Some Background To Sea Level Measurements
By Paul Homewood
Some comments on Tuesday’s post, UCS Forget To Mention Subsidence, made me wonder if there is a bit of confusion around about sea level changes.
So a quick recap on the basics, and clarification about some of the terminology used. ( Pardon, if I am stating the obvious).
A) Tidal Gauges
There is a worldwide network of tidal gauges, from which measurements are collated by the PSMSL, Permanent Service for Mean Sea Level, based in Liverpool.
Gauges measure sea levels relative to the land. This is sometimes referred to as Relative Sea Level or RSL.
These sea levels can change essentially for two reasons.
1) Eustatic Change
In simple terms, this refers to the volume of water in the ocean, which can increase, for instance, because of glacial melting and thermal expansion.
The other mechanism mentioned is a change in the size of the ocean basin that contains it, but this is naturally a factor that only changes over extremely long periods of time.
Eustatic sea level is also referred to as Absolute Sea Level, or ASL.
2) Isostatic Change
This relates to the sinking/rising of land, and can be due to a number of factors, such as:
a) Post glacial rebound – land mass which used to lie under ice age glaciers is gradually rising, while peripheral areas outside the glaciation, which had “bulged” during the ice age, are now sinking back again.
b) Tectonic movements.
c) Localised subsidence, due, for instance, to extraction of water.
d) Regional geologic factors, such as Chesapeake Bay, which is the site of an ancient comet impact crater.
A combination of all these factors can lead to vast differences between sea level change from one part of the world to another.
Sea Level Measurements
All sea level measurements taken from tidal gauges are recorded as “actuals”, and not adjusted for the effect of land sinking/rising.
B) Satellite Measurement
Measurements of sea level height have been taken from satellite systems, such as Jason-2, since 1992. As this suggests, it is the height of the sea surface which is measured, by calculating the distance between the ocean and the satellite.
Therefore, any rising or sinking of land is irrelevant, and not adjusted for.
The estimates of Global Mean Sea Level, however, by the University of Colorado, and I believe all of the other major sets, do include an estimate for the effect of the ocean basins getting larger since the end of the ice age. As UC explain:
The correction for glacial isostatic adjustment (GIA) accounts for the fact that the ocean basins are getting slightly larger since the end of the last glacial cycle. GIA is not caused by current glacier melt, but by the rebound of the Earth from the several kilometer thick ice sheets that covered much of North America and Europe around 20,000 years ago. Mantle material is still moving from under the oceans into previously glaciated regions on land. The effect is that currently some land surfaces are rising and some ocean bottoms are falling relative to the center of the Earth (the center of the reference frame of the satellite altimeter). Averaged over the global ocean surface, the mean rate of sea level change due to GIA is independently estimated from models at -0.3 mm/yr. The magnitude of this correction is small (smaller than the ±0.4 mm/yr uncertainty of the estimated GMSL rate), but the GIA uncertainty is at least 50 percent. However, since the ocean basins are getting larger due to GIA, this will reduce by a very small amount the relative sea level rise that is seen along the coasts.
Basically, if the ocean bottom was not sinking, sea level rise as measured by tidal gauges would be greater.
With the help of this adjustment, they manage to get the rate of sea level rise, according to their figures, up from 2.8mm/yr to 3.1mm/yr, since 1993. So clearly this is not the “small” adjustment they claim, as it is more than 10% of the change!
However, the real issue comes when the satellite figure of 3.1mm, incl GIA, is compared with sea level rise prior to 1993, based on tidal gauges. The latter, of course, as we have seen measure sea level relative to the shoreline, and take no account of ocean bottoms falling and holding more water.
For a proper comparison to be made, the GIA needs to be excluded, which never seems to happen.
In any event, as NOAA state “One of the most significant potential impacts of climate change is sea level rise that may cause inundation of coastal areas.”. It is the SEA LEVEL which concerns us, and not more water accumulating in the bottom of the ocean. The clue is in the name – Global Mean Sea Level!
Scientists may find it useful to include an estimate for GIA, when they do their own analysis. But the official, published number should not include this adjustment, as it is grossly misleading.