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Over 6 feet sea level rise by 2100 is most unlikely
Sea level rise is not to be confused with King tides. That is a totally different phenomenon I covered in an earlier article.
On January 12, the Marin IJ published a cover page article Sea-level protection plan can guide Marin strategy. And, the article stated: "The water is also expected to rise more than 6 feet by 2100." The article's source was the State of California Sea Level Rise Guidance 2024 from the California Ocean Protection Council (COPC).
The Marin IJ statement way overstated the prospective rise in sea level.
First, let's look at the COPC sea level scenarios for San Francisco.
Source: COPC
Now, let's look at the scenarios' probabilities.
Source: COPC
Within the table above, the over 6 feet scenario (High) mentioned by the IJ has essentially a 0% probability of materializing even if the temperature would rise by 5 degree Celsius.
According to the COPC, we may be heading towards a 3 degree Celsius increase by 2100.
Let's rearrange the table above so that the sum of probabilities for each temperature scenarios equals 100%.
Source: COPC
Given a 3 degree Celsius increase in temperature, the most likely scenario is the "Int-Low" at 1.6 feet with an 82.0% probability. The second most likely scenario is the "Low" one at 1.0 foot with a 12.9% probability.
What is the current pace of sea level rise for San Francisco?
Below see a long term time series from the NOAA for San Francisco. It shows that sea level has risen by about 0.30 meters since 1855. That is about 300 millimeters over 70 years or about 4 millimeters per year.
Source: NOAA
The most recent NOAA estimate is that San Francisco sea level rises by just under 2 millimeters per year, which is about half of its long term trend of 4 millimeters.
Source: NOAA
Comparing the scenarios' sea level rise pace vs the current one
As shown below, all the COPC scenarios are associated with a pace of sea level rise that is often a high multiple of the current sea level rise. For instance, the Intermediate scenario is associated with a decadal pace of sea level rise that is over 3 times faster than the current pace in the early decades. And, it reaches over 10 times the current pace in the last decade of the century.
This projected acceleration in sea level rise pace contrasts with historical trends as the current pace is about half as much as the long term trend going back to 1855.
Benchmarking the Low and Int-Low scenarios with an alternative approach
Let's see by how much sea level rises if we assume that the pace of sea level rising reaches 2 x, 3 x, and 4 x the current pace by the last decade of this century. The table below maps out the intermediary multiples for each decade. For instance under the "2 x" column you can see how the pace increases exponentially from 1 x in the current decade to 2 x by the last decade of the century. And, the pace accelerates by 9.1% per decade.
1.00 (1 + 9.1%)^8 = 2.00
Next, let's compare the COPC Low and Int-Low scenarios vs the estimates using the multiple approach (2 x, 3 x, and 4 x).
As shown above the Low scenario fits nicely in between the 2 x and 3 x estimates.
The Int-Low scenario is quite a bit higher (1.6 feet) than the 4 x estimate (1.3 feet). To match the Int-Low scenario you would have to use a multiple of 5.5 x. I am not familiar with any natural variable whose change would accelerate by that much over the next few decades (especially when such a variable is currently running at half the speed of its own long term trend).
The above suggests that the range of sea level rise between 1 and 1.6 feet by 2100 is already quite high. The over 6 feet mentioned in the IJ article reflects a misunderstanding of the mentioned COPC report.
THE END
Gaetan Lion is an independent researcher with expertise in economics, econometrics, investments, risk management, statistics, and quantitative analysis. His areas of interest include demographics, economics, investments, Climate Change, health care, sports, and politics.