Lately there have been numerous claims of a coming sea level rise catastrophe resulting from the “climate crisis” supposedly driven by ever increasing fossil fuel related emissions of carbon dioxide. The claims include coastal sea levels rising a meter or more over coming decades and inundating many coastal cities. I decided to examine the actual coastal sea level measurements to see what they show regarding these extraordinary claims. What I found is that coastal sea level measurements indicate that the long-term absolute coastal sea level rise is not accelerating and is not alarming. In fact, it is consistent with what humans have dealt with over the last 150 years and nothing worse.
Sea level changes are very important to human activities and interests at coastal locations, especially in populated areas. However, what happens in the middle of oceans is of little consequence in this regard. Thus, I have focused on coastal measurements. Sea level monitoring at some coastal locations began over 100 years ago. From these longer periods of record, coastal sea level trends can be evaluated over time and compared among locations.
First, there are several important factors to consider for coastal sea level measurements. Most importantly, the land can rise or fall over time relative to the center of the earth and this change can significantly affect the relative sea level change measured at a given location over time. A lesser, but sometimes important effect can occur from change in elevation of the measurement station relative to the overall nearby land surface, such as from a heavy concrete dock slowly sinking into the surrounding mud over time for instance. Also, measurement methods vary from station to station and over time which can introduce artificial differences. Another factor is that sea level at some locations is much more variable over time because of tides and/or storm surge and associated variations over time. Furthermore, sea level at some locations is influenced by changing regional sea surface temperature patterns, such as tropical locations where sea level is affected by El Niño and La Niña related patterns.
The plots provided in this post come from the Sealevel website and have seasonal fluctuations removed. I looked for measurement locations with continuous or nearly continuous 100-year or longer periods of record extending to the present. Then I checked for locations with recently stable vertical land motion. Locations with stable vertical land motion are most likely to better represent absolute coastal sea level change. Stations where the land is rising or falling are measuring relative sea level change and the vertical land motion must be considered to estimate the absolute sea level change. I found 15 long-term sea level measurement sites with nearby or collocated Continuous Global Positioning System (CGPS) measurements. Sea level measurements from seven of these sites are presented below.
To estimate the vertical land motion, I looked at nearby CGPS estimates of land motion trends using the Sonel website. For some locations there were no nearby CGPS measurements available, so I disregarded those sites for this analysis. For the remaining sites, most had CGPS measurements from nearby locations that were not collocated with the sea level monitoring site, which increases uncertainty a bit. Fortunately, the site with the smallest vertical land motion has the CGPS monitor collocated with the sea level monitoring site at Honolulu, Hawaii. Measurements from this site, as displayed in Figure 1 below, indicate a sea level trend of about 1.49 +/- 0.21 millimeters per year (mm/yr) for 1905-2017. The recent CGPS land vertical motion trend for 1999-2014 was -0.23 +/- 0.18 mm/yr or practically stable considering the uncertainty. These measurements indicate a global coastal absolute sea level rise of about 15 centimeters (6 inches) over 100 years with no acceleration – certainly not alarming and no different than what has already been handled over the last 100 years.
The graph in Figure 1 also includes a plot of ice core and Mauna Loa carbon dioxide (CO2) levels in conjunction with the sea level measurements. As can be seen, there is no indication of acceleration in the sea level rise associated with the accelerating rise of atmospheric CO2. The implication is that we are not likely to see an effect on sea level rise from increasing CO2 levels any time soon, or we already would be seeing the effect.
The site with the second most stable land motion was at Trieste, Italy where the indicated sea level rise for 1875-2016 was 1.30 +/- 0.15 mm/yr as seen in Figure 2 below, with a recent land vertical motion trend of 0.32 +/- 0.26 mm/yr. Here the CGPS site was not collocated with the sea level monitoring site and thus there may be a larger uncertainty about the land motion at the sea level monitoring site. This uncertainty is probably enough to account for the difference in the sea level rise from that measured at Honolulu. However, once again there is no indication of any significant sea level rise acceleration over the last 100 years at Trieste, despite the accelerating rise in CO2 levels.
Of the remaining sites, nine had nearby or collocated CGPS measurements indicating land motion trends of -0.53 to -4.59 mm/yr (subsiding land) and four sites had land motion indicated in the range from 0.59 to 7.88 mm/yr (rising land). Figures 3, 4, and 5 show sea level trends for locations with subsiding land: San Francisco, California; New York City (The Battery), New York; and Galveston, Texas. Nearby CGPS measurements indicated vertical land motion trends of -0.84 +/- 0.18 mm/yr at San Francisco, -2.12 +/- 0.62 mm/yr at New York City, and -4.59 +/- 0.78 mm/yr at Galveston (all with subsiding land). The measured relative sea level rise rates are listed in the graphs.
The relative sea level measurements at two locations with CGPS indicated rising land motion are displayed in Figures 6 and 7 for Oslo, Norway and Vaasa, Finland. Nearby CGPS measurements indicated recent rising land motion of 5.33 +/- 1.12 mm/yr at Oslo and 7.88 +/- 1.14 mm/yr at Vaasa.
Yet again, there is no indication of accelerating sea level rise at San Francisco, New York City, or Galveston or corresponding decelerating sea level fall at Oslo and Vaasa as might be expected if accelerating atmospheric CO2 levels were having an impact. Sea level rise will be enhanced at locations where land is subsiding. Galveston is a good example, where the land is subsiding faster than the sea is rising – a bad combination. And there are many other coastal locations in the same predicament, but it’s the land subsidence that is the main problem at these locations.
Extraordinary claims require extraordinary evidence, and the evidence clearly does not support the claims. The claims of dire sea level rise in coming decades are pure speculation at best and fearmongering at worst.