat on the ivory tile of our kitchen counter. My siblings and I were sent to wrestle with the avocado picker, twice as tall as us and leaning precariously out over a patch of grass we feared was slithering with rattlesnakes. Now, the orange tree suffers, likely ridden with a blight of citrus-killing disease and choking from drought. When I returned home after seven years in the Pacific Northwest, the hometown I grew fond of had changed—and climate change was one culprit.

While scientists have warned of the impending climate disaster for decades, these events have begun to manifest in real-time. Local residents have been exposed to heat waves, wildfires, and smoke time and time again. A 2020 paper published in Geophysical Research Letters by climate scientists at UCLA states that California has been experiencing more extreme weather patterns by way of more concentrated precipitation. An unfortunate series of events caused by drought can lead to increased flooding when it does rain. According to, 100 percent of San Diegans are affected by the current drought. Combined with existing crises of COVID-19 and inequity, this is an environmental, agricultural, and public health disaster brewing on the boundless Pacific horizon, threatening vulnerable communities the most.


When I moved back home in September of 2021, I flew into a heat wave. It is not uncommon for September and October to be hot in Southern California, but last year the thermometer pushed 90 degrees, and 100 in more inland areas. A weather forecaster in an article from The San Diego Union-Tribune said daytime temperatures during this heat wave were about seven degrees above normal.

Heat waves during the fall season receive their fair share of attention because of how dangerous they are: when 90-degree days become 100-degree days, we can’t release our body heat through sweating, resulting in heat strokes that can endanger the population. Dry and hot environments make wildfires worse. However, heat waves during the winter can be overlooked because cool 40-degree days becoming mild 50-degree days isn’t life-or-death. Across the Southern California coast, winters are heating faster than summers, with abnormally hot days in the winter happening more than twice as often than in the summer.

Warmer winters mean that some crops do not get the necessary chilling time, ultimately yielding smaller harvests. This also means pests and diseases can spread faster, plants get confused, or the heat straight up kills crops. Take the notoriously fickle avocado as an example: A 2018 study carried out by researchers across various University of California institutions and published in the journal Agronomy predicted that by 2050, avocado yields could decline by 40 percent. This is alarming not only because San Diego County has over 5,000 farms, more than any other county in the United States—many of them being small, family-run farms—but also, San Diego County leads the state and nation in the annual production value of avocados at a whopping 153 million dollars. My father, whose sprawling garden boasts 12 beds, has given up on growing lettuce because the perpetual heat prevents it from thriving. Lettuce thrives between 70-80 degrees, but can only tolerate a few days of 80-85 degree temperatures.


On the other hand, a perfect storm brews when wildfires blaze across the state concomitantly with hot Santa Ana winds during the hot and dry season. Tarik Benmarhnia is an environmental epidemiologist with a joint appointment at UC San Diego’s Scripps Institution of Oceanography and the Herbert Wertheim School of Public Health and Human Longevity Science. He studies how environmental science, epidemiology, and public health together can inform the impact of global climate events. In 2021, he was involved in a study on Santa Ana Winds published in the journal Climate Dynamics.

Benmarhnia and his colleagues looked at two “flavors” of Santa Anas: Hot Santa Anas are dry desert winds that blow in from the Great Basin, an area that covers most of Nevada and some of its surrounding states. Not all extreme heat events along the Southern Californian coast are caused by hot Santa Anas, but the two are moderately correlated, and hot Santa Anas exacerbate the worst effects of heat waves when they both occur. Cold Santa Anas are named so because of the extreme cold they bring from a more northerly direction in the Pacific Northwest, preceded by widespread precipitation over the West and accumulation of snow over the Great Basin.

Both Santa Anas’ strong gusts can increase a fire’s reach as the fire licks clean the vital Sierra Nevada forests and levels towns. They also carry wildfire smoke into coastal areas. Yet, unsurprisingly, hot Santa Anas, with their correlation to extreme heat, are ten times as likely to drive wildfires and burn down an area twenty times as large. Benmarhnia’s research shows that hospitalizations increase during Santa Ana periods, with respiratory complications usually involved.

In August and September of 2020, California suffered devastating wildfires at the same time COVID-19 cases spiked. Benmarhnia wanted to know whether these dual events would have compound impacts—he studies interactions between heat events and disease, especially among vulnerable populations. He found that the wildfires worsened COVID-19 symptoms and COVID-19-positive people who were exposed to wildfire smoke had an increased risk of dying.

“There are systematic reasons why some communities will be impacted more than others,” Benmarhnia says. The same populations ravaged by COVID-19 are also most impacted by wildfire smoke, pollution, and extreme heat. “We need to think about inequality, structural racism, and access to healthcare when we consider ways to tackle climate change,” says Benmarhnia. “All of this is not a coincidence.”

Benmarhnia moved from Montreal to California five years ago, and he noticed that California’s five largest wildfires on record have all occurred in those five years. “That’s not random; that’s due to climate change,” he says. “When you see a red sky, that’s very concerning.”

Sara McElroy was one of Benmarhnia’s first students when he started teaching at UCSD, and she joined his lab shortly after. In 2020, McElroy, Benmarhnia, and their colleagues published a paper in Science of the Total Environment on heat wave classifications and found flaws in the current San Diego classification methods. Using meteorological data, they showed that San Diego comprises three different climate zones—coastal, inland, and desert—each of which responds to heat waves differently.

San Diego only uses one temperature threshold to initiate county-wide early warning systems to prepare residents for heat waves. McElroy and Bemarhnia’s research shows this overestimates heat effects for the desert zone, as residents there are more adapted to high heat, and underestimates the heat effects of coastal zones, which sees more hospitalizations during heat waves. “You have to take into account that there might be different climate zones or geographic areas that are affected differently [by heat waves],” McElroy explains.

“We need to think about inequality, structural racism, and access to healthcare when we consider ways to tackle climate change. All of this is not a coincidence.” -- Tarik Benmarhnia, Associate Professor, UCSD Scripps Institution of Oceanography and School of Medicine

Her research studying San Diego’s early warning systems inspired her to pursue more direct methods of environmental research in a new job position. “Sometimes as a researcher, you feel disconnected because you're not physically out in the field talking to people,” said McElroy. Still, these findings are crucial to keeping residents safe during climate events, and McElroy recommends a full-scale update that incorporates climate zones and expresses heat-related statistics in ways that spur action. “If you just say a heat wave has a relative risk of 1.75, what does that actually mean?” McElroy asks. She has found that communicating risk as a percentage of hospital visits has more meaning to local officials who put policies in place.


This past Christmas, while many were shuttered inside due to rising coronavirus cases, Southern California was getting drenched. Outside my back window, the ice plant glistened, and tufts of weeds emerged from the dirt. During childhood, I remember there being a few minutes of drizzle until the clouds parted. Never had I seen so many consecutive days of rain. I thought, why did I leave Washington state?

San Diego was projected to have more rain in just one day through Christmas Eve in 2021 than their monthly average total rainfall (1.67 inches). (National Weather Service San Diego)

Downpour events in Southern California are becoming more extreme. Researchers at the Institute of the Environment & Sustainability of UCLA are using global climate models to predict how weather patterns might be affected by human-induced climate change. They say that both the dry and wet events will get more extreme, with more time in between them, which Benmarhnia seconded. Longer periods of drought dry out vegetation which then becomes tantalizing tinder for wildfires, and more concentrated downpours lead to increased risk of flooding, potentially challenging Southern Californian infrastructure.

Atmospheric rivers, which are narrow corridors of moisture that travel in from the ocean, dump heavy precipitation in a short amount of time. The 2020 paper published in Geophysical Research Letters states that atmospheric rivers, specifically over the Sierra Nevada region, could become more extreme. Benmarhnia also mentioned atmospheric rivers and added that California is not seeing more precipitation per se, but that instead of having rain events spread out over the year, they have been happening all at once.

Courtesy of the UCLA Institute of the Environment & Sustainability

What is there to do, when millions of acres of forests burn, the air dense with noxious smoke, and downpours slick the streets? How can we ensure our local communities are safe during climate events? Benmarhnia’s research shows that implementing early warning systems saves lives. This may include a city setting up cooling stations at the local library or recreation center. Neighbors or city employees could pass out water bottles to those experiencing houselessness and learn to recognize the signs of heat-related illness. What if the city had workers come to people’s houses to check in, ensuring they were safe and equipped during a heat wave?

“The good news is when we implement policies like early warning systems, they work,” Benmarhnia says. “We can reduce hospitalizations. Whatever we do right now will have a big impact in 70 years … but there are a lot of things that can make a difference.”

Brenda Garcia Millan works to promote community education and engagement about climate policies with the Climate Action Campaign, a local nonprofit that works with officials on climate strategies. On January 28th, 2022, Garcia Millan set up a workshop in San Ysidro to educate the community about the Climate Resiliency Plan, which intends to confront climate events that are already happening, such as heat waves and wildfires. As part of the plan, the city wants to plant more trees and vegetation. This will provide more cooling spaces and clear out dead vegetation more likely to burn in wildfires. The city also wants to build a sea wall and set up “green roofs” to harvest rainwater and prevent ocean pollution from runoff. Garcia Millan says San Diego is the first city in the region to implement climate-resilient policies so widely.

San Ysidro is a multicultural community bordering Tijuana, Mexico; 93% percent of its residents are Hispanic and 87% speak Spanish at home, more than twice the rest of San Diego. However, its Tree Equity Score is among the lowest in San Diego, according to an analysis by the nonprofit American Forests: with an average tree canopy cover of 12.6% and as low as 3% along the U.S.-Mexico border, it also has fewer trees and cool zones to escape the heat. During the September 2021 heat wave, San Ysidro had 10 days straight over 90 degrees.

"You cannot combat climate crises without addressing ongoing social disparities." -- Brenda Garcia Millan, Research and Policy Analyst, Climate Action Campaign

Garcia Millan gave the presentation in Spanish and answered questions from the community about the Climate Resiliency Plan and how it may impact the city. “There is a history of underfunding in [vulnerable] communities,” Garcia Millan says, “and we want to ensure the city tackles climate issues in an equitable way.” She relayed their feedback to the city of San Diego, ensuring communities—especially those hit the hardest by climate crises—are not left behind.“You cannot combat climate crises without addressing ongoing social disparities,” she says.

My city is changing; that is inevitable. But I can still have faith that climate scientists at top universities and advocates around the county are devising actionable solutions for policy change. While we might not be able to prevent heat waves or severe wildfires altogether, we can start putting policies in place to protect us against their worst effects. For now, I’ll admire the lilac sky as the sun dips underneath the steady horizon.

Hanna Webster for The Xylom

Correction: A previous version of the story stated that Dr. Benmarhnia is affiliated with the UCSD School of Medicine. He has a joint appointment at the UCSD Scripps Institute of Oceanography and the Herbert Wertheim School of Public Health and Human Longevity Science.


Support Student-Led Science News

The only student-run newsroom focused on science and society. Our in-depth, data-driven approach, mentorship for early-career storytellers, and multicultural content take time and proactive planning, which is why The Xylom depends on reader support. Your gifts keep our unbiased, nonprofit news site free.

Hanna Webster

Hanna Webster is a San Diego-based science writer and graduate student in the Johns Hopkins University Science Writing Program. With a degree in behavioral neuroscience and creative writing from Western Washington University, she enjoys writing stories about public health, psychology, and sociology, with bylines in, The Science Writer, Leafly, and more. When Hanna is not writing, you can find her frequenting live music, taking photos, and attempting The New Yorker crossword.

bottom of page