Sea Level

Sea levels have risen spectacularly since the end of the last glacial maximum, rising some 140 m in total. But in the last 8,000 years, sea levels have risen slowly and steadily. Actual measurements from tidal gauges in many places around the world averages 1.4-1.7 mm/yr , which equates to 6 inches per century and there is no evidence of an increase in the rate of sea level rise in recent years, though there is constant variability from year to year. This is well within what would be considered normal and would need to rise an additional 5 to 10 m to reach levels in previous inter-glacials.

Compare the .17m rise over 100 years to what is regularly experienced from lunar tides:

  • Coastal tidal ranges vary globally and can differ anywhere from near zero to over 11 metres (38 feet).[2](Bay of Fundy can be as much as 16.3m or 53.5ft)
  • Perigean and Apogean spring tides can add 5cm or 2in to normal tidal range.
  • Storm Surge can also he high, such as with Hurricane Katrina in 2005, which produced a maximum storm surge of more than 25 ft (8 metres)
IPCC Claims

As one possible reference, we can look at what the UN IPCC said about sea level rise in their last report:

  • Current rate of sea level rise is .17m per century for the last 100+ years. {IPCC AR5 pg 46}
  • During the last interglacial (116,00-129,000 years ago) sea level was 5 to 10m higher than present. {IPCC AR5 pg 385}

The Eemian period (the last Interglacial, 125,000 years ago) is probably a more useful analogue for the future. Global average temperature was about 2 degrees higher than now, whereas sea level was 4-6 metres higher than now (“with individual maxima up to +7 or +9m“).

 

Historical Perspective

The following two graphics show the rise of sea level during the current inter-glacial. This first graph captures the rise in ocean levels since the beginning of the current inter-glacial that started 14 thousand years ago. The second graphic zooms in on the last 8,000 years where sea levels have been remarkably stable, with a steady but gradual rising trend.

post-glacial_sea_level-incl-3-mm-yr-1-trend
Sea level since the last ice age. As the ice from the last ice age was melting, sea levels rose by some 120 metres over the course of about 8000 years, before it flattened out ~6000 years ago.
holocene_sea_level-incl-trend
Zoom in on the last 9000 years (covering most of the Holocene epoch). The strong sea level rise at the end of the last ice age is still visible on the left hand side, slowing down 7000 years ago and even more so 4000 years ago.
Sea level increase in recent years.

The rate of coastal sea-level rise, as measure from tide gauge measurements averages  about 1.4-1.5 mm/yr (under six inches per century), and that rate hasn’t increased since the late 1920s. It is well worth examining the individual tidal graphs as they are all inconsistent, some showing positive rise, some showing a negative drop.

However, Nasa now reports sea level from satellite measurements but does so by offering adjusted rates and seems to give contradictory information.

nasa-sea-level

Their graphic is quite misleading. The 3.42 mm/yr rate does not apply to both graphs. Look closer at the scales, and do the arithmetic, and you’ll realize that the 2nd graph is actually showing a slope of only about half that. It shows a slope of about 1.8 mm/yr, with no evidence of acceleration.

What’s more, the second graph does not represent tide gauge data; it’s from tide gauge data inflated by a +0.3 mm/yr GIA “adjustment,” to subtract off the rate by which the sinking ocean floor is hypothesized to reduce sea-level rise. The real rate of coastal sea-level rise from averaged tide gauge measurements is only about 1.4-1.5 mm/yr (under six inches per century), and that rate hasn’t increased since the late 1920s.

Even NASA’s first graph, of satellite altimetry measurements, is deceptive. It also uses calculated values, not actual observational data and also adds the model-derived GIA, plus by combining the various satellite measurements it also hides the wide variations in rates measured by different satellites as show below:

MSL_Serie_ALL_Global_IB_RWT_NoGIA_Adjust

 

Rate of change of Sea Level Rise

So, is NASA right? Are sea levels rising faster than before?

Here is a graph showing the changes to sea level rise from one year to the next for the last century. We are not seeing any appreciable acceleration in sea level. Plenty of fluctuation for sure. And if you were to take the period 1910 to 1940 you could argue that sea level rise was accelerating. But that would be misleading of course.

sea-level
Can someone then tell me how you can infer that sea level rise is accelerating due to AGW, when compared with sea level rise for the first half of the 20th century? It is clear that natural variability has dominated sea level rise during the 20th century, with changes in ocean heat content and changes in precipitation patterns.

Here are some references if you would like to explore this issue further.
NOAA tide gauge data shows no coastal sea level rise acceleration

is sea level rise accelerating

Slowing sea level rise

An answer to is the rise in sea levels accelerating

 

This science site says 1/8 inch per year which equates to 12.5 inches over 100 years.

https://oceanservice.noaa.gov/facts/sealevel.html

https://www.skepticalscience.com/sea-level-rise-intermediate.htm

 

Sea Level