In recent years there has been much press regarding global warming (climate change) and how this will affect sea level rise, including claims that the oceans could rise up to 3 meters (10 feet) or more by 2100. Just how much weight should we give these high-end projections? If true, then vast areas of the world’s coastline could be flooded with increasing frequency during storm surges or even with king tides affecting huge numbers of people.
The National Oceanic and Atmospheric Administration (NOAA) states that the global temperatures are rising at a rate of about .08C per decade since records began in the mid to late 1800s (1) resulting in a total warming of just over 1 degree C as of 2021. The warming has not been continuous, however, as there was a cooling trend between 1880 and 1910 followed by significant warming of about 0.25 C from about 1910 to 1945. Temperatures remained stable or possibly even cooled slightly between 1945 until about 1978 which was followed by another period of stronger warming from 1978 to the present.
Much of the debate centers on just how accurate are the global temperature records and also how reliable are the long term climate projections made from these data. Are the projections of sea level rise reasonable or are the uncertainties so great that the predictions regarding sea level are unreliable?
Causes of Sea Level Rise
Sea level rise is caused primarily by two main factors related to global warming:
- the added water from melting ice sheets and glaciers and
- the expansion of seawater as it warms.
Other factors that influence sea level include land water use, glacial isostatic adjustment (ongoing movement of land due to past ice-age glaciers), land subsidence, plate tectonics, oceanic volcano activity and the rise or fall of the sea bottom.
Many coastal regions are particularly vulnerable to sea level rise as they are also experiencing significant land subsidence at the same time. Land subsidence often occurs in regions where large amounts of groundwater have been withdrawn from the aquifers below ground. In addition, extraction of petroleum and natural gas, earthquakes, and some types of subsurface mining can also cause land subsidence.
In coastal areas, sinking land leads to much higher relative sea-level rise rates and will result in significantly elevated flood risk. In the United States, the coastal zone of Louisiana is sinking at a rate of 6-12 mm/yr while the lower Chesapeake Bay sinks at 1-5 mm/yr. In Asia it is even much worse with cities like Jakarta and Shanghai having subsidence rates as high as 200 mm/yr.
How do we measure sea level?
Traditionally, global sea level change has been estimated from tide gauge measurements collected over the last century. Tide gauges, usually placed on piers, measure the sea level relative to a nearby geodetic benchmark. (2). Tide gauges are surveyed regularly to check for any settling of the instrument relative to the benchmark. Gauges can also move vertically as the underlying bedrock moves up or down due to post-glacial rebound. Tectonic uplift and local subsidence also can complicate the picture. Tide gauge data suggests that global sea level has risen approximately 10-25 cm during the past century.
Since 1993, satellite altimetry missions have been monitoring the Global Mean Sea Level (GMSL): TOPEX/Poseidon, Jason-1, Jason-2, Jason-3 and the Sentinel-6 MF. After extensive calibration, data from these missions indicate that global sea level has risen at a rate of about 3.0 mm/yr (+ or – 0.4 mm) (4).
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. By comparison, the 20th-century tide gauge based GMSL reconstruction by Dangendorf et al. (7), showed an acceleration rate of 0.018 ± 0.016 mm/yr. Given the relatively large range of estimates, there is still a good deal of uncertainty here regarding the actual rate of acceleration.
Inter-annual VariationTide gauge data also suggests that sea level rates began to rise significantly after about 1920 (3) but the data also shows annual rates have varied quite a bit with 18-year averaged rates varying between 1 and 4 mm/yr, even before 1950, prior to when most of the CO2 emissions occurred. This suggests that the current rates may not be solely driven by greenhouse gas emissions and will add more uncertainty to the various sea level projections. (5).
Tide gauge data show that rates of global mean sea level rise between about 1920 and 1950 were comparable to recent rates as determined by satellite since 1993. The question is whether the elevated sea level rise rates during recent decades represent an acceleration in the longer term rate due to manmade warming, or just a temporary increase due to natural climate variability?
Sea Level Projections
Current projections by the IPCC and NOAA are based on 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. Depending on which scenario is used, the resulting projections of sea level rise can have a large differences over time from 0.3 meters (1 foot) to 2.5 meters (8.2 feet) by 2100, with a corresponding difference in the risk to coastal populations.
Below is the IPCC AR6 table showing the break-down of projected warming for each scenario which results in a wide range of values by 2100 from as little as 1.0 C to as much as 5.7 C.
In the past, 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).
Sea level rise projections for the 21st century are only as valid as the climate model simulations upon which they are based. Bottom line, sea level is rising and the rate appears to be increasing, however, there is still significant uncertainty regarding predicting future warming rates and thus sea level estimates for the next 50-100 years are problematic.
If the rate of sea level rise would double, for example, over the next century from the current satellite estimates, we would expect a total sea level rise of about 0.4 meters (1.3 feet) by 2100. Even if we could reach similar rates as encountered after the last glaciation of 10-13 mm/yr over the next century (more than triple the current rate) then by 2100, the total rise will be about 0.6 meters (2.0 feet).
If you look at the sea level from actual tide gauge data vs. the projected sea levels made by NOAA (9), you will see that the trend since 1996 indicates that the actual levels have been rising at rate at or below the “Intermediate” projection and often even below the “low” projection (see examples below). If this trend continues, then the most likely global sea level rise projection for 2100 would be between 0.3 meters and 1.0 meters (1 foot and 3.3 feet) with the most likely outcome about 0.5 to 0.6 meters (1.5-2 feet). This assumes, of course, that there are no catastrophic meltwater events prior to 2100.
Risk for a Catastrophic Sea Level Rise Event
The question then is how could we get to a 6 to 10 foot sea level as often reported in media accounts by 2100? In order to reach this high level of sea level rise, a sudden enormous pulse of meltwater would be needed. Sudden meltwater pulses are found in the historical record after the end of the last glaciation. The largest of these is known as “Meltwater pulse 1A” (MWP1a) which was a period of rapid sea level rise, between 13,500 and 14,700 years ago, during which global levels rose between 16 meters (52 ft) and 25 meters (82 ft) in about 400–500 years, giving mean rise rates of roughly 40–60 mm (0.13–0.20 ft)/yr (8).
One way this could happen in the current century is if there was a rapid collapse of the Western Antarctic Ice Sheet . The western half of the Antarctic has its ice sheet base lying below sea level so as global temperatures rise, warmer ocean water will melt the underside of the ice sheets.
Modeling suggests that a significant sea level rise is possible from Antarctica ice melt alone by 2100. The key assumption here is that greenhouse gas emissions will boost the planet’s temperature by about 4 degrees C (7 degrees F) which is considerably more than the present 1 degree C of observed warming. This type of rapid collapse is highly uncertain as to timing and dependent on higher warming rates than currently projected.
In order for a sea level rise of 3 meters (10 feet) by 2100, the sea level rise rate would have to accelerate by a factor of more than 10 times the current rate which in my opinion is very unlikely.
Sea level has been rising for some time 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 50-100 years are problematic. Based on observed sea level trends since 1996, the best estimate for sea level rise by 2100 would be between 0.3 meters and 1.0 meters (1 foot and 3.3 feet) with the most likely outcome about 0.5 meters (1.64 feet).
- NOAA State of the Climate
- Sea Level Research Group. University of Colorado, Tide Gauge Sea Level
- Cliff Mass Weather Blog, The Paradox of Sea Level Rise, December 2017
- Ablain, M., Legeais, J.F., Prandi, P. et al. Satellite Altimetry-Based Sea Level at Global and Regional Scales. Surv Geophys 38, 7–31 (2017).
- Climate.gov, 18 year sea level rates [IPCC AR5 WG 1 Chapter 3 Figure 3.14]
- Global Mean Sea Level – Graph, NOAA.gov
- Sönke Dangendorf, Marta Marcos, Guy Wöppelmann, Clinton P. Conrad, Thomas Frederikse, et al. Reassessment of 20th century global mean sea level rise. PNAS (2017)
- Wikipedia, Meltwater pulse 1A
- NOAA Technical Report NOS CO-OPS 083: GLOBAL AND REGIONAL SEA LEVEL RISE SCENARIOS FOR THE UNITED STATES, 2017