Global warming created San Francisco Bay, though at a much more leisurely pace than is fueling expansion of its shoreline today.
At the peak of the last Ice Age 18,000 years ago, San Francisco Bay was empty because sea level was about 400 feet lower than it is today. The shore of the Pacific Ocean lay west of the Farallon Islands. Then sea level began to rise as global temperatures increased gradually. All of this was part of a common cycle that had lasted about 2 million years, although it was not caused by human activities, but by naturally occurring ecological processes set in motion by variations in earth’s orbit. There have been numerous warming and cooling periods on earth, during which at least four San Francisco Bays were formed, and possibly as many as seven, as ice ages succumbed to periods of global warming and sea levels fell and rose. By 8,000 years ago, the Pacific shoreline had reached the Golden Gate and by 3,000 years ago sea level rise tapered off, leaving today’s familiar shoreline. The most recent Ice Age episode came to a close and another one was not expected for about 20,000 years.
However, something radically different had occurred during this last Ice Age episode—something never before seen on the planet. Human beings had taken advantage of land bridges and traversable coastlines exposed by lower sea levels to walk and paddle all the way to the New World. Having first left Africa 70,000 years ago, the new species arrived in North America from Asia about 14,000 years ago. They may have been in the (now) Bay Area since then. We don’t know for sure, because the rising sea submerged evidence of their cultures.
In the early 1900s archaeologist Nels Nelson documented over 400 shellmounds around the Bayshore, testament to at least several thousand years of human habitation. These shellmounds signified the presence of villages that had been occupied for the longest duration since humans arrived in the region, as earlier rapid rates of sea level rise would have inundated previously established settlements more regularly. The famous Emeryville shellmound was the largest to be documented by Nelson, at about 45 feet high and 300 feet in diameter. It undoubtedly signified the location of a major population center. Nelson excavated it and discovered that, along with shells and bones from food items, there were carefully interred human remains. It was razed in 1924 to make way for a paint factory.
Stable sea levels also enabled San Francisco Bay’s 200,000-acre tidal marsh complex to form. Scientists estimate the age of the marshes at between 4,000 and 2,000 years. Since tidal marsh plants can survive only in the shallow margins of the Bay, they could not colonize until sea level stabilized.
Human-caused global warming since the beginning of the Industrial Revolution has restarted sea level rise, adding almost 8 inches to mean sea level since 1900—an alarmingly rapid rate. Scientists estimate that sea level will rise another 14 inches by 2050, and another 55 inches by 2100. Well before then, storms arriving on higher tides will inflict increasing damage on the developed shore, including vital highways, railroads and public utilities. Look to Hurricane Sandy for a chilling illustration of the damage higher sea level is already causing today.
The original human occupants of the Bay Area probably lived here for well over 10,000 years. In less than 300 years, we in the second big wave of immigration have set in motion planetary processes that previously could only be triggered by celestial orbits. It makes one wonder if we really have any idea about how we’re going to manage things from here on out.