By Dr Sebastian Lüning and Prof Fritz Vahrenholt
(Translated, edited and condensed by P Gosselin)
In recent posts we examined sea level rise, and in our last segment here we will look at the current sea level rise from a historical context and projections for the future.
Last 180 million years
On a geological scale, sea level over the last 180 million years was higher than it is today about half of the time (Figure 1).
Figure 1: Seal level over the last 180 million years. Source: David Middleton.
The last 12,000 years
Since the end of the last ice age, we see a rapid rise of more than 10 mm per year until about 8000 years ago. Then came a slowdown 4000 years ago (Figure 2). Today’s sea level rise of 1-3 mm per year is what we are seeing at the end phase of this interglacial period.
Figure 2: Sea level rise over the last 14,000 years. Source: David Middleton.
Today we find ourselves deep into a post-ice age interglacial warm period. During the last ice age the sea level was 120 m lower than today’s level. During the last interglacial, the Eem interglacial of 126,000 to 115,000 years before present, sea level was up to 9 m higher than today – without any help from man. This has been shown by a number of studies (Dutton & Lambeck 2012 in Science, Muhs et al. 2012 in Quaternary Research, O’Leary et al. 2013 in Nature Geoscience).
Last 300 years
The overall general trend shows cyclic behavior that follows the rhythm of warm and cold periods. Ice increased during the Little Ice Age, and so sea level rise stopped (Figure 3). During the transition to the current Modern Warm Period, which began around 1850, sea level began to rise once again and reached its top speed by 1920. The overall speed has not changed in the last 100 years, and thus it is difficult to see any evidence of a man-made impact in this development. There was a rapid increase from 1860 to 1879, but because industrialsation was barely beginning, the cause cannot be man-made. Today’s rate of rise is similar. In fact man-made global CO2 emissions did not begin in earnest until 50 years ago.
Figure 3: Sea level over the last 300 years. Source: Roger Andrews.
Although we keep hearing media reports of an accelerated sea level rise, hard measurement data show that it is merely a resumption of the post little Ice Age rise. Interestingly the IPCC in its first 1990 assessment viewed this similarly. There they wrote:
Excerpt from the IPCC first assessment report of 1990 concerning sea level development. Thanks to Climate Depot.
The estimate of the IPCC from 1990 is still solidly based – as Judith Curry confirmed at Climate Etc. in 2012:
The evidence for accelerating anthropogenic sea level rise is pretty weak, and lost in the noise of natural variability.”
And when one looks at the approximately 200 existing coastal datasets with long measurement periods, one comes up with an average rise of only 1.1 mm per year. This is much less than the values from satellites, which are more than 3 mm. One reason may be calibration problems. Satellite data has been corrected upwards as the versions from 2004 and 2013 show (Figure 4).
Figure 4: Satellite sea level data were adjusted considerably upwards. Source: The Hockey Schtick.
Currently the Jet Propulsion Laboratory of NASA is investigating possible sources of error in satellite sea level measurement.
The last decades
We observe that sea level rise is pulsing with an about 60-year cycle (Figure 5), which we have discussed on several occasions in the past. This has to do with the PDO, AMO and NAO ocean cycles (see our blog article: Forscherteam der University of Colorado Boulder: Ozeanzyklen haben Meeresspiegelanstieg in den letzten 20 Jahren verstärkt).
The impact of ocean cycles was recently examined by a team led by Benjamin Hamlington of the University of Colorado in Boulder. The authors reported in an article appearing in the Geophysical Research Letters October 2013: “Contribution of the Pacific Decadal Oscillation to global mean sea level trends”. According to the authors, over the last 20 years the PDO contributed about 0.5 mm per year to the observed sea level rise. The scientists say that the PDO’s contribution have to be taken into consideration over the long term and taken into account in prognoses. The PDO will act to reduce the long-term rise, the scientists write.
Figure 5: Sea level rise over the last 90 years. Source: David Middleton.
In January 2014 a group of scientists led by Xianyao Chen published an analysis of global sea level trends based on satellite data in the journal Global and Planetary Change. Here they detected a deceleration in sea level rise, from 3.2 mm in the period 1993 – 2003, to only 1.8 mm per year in 2008. The reason: Pacific ocean cycles.
Figure 6: Sea level development over the last 20 years based on satellite data in comparison to prognoses from the Rahmstorf group.
Under the bottom line: sea level rise has been pretty much constant over the last 80 years. Using the same rate, sea level can be projected to rise only 20 – 30 cm by the year 2100.
Spectacular sea level rise projections unscientific
The IPCC bases its 1-meter plus sea level rise on assumptions that sea level rise will accelerate in the future (Figure 7).
Figure 7: Sea level rise over the last 100 years and prognoses for the coming 85 years. Source: Roger Andrews.
The apocalyptic scenarios come from a group of scientists led by Stefan Rahmstorf. Interestingly Rahmstorf and his colleagues even float prognoses of 2.3 meters by 2300, even though the figure scientifically cannot be taken seriously. Forecasts for the year 2100 or 2300 can never be verified and so those making projections will never have be held accountable. So easily can spectacular projections be made.
Sea level rise far below earlier projections
Things are different for near and mid-term prognoses. Here as time goes by, earlier projections can be compared to observations. In its first report in 1990, the IPCC predicted a sea level rise of 120 mm by 2014. Today, 24 years later, we are able to see how accurate that 1990 prediction really was: It turns out to be only about one fifth of what was projected. In a nutshell: the IPCC 1990 projection was off by a mile.
Scientists made a number of false assumptions, according to Alberto Boretti of the University of Ballarat in Australia. In a study in the journal of Coastal Engineering, Boretti expressed deep doubts over the models from the Rahmstorf group. Overly simplistic models were also criticized by a group of 18 scientists led by Jonathan Gregory of the University of Reading in an article appearing in the Journal of Climate, July 2013. Gregory and his team found no accelerated glacial melting over the last decades. More info on this at The Hockey Schtick. The abstract reads:
Semiempirical methods for projecting global-mean sea level rise (GMSLR) depend on the existence of a relationship between global climate change and the rate of GMSLR, but the implication of the authors’ closure of the budget is that such a relationship is weak or absent during the twentieth century.
To sum it up, the following seafaring saying aptly applies in the current climate discussion:
The rat that leaves the sinking ship is smarter than the captain who goes down with it. Jonathan Swift (1667-1745)