Is The Rate Of Sea Level Rise Accelerating?
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By Paul Homewood
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Sea level alarmists often claim that the rate of sea level rise is increasing. They need to do this to convince the public that the innocuous 7” or so of sea level rise experienced in the 20thC is suddenly going to turn into meters by 2100.
They use two tricks to back up this claim:
1) They splice the satellite record, which only started in 1993, onto the tidal gauge records.
According to satellites, sea levels have been rising at 3.4mm/yr. Whether this figure is right or not, no half competent scientist would dream of splicing two totally different sets of data together in such a way.
Worse still, their banner figure of 3.4mm includes what is known as glacial isostatic adjustment (GIA), which accounts for the fact that the ocean basins are getting slightly larger since the end of the last glacial cycle.
In other words, if the basins were not getting larger, sea levels would rise more. To account for this, they add 0.3mm a year to their sea level figures.
This is all well and good, if it were not for the fact that tidal gauges do not include such an adjustment, so the comparison of satellites and gauges becomes incompatible.
2) They compare recent sea level rise with the 20thC average.
However, sea levels were not rising at an even pace during the last century. There were times when it was rising at rates similar to today, and others, notably between 1950 and 1980 when global temperatures were falling, which saw a lower rate of rise.
As the IPCC stated in its 2013 AR5 report:
It is very likely that the mean rate of global averaged sea level rise was 1.7 [1.5 to 1.9] mm/yr between 1901 and 2010 and 3.2 [2.8 to 3.6] mm/yr between 1993 and 2010. Tide gauge and satellite altimeter data are consistent regarding the higher rate during the latter period. It is likely that similarly high rates occurred between 1920 and 1950
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So, the current rate of rise is not unprecedented, and does not “prove” that the rise will continue to accelerate. Indeed, if the 20thC record is anything to go by, it could well slow down again, as part of a natural cycle.
The year 1993 is also not a very reliable place to start from, because it was just after the Pinatubo eruption in 1991. This lowered global temperatures, leaving sea levels also lower than they would otherwise have been, consequently increasing the subsequent upward trend.
Interestingly, University of Colorado sea level page refers to this, in a paper by John Fasullo:
Abstract:
Global mean sea level rise estimated from satellite altimetry provides a strong constraint on climate variability and change and is expected to accelerate as the rates of both ocean warming and cryospheric mass loss increase over time. In stark contrast to this expectation however, current altimeter products show the rate of sea level rise to have decreased from the first to second decades of the altimeter era. Here, a combined analysis of altimeter data and specially designed climate model simulations shows the 1991 eruption of Mt Pinatubo to likely have masked the acceleration that would have otherwise occurred. This masking arose largely from a recovery in ocean heat content through the mid to late 1990 s subsequent to major heat content reductions in the years following the eruption. A consequence of this finding is that barring another major volcanic eruption, a detectable acceleration is likely to emerge from the noise of internal climate variability in the coming decade.
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So we learn two things:
a) The rate of sea level rise has actually slowed down in the last decade.
b) The rate of rise in the first decade of satellite data was artificially high, because of the recovery in ocean heat content in the mid 1990s, when the effects of Pinatubo had disappeared.
A 2013 paper by Chen et al also found a similar slowdown in sea level rise:
It is found that the GMSL rises with the rate of 3.2 ± 0.4 mm/yr during 1993–2003 and started decelerating since 2004 to a rate of 1.8 ± 0.9 mm/yr in 2012.
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In any event, expert oceanographers know full well that measuring trends over just a decade or two is meaningless. Top ocean scientist, Bruce Douglas, wrote a paper, “Global Sea Rise: a Redetermination”, in 1996, with the Abstract stating:
It is well established that sea level trends obtained from tide gauge records shorter than about 50-60 years are corrupted by interdecadal sea level variation.
In particular, sea levels are particularly influenced by ENSO changes, as CU show.
During El Ninos sea levels rise:
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One more complication is groundwater depletion, via irrigation, water use etc. A study by Yoshide Wada in 2012 estimated that this had added 0.44mm/yr to sea levels since 1993.
So, far from the scary headlines put about, the rate of recent sea level rise has not been unprecedented, is not accelerating, and shows no sign doing so in years to come.
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June 29, 2017 at 01:33PM
Iowa Climate Science Education 