Above the streets of San Francisco, an old man lies dying. Painting the cityscape with the final strokes of his ethereal brush, slipping silently between the towering skyscrapers, and curling around the Golden Gate Bridge as if to hug an old friend is none other than Karl the Fog, the enigmatic shroud of San Francisco.

Locals have a love-hate relationship with Karl. Some mornings he swirls around the feet of grumbling commuters, an uninvited dance partner on their journey to work. But, as the sun dips below the horizon, he retreats, pulling back his misty tendrils to reveal a sunset that paints the sky in strokes of orange, pink and gold.

Karl, the fog of San Francisco, is more than just a meteorological phenomenon – he is a character woven into the very fabric of the city, holding the hearts of San Francisco in his ephemeral hands.

“I have a real fondness for the fog – it’s a kind of identity for this city that’s just taken on its own meaning,” said Todd E. Dawson, a professor of Integrative Biology and ESPM at UC Berkeley.

In fact, when typing “fog” on an iPhone, it will suggest an emoticon of the Golden Gate Bridge covered in fog – Karl is that iconic to the city of San Francisco.

Above the Pacific Ocean, high-pressure winds spin clockwise, pulling ocean currents westward, away from the California coast. This churns up the deep, cold water in a process called upwelling. When these cold coastal waters meet the moist air above, it forms fog. This fog is further influenced by the Marin Headlands that line the coast, which act as a barrier and keep the fog predominantly offshore. However, Karl always finds a way through. Every morning, warm air rises above inland California, creating a vacuum and pulling the fog through the gap in the mountains marked by the Golden Gate Bridge. This daily cycle is the reason why Karl makes his entrance late in the day only to retreat back in the morning.

Dawson, who studied Bay Area fog using airport data and temperature records, has termed this phenomenon the “fog conveyor belt.” This mechanism is responsible for 30 to 40 percent of all the moisture that enters coastal California, a crucial factor during drought conditions often seen in California.

Karl plays a critical role in shaping the ecological and cultural landscape of the region. Fog is a crucial source of water for the survival and prosperity of coastal grasslands and redwood forests that lie within the fog belt. The unique climate of the Bay Area, characterized by its microclimates and varied topography, owes much to the presence of fog. This natural phenomenon creates environments where diverse flora and fauna can thrive, selecting microhabitats that suit their specific needs.

This is true for both natural as well as man made ecosystems. Karl plays a pivotal role in  keeping the vibrant tapestry of plants alive in Golden Gate Park in San Francisco. As Ryan Guillou, Director of Collections and Conservation at the Gardens of Golden Gate Park explains, “San Francisco has its own climate, but then within San Francisco, there are many microclimates due to its geography, lots of its topography, whether you're on the Bay side or the ocean side.”

Golden Gate Park, stretching approximately two miles from east to west, embodies this complexity. Its eastern end experiences warmer and sunnier conditions, making it less windy compared to its western counterpart. This contrast is stark as one moves towards Ocean Beach, where it becomes windier, foggier, and cooler. These conditions are not merely atmospheric nuances; they play a critical role in determining which plant species thrive and where. The variation in microclimates within Golden Gate Park means that plant communities are not uniformly distributed. Some plants find a conducive environment in the warmer eastern areas of the park, flourishing in the abundant sunshine and relative warmth. Conversely, the cooler, foggier conditions of the park's western edge may favor different species, adapted to thrive in such environments. This diversity is pivotal to the park's ecological health and its ability to support a wide array of plant life.

“Redwoods were kind of where I really got into working on fog,” Dawson noted, “the entire coastal California— all the vegetation, grasslands, chaparral, the redwoods themselves.”

California's Mediterranean climate, characterized by wet winters and dry summers, presents a challenging environment for many plant species. However, the coastal redwood trees have adapted to leverage the summer fog, drawing more than half of their moisture from this source. “The important thing is that when we don’t have the rain, we do often have the fog,” Dawson said “one of the keys that we’ve discovered in understanding the ecology of the redwood forest is that we think one of the reasons why the redwood trees can be so large is because they get this water subsidy during the summertime when there’s otherwise no rainfall.”

Dawson's research has unveiled surprising mechanisms by which redwoods absorb moisture directly from the fog through their leaves—a revelation that challenged the conventional wisdom of water uptake in plants. “No one had ever made that observation before... it was one of those aha moments going ‘Oh my goodness, there’s like two root systems–one in the soil and one in the atmosphere’,” Dawson explained, highlighting the dual pathways through which redwoods access water. These findings help explain the trees' remarkable adaptation to their environment, utilizing fog as a crucial resource for survival and growth.

Expanding on this topic, Laura Lalemand, a Senior Scientist at Save the Redwoods League, elucidated the multifaceted relationship between redwoods and fog. “Redwoods capture fog in a couple of different ways. First, they capture it in the form of fog drip,” which she credited for up to 45 percent of their annual water. This process not only sustains the trees themselves, but also supports the broader ecosystem, contributing to the moisture available for other plants, animals, and freshwater streams within the redwood forests. The presence of fog fosters unique arboreal communities within old-growth redwood canopies, housing fern mats, salamanders, and other species that contribute to the biodiversity and ecological richness of these forests. As Lalemand pointed out, “You only see those really complex and decadent old-growth canopy structures in forests that have a significant amount of fog.”

Moreover, the fog's impact is not limited to purely vegetative ecosystems. Research has revealed its benefits for agriculture, particularly in the fog belt regions of Santa Cruz county and Watsonville, where crops like strawberries and artichokes thrive in foggy conditions. “When the fog comes in and is intercepted by the foliage, the fog sometimes can get directly absorbed into the plant,” a discovery made by Dawson and his team. This direct absorption provides a significant advantage, enhancing crop productivity on foggy days. However, the elusive nature of fog has long posed challenges for scientists seeking to understand and measure its occurrence and impact. Despite its significant role in certain ecosystems, particularly in regions like San Francisco, fog remains one of the less studied meteorological phenomena. The lack of study can be attributed to its relatively localized geographical impact, and the complexity of its formation and behavior.

Fog measurement is complicated by the very nature of fog itself. As Dawson explained, “It's a challenge because it's hard to standardize. And not every place has every sensor. So, the data is really patchy from different places around.” The introduction of remote sensing through aircraft or satellites has offered new ways to estimate fog density, yet these too are limited by the variability in fog's physical properties and the technological limitations of sensors.

However, in 2010, Dawson and his team managed to piece together airport data and weather station temperature records dating back to 1951 to make some concerning discoveries. Over the years, trends in fog occurrence have shown a decline, particularly along the California coast. Dawson's research indicates a decrease in fog since the 1950s, a trend closely tied to global warming and the diminishing temperature difference between the coast and inland areas. “As that temperature difference went down, the power of pulling the fog on shore also went down,” Dawson noted, pointing to the warming of the coast at a slightly faster rate than inland areas. This reduction in temperature difference diminishes the vacuum effect that traditionally pulls fog ashore, leading to less frequent and dense fog events.

The implications of these trends are significant, especially for ecosystems reliant on fog for moisture during dry summer months. The decline in fog could have dire consequences for species adapted to these microclimates, including the iconic redwoods of California. These discoveries set off warning alarms for a future where fog and its life-sustaining properties for certain ecosystems may become increasingly scarce.Lalemand pointed out the uncertainty surrounding the changes in fog patterns, noting that “Climate models show a lot of different scenarios,” with some areas experiencing increases and others decreases in fog density. This variability complicates efforts to predict and manage the impacts on local ecosystems. However, the consensus leans towards a decline in fog in many areas, especially during the critical summertime nighttime periods when redwoods undergo most of their growth.

This decline is not just detrimental for our ecosystems. Dawson amplified the concern over declining fog patterns, likening fog to “the big air conditioner” for the region. Its loss, he suggested, not only reduces the crucial moisture supply during dry summer months but also leads to warmer coastal temperatures. This warming trend has a cascading effect on energy consumption, as more people may opt for air conditioning to cope with the increased heat, thereby increasing power demand and exacerbating the climate crisis. “We’re really perturbing the climate system... That means that changes the way our storms behave, the severity of the storms and other things like fog formation and duration.”

Broad-based societal changes are vital in addressing the root causes of climate change. “Mitigating the effects of changes in fog is again mitigating the overall effects of changing the global temperatures on the planet,” Dawson continued, “Until the society at large around the entire globe gets away from using polluting gasses and using fossil fuels...we’re not going to be able to slow things down.” Just across the bay, the League leads their conservation efforts through a drastically different method. Using the genome of redwood trees, they aim to identify sections of genetic code that are associated with adaptations. This genetic mapping can pave the way for targeted conservation efforts, enabling the identification and propagation of redwood genotypes that exhibit greater resilience to changing climate conditions, including variations in fog patterns.

Moreover, Lalemand highlighted the significance of restoration practices tailored to enhance forest resilience and the importance of community engagement and support for conservation initiatives. Engaging with local organizations, participating in restoration projects, and supporting research and conservation efforts financially or through volunteer work are actionable steps individuals can take. Such community-driven efforts are crucial for the preservation of the Bay Area's unique ecosystems and for fostering a wider cultural and societal commitment to environmental stewardship.

We need to work together to ensure that Karl can keep up his magic.

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