Many coastal resources in the Southwest have been affected by sea level rise, ocean warming, and reduced ocean oxygen—all impacts of human-caused climate change—and ocean acidification resulting from human emissions of carbon dioxide. Homes and other coastal infrastructure, marine flora and fauna, and people who depend on coastal resources face increased risks under continued climate change.
At the Golden Gate Bridge in San Francisco, sea level rose 9 inches (22 cm) between 1854 and 2016 (Figure 25.5),236 and in San Diego, sea level rose 9.5 inches (24 cm) from 1906 to 2016.237 Tidal gauges around the world show increases in sea level,238,239 and analyses show that climate change caused most of this rise by melting of land ice and thermal expansion of ocean water.21,240,241 Non-climate-related land level changes influence relative sea level change. For example, between Cape Mendocino, California, and the Oregon border, lifting of the land at the San Andreas Fault has caused a drop in relative sea level between 1933 and 2016. Past earthquakes in the northern California coastal zone have abruptly lowered the shoreline and raised relative sea level.242
Under the higher scenario (RCP8.5), continued climate change could raise sea level near San Francisco by 30 inches (76 cm) by 2100, with a range of 19–41 inches (49–104 cm). 242 Currently, 200,000 people in California live in areas 3 feet (0.9 m) or less above sea level.9 Projections of sea level rise show that this population lives in areas at risk of inundation by 2100.9 Storm surges and high tides on top of sea level rise would exacerbate flooding.242 In Redwood City, one-fifth of houses and one-quarter of roads are at risk of flooding under the higher scenario (RCP8.5) by 2100.243 Sea level rise and storm surge could completely erode two-thirds of southern California beaches by 2100244 and cause saltwater infiltration that would spoil groundwater at Stinson Beach in Marin County, California.245 Major seaports in Long Beach and Oakland and the international airports of San Francisco, Oakland, and San Diego are vulnerable. Projected sea level rise and storm surges could cause as much as $5 billion (2015 dollars, undiscounted) in damage to property along the California coast from 2000 to 2100 under the higher scenario (RCP8.5).178 In Point Reyes National Seashore, sea level rise threatens to inundate habitat for the endangered western snowy plover, harbor seals,246 and northern elephant seals,247 as well as archaeological Indigenous sites.
Governments and private landowners along the California coast have built seawalls, revetments, and other structures to protect against sea level rise and storm surge, armoring 10% of the coastline.248 Because hard structures often alter natural water flows and increase coastal erosion, many parties are now exploring how to restore dunes, reefs, wetlands, and other natural features to protect the coast by breaking wave energy, to increase wildlife habitat, and to preserve public access to the coast.249
Local governments on the California coast are using projections of sea level rise to develop plans to reduce future risks. The City of San Francisco250 is implementing a plan that limits building in low-lying areas, constructs terraced wetlands at India Basin to facilitate upland migration of marsh habitat, and protects San Francisco International Airport with berms and seawalls along the 8-mile (13 km) shoreline. Golden Gate National Recreation Area has produced a detailed spatial analysis of the vulnerability of the marsh, paths, and buildings at Crissy Field to sea level rise and storm surges and has developed adaptation options, including moving infrastructure and establishing protective wetlands on inundated land.251 In 2016, residents of the nine counties of the San Francisco Bay passed Measure AA, which provides funding for wetlands restoration to naturally reduce risks of flooding and inundation due to sea level rise and storm surge.
Ocean waters off the California coast and around the world warmed 0.6° to 0.8ºF (0.3° to 0.5ºC) from 1971 to 2010,252 mainly due to human-caused climate change.21 Over the past century, sea surface temperatures in the northeast Pacific Ocean (including those off the coast of California) also experienced large year-to-year and decade-to-decade variations in response to changes in wind and weather patterns that altered the exchange of heat between the ocean and atmosphere and within the upper ocean,253 but showed overall warming from 1920 to 2016 (Figure. 25.6).
The marine heat wave along the Pacific Coast from 2014 to 2016 occurred due to a combination of natural factors and climate change.254 The event led to the mass stranding of sick or starving birds and sea lions and shifts in pelagic (open water) red crabs and tuna into the region.255 The ecosystem disruptions contributed to closures of commercially important fisheries and substantial reductions in California salmon catches in 2016 and 2017.256,257,258 Ocean warming also contributed to an increase in harmful blooms of algae along the Pacific Coast.259,260,261,262 These harmful algal blooms have produced domoic acid, which can kill people who eat tainted shellfish261,263 and kill California sea lions.261,264,265 Harmful algal blooms and shellfish contamination in the record warm year of 2015 delayed the commercially important Dungeness crab fishery, which contributed to a substantially reduced catch. Shifts in the timing of Dungeness and rock crab fisheries into whale migration season in 2016 contributed to increases in whale entanglements in fishing gear.266
Continued climate change could warm California Current waters 4°–7°F (2°–4°C) above the 1980–2005 average by 2100 (Figure 25.6).267 This could contribute to more harmful algal blooms,259,261 deaths of birds and sea lions, closures of fisheries, and economic loss to sectors dependent upon coastal marine resources. Under higher emissions (SRES A2), 28 fish species, including coho salmon and steelhead, could shift northward more than 180 miles (300 km) by 2050 due to higher sea surface temperatures.268 Marine heat waves may also increase in frequency, possibly causing local disappearance of some fish and economic losses.269
Observed ocean water acidity off the coast of California increased 25% to 40% (decreases of about 0.10 to 0.15 pH units) from the preindustrial era (circa 1750) to the early 2000s270,271 due to increasing emissions of carbon dioxide from human activities.21,272 Modeling studies show that human-caused changes in ocean acidity have increased beyond what would be expected from natural variations in the early-to-mid-20th century.273 Along the California coast, during some episodes of naturally acidic spring/summer upwelling of deeper ocean water, ocean acidity has quadrupled (a decrease of 0.7 pH units) to some of the most acidic values in the world.274 Increased ocean acidity along California’s coast has dissolved shells of some small planktonic sea snails (pteropods), exceeding their adaptive capacity, which was developed from evolution in natural acidic upwellings.275,276,277 In contrast, nearshore kelp forests in the northern Channel Islands off the California coast experienced few acidic events compared to local mainland sites in one three-year study.278
Higher carbon emissions (SRES A2) could increase the acidity of California coastal waters 40% (a decrease of 0.15 pH units) above 1995 levels by 2050.270 In addition to damaging marine ecosystems, ocean acidification increases risks of economic losses in the shellfish industry. One ecosystem modeling study suggests negative effects of projected ocean acidification on California’s state-managed crab, shrimp, mussel, clam, and oyster fisheries, but an increase in the urchin fishery.279 Warming of ocean waters has reduced oxygen concentrations in the California Current System by 20% from 1980 to 2012.280,281 Dissolved oxygen variations in waters far offshore affect oxygen concentrations in the California Current System nearshore.280,282 This deoxygenation contributed to an expansion of Humboldt squid, a species that thrives in deoxygenated water, in the northeastern Pacific Ocean in the late 1990s.283,284 Invading Humboldt squid prey on hake and other fish that are commercially important to coastal fishing communities.283
Climate change may reduce ocean oxygen in Pacific Ocean waters to levels lower than any naturally occurring levels as early as 2030285 or 2050.273 Reduced oxygen could decrease rockfish habitat off southern California by 20% to 50%.286 Further deoxygenation may harm bottom-dwelling marine life, shrink open-water habitat for hake and other economically important species,287 and increase the number of invasions by squid. Tracking the variability of ocean waters and fish populations and adjusting catch quotas accordingly can reduce pressures on fisheries stressed by climate change,288 actions that have been identified as parts of the National Oceanic and Atmospheric Administration’s (NOAA) Fisheries Climate Science Strategy.289
With continued climate change, risks would cascade from one area to another. For example, projected warmer winter temperatures in the Sierra Nevada would increase winter runoff, reduce spring and summer freshwater inflows into San Francisco Bay, and increase salinity in the Bay 3 to 5 grams per kilogram of water by 2100.290,291,292 Also, sea level rise and storm surge would compound effects inland of river and stream flooding, putting houses and roads at risk of inundation and damage.293,294