SEA LEVEL RISE IS NOT THE CAUSE OF OUR COASTAL EROSION
There is no such thing as a “global sea level”
The concept of “global sea level” may be useful when describing global conditions but become less useful when making important decisions on a local level. The actual level of the ocean varies across the world ocean by as much as a foot due to the rotation of the earth, global winds and currents, and water temperature. Locally, the level of the ocean varies by several feet over minutes or hours due to waves, wind, and tides.
San Clemente tends to have relatively colder waters, making our sea levels relatively lower, although warm El Nino years can raise water levels by a significant amount. Our currents and wind patterns also tend to push water offshore, resulting in relatively lower sea levels.
Sea levels have risen and fallen throughout Earth history
Sea levels have changed by hundreds of feet throughout geologic history due to a variety of causes. Regular variations in the earth’s orbit caused cold glacial periods and warmer interglacial periods during the Ice Ages; the earth has been on a warming trend for the last 12,000 years. More recently, over the last 100 years, global warming has contributed to the natural rise in sea level.
In our area, sea level has been rising about 0.08” per year for the last century
The U.S. National Oceanographic and Atmospheric Agency (NOAA) has tracked tidal gauge data in many locations across our country’s coast and our closest station (San Diego) has shown a steady rate of 0.08 inches per year over the period 1906-2016. This translates to an inch of sea level rise over a twelve-year span.
Tidal gauge studies have demonstrated that our local coastline has experienced about six inches of sea level rise over the past 100 years. This small, gradual, and consistent rise in sea level could explain a shoreward movement of our shoreline by a few feet. Our coastline has experienced 50 to 200 feet of coastal erosion over the last few years — obviously not caused by by this small amount of rising sea level.
Rates of sea level rise vary along our coastline due to effects of land subsidence or emergence
NOAA tracks sea level by the level of water in a tidal gauge. Water in the gauge that can rise or fall due to two causes: 1) the level of the ocean water can rise or fall; 2) the level of the land can rise or fall due to tectonic or other influences.
The figure (above or on the left) shows rising sea levels in La Jolla and San Francisco, but falling sea levels in Crescent City as a result of tectonic uplift of land in Northern California.
San Clemente has a geomorphic character consistent with an emergent coastline (land is rising relative to the ocean level), which is one of the reasons why our rate of sea level rise is lower relative to that of “global” sea level rise.
Our coast is not experiencing a recent “acceleration” in the rise of sea level
Recent reports on global sea level have made the claim that the rate of sea level rise has recently accelerated. Such acceleration is not apparent on our coastline. The figure below (from NOAA) depicts sea level rise over the most recent period (1993-2020) showing that our coastline continues to experience a sea level rise of around 0.08 inches per year. Note that the Atlantic and Gulf coasts are experiencing rates of sea level rise two to three times faster than ours, due to increasing water temperature and general subsidence of land in these areas.
The models used to predict future sea level rise are not useful
The Intergovernmental Panel on Climate Change (IPCC) uses models to predict a sea level rise of between 1 and 3 feet (averaged globally) by 2100. Predicting tomorrow’s weather is difficult; predicting future climate changes is extremely difficult. The models currently relied upon to predict future sea level rise are actually a series of nested models to predict: future carbon emissions -> resultant concentrations in atmosphere -> resultant increase in temperature -> resultant change in melting ice and thermal expansion of water -> resultant sea level rise.
Due to inherent difficulties in climate modelling, extremely small changes in the input parameters can result in very large differences in the predicted sea level. The amplitude of the differences is so large that the results should not be relied upon to make long-term decisions about our natural environment.
The California Ocean Protection Council took the data and models from IPCC, and predicted potential sea level rises in California between 1 to 10 feet. This huge range in predicted sea level illustrates the unreliable nature of the models.
We believe that these predictions are not justified by the currently-available data, and do not provide useful information upon which to base major infrastructure investments or to give up trying to protect our most valuable asset — our beaches — from coastal erosion.
Instead, we firmly believe that we should use the past historical and current rates of sea level rise at our local coastline to base our future coastal protection plans. We should not allow unreliable models to deter the preservation of our natural environment.
In 2018, California Ocean Protection Council predicted sealevel rise of 8”-14” by 2050; they just revised this estimate to 9” most likely
The California Ocean Protection Council has just revised their predictions of sea level rise for 2050, from 8 to 14 inches, to 9 inches. This includes an assumption that there will be an acceleration in sea level rise, which has not been observed on our coast. This relatively large scale change in the estimates is a reflection of the state of the science — which is still very unsure. Climate change predictions are difficult.
However, this new estimate is more useful for planning purposes. An additional eight inches of sea level is not expected to significantly impact our coastline, which is steeply sloping. The most resilient response to this slow rate of sea level rise is to work on the real cause — lack of sand — by placing more sand on our beaches and maintaining that sand with sand retention devices.