Projecting Future Sea Level Rise

California’s coastal observations and global model projections indicate that California’s open
coast and estuaries will experience increasing sea levels over the next century. Sea level rise has
affected much of the coast of California, including the Southern California coast, the Central
California open coast, and the San Francisco Bay and upper estuary. These trends, quantified
from a small set of California tide gages, have ranged from 10–20 centimeters (cm) (3.9–7.9
inches) per century, quite similar to that estimated for global mean sea level. So far, there is little
evidence that the rate of rise has accelerated, and the rate of rise at California tide gages has
actually flattened since 1980, but projections suggest substantial sea level rise may occur over
the next century.

Climate change simulations project a substantial rate of global sea level rise over the next
century due to thermal expansion as the oceans warm and runoff from melting land-based
snow and ice accelerates. Sea level rise projected from the models increases with the amount of
warming. Relative to sea levels in 2000, by the 2070–2099 period, sea level rise projections range
from 11–54 cm (4.3–21 in) for simulations following the lower (B1) greenhouse gas (GHG)
emissions scenario, from 14–61 cm (5.5–24 in) for the middle-upper (A2) emission scenario, and
from 17–72 cm (6.7–28 in) for the highest (A1fi) scenario. In addition to relatively steady secular
trends, sea levels along the California coast undergo shorter period variability above or below
predicted tide levels and changes associated with long-term trends. These variations are caused
by weather events and by seasonal to decadal climate fluctuations over the Pacific Ocean that in
turn affect the Pacific coast. Highest coastal sea levels have occurred when winter storms and
Pacific climate disturbances, such as El Niño, have coincided with high astronomical tides.
This study considers a range of projected future global sea level rises in examining possible
impacts at California coastal and estuarine stations. Two climate models and three scenarios
considered in this scenarios study provide a set of possible future weather and short-period
climate fluctuations, and a range of potential long-term sea level rise values. A range of mean
sea level rise was considered in combination with weather and El Niño fluctuations extracted
from two global climate models and two GHG emissions scenarios. The mean sea level rise
values, determined from a survey of several climate models, range from approximately
10–80 cm (3.9–31 in) between 2000 and 2100. The middle to higher end of this range would
substantially exceed the historical rate of sea level rise of 15–20 cm (5.9–7.9 in)per century
observed at San Francisco and San Diego during the last 100 years. Gradual sea level rise
progressively worsens the impacts of high tides and the surge and waves associated with
storms. The potential for impacts of future sea level rise was assessed from the occurrence of
hourly sea level extremes. The occurrence of extreme events follows a sharply escalating pattern
as the magnitude of future sea level rise increases. The confluence of Low barometric pressures
from storms and the presence large waves at the same time substantially increases the
likelihood of high, damaging sea levels along the California coast. Similarly, astronomical tides
and disturbances in sea level that are caused by weather and climate fluctuations are
transmitted into the San Francisco Bay and Delta, and on into the lower reaches of the
Sacramento River. In addition to elevating Bay and Delta sea levels directly through inverse
barometer and wind effects, storms may generate heavy precipitation and high fresh water
runoff and cause floods in the Sacramento/San Joaquin Delta, increasing the potential for
inundation of levees and other structures. There may also be increased risk of levee failure due
to the hydraulics and geometry of these structures. Rising sea levels from climate change will
increase the frequency and duration of extreme high water levels, causing historical coastal and
San Francisco Bay/Delta structure design criteria to be exceeded.