A new Sea Level Rise report from NOAA has been issued to project sea level rise and coastal flood hazards for the United States out to 2150 for 5 sea level scenarios: low, intermediate-low, intermediate, intermediate-high and high. The various scenarios are based on output directly from the United Nations Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report where global temperature projections are made based on various greenhouse gas emissions estimates.
There are 5 different Shared Socioeconomic Pathways (SSP) for CO2 emission scenarios (ie. SSP1-1.9, SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5). Many, if not most, of the high sea level projections reported by the media have been based on the SSP 8.5 scenario which has been described as the “business as usual” scenario. This scenario, far from being the “business as usual” scenario, can more accurately be labeled as the “extreme scenario” and not very likely, in my opinion.
What is Projected for Tampa Bay?
For Tampa Bay, the tide gauge with the longest record is located at St. Petersburg, Florida. The new NOAA report projects sea level rise by 2050 of between 0.28 meters (0.9 feet) for the low estimate, 0.36 meters (1.2 feet) intermediate estimate, and up to 0.49 meters (1.6 feet) for the high estimate relative to a baseline of year 2000. Extending the projection out to 2100, the report projects a sea level rise at St. Petersburg from a low estimate of 0.48 meters (1.6 feet) to a high estimate of 2.15 meters (about 7 feet).
If these projections are accurate, then there will be significant increases in the frequency of flooding events by mid-century and possibly catastrophic flooding by the end of this century in the Tampa Bay Region.
Just how much credibility should we place in these projections?
For the Tampa Bay area, the actual tide gauge data at St. Petersburg shows an increase in sea level of about 150-170 mm over the past 50 years which equates to an average rate of 3 to 3.4 mm/year. This rate is about the same as the satellite-based global sea level rise estimate between 1993 to 2020 of 3.4 mm/yr.
In order to reach the report’s intermediate estimate for St Petersburg of 0.36 meters (1.2 feet) by 2050, a sea level rate increase from the present 3-3.4 mm/yr to an average rate of 12 mm/yr for the next 30 years is required. Given the current rate of 3.4 mm/yr the rate would have to increase to something near 20 mm/yr by 2050 in order to achieve an average rate of 12 mm/yr for the 30 year period in question. Even worse, in order to reach the high estimate of 0.49 meters (1.6 feet) by 2050, the sea level rise rate must average over 16 mm/yr over the next 30 years, requiring the rate to soar to over 25 mm/yr to get to the 16 mm/yr average!
Historically, global sea level rose by a total of about 120 meters over a period of about 8,000-9,000 years as the vast ice sheets of the last glaciation melted away. This equates to an average rate of sea-level rise during this period of roughly 1 meter (3.3 feet) per century or about 10 mm per year (some studies suggest the average rate was up to 13 mm per year). In order to achieve the NOAA projected intermediate to high sea level rises, rates must increase well beyond those experienced during most of the post glaciation period.
Is Sea Level Rise Accelerating?
Various studies of the satellite altimeter data have shown that GMSL rise is accelerating somewhere between 0.015 mm/yr and about 0.10 mm/yr so there is still a considerable amount of uncertainty as to the actual rate. Taking the high end projection of 0.1 mm/yr we should see the sea level rise rate increase from 3.4 to 6.4 mm/yr by 2050 which should result in a total rise of about 150 mm or 0.5 feet by 2050 (less than half of the NOAA intermediate estimate of 0.36 meters (1.2 feet).
How likely is this?
If you look at the sea level from actual tide gauge data vs. the projected sea levels made by the NOAA report, you will see that the trend since 1996 indicates that the actual sea level at St Petersburg has been rising at rate equal to or below the “Intermediate” projection and often even below the “low” projection (see below).
In order to achieve these high sea level rise rates presented in the NOAA report, it would require rapid melting of portions of the Antarctic ice sheet. Modeling suggests that a significant sea level rise is possible from Antarctica ice melt, however, the key assumption is that greenhouse gas emissions will boost the planet’s temperature by about 4 degrees C (7 degrees F). This type of rapid collapse is highly uncertain as to timing and dependent on higher warming rates than currently projected for the end of this century.
Sea level is rising and the evidence suggests that the rate of rise has increased. There remains, however, a significant amount of uncertainty regarding predicting future warming rates and thus sea level estimates for the next 30-80 years are problematic. Based on observed sea level trends since 1996, the best estimate for sea level rise in Tampa Bay by 2050 is more likely to be near 150 mm (about 6 inches) and by 2100 roughly 1.5 to 2 feet.