Is sea level rise accelerating?

by Judith Curry
Estimates of the rate of sea level rise are diverging.

For background on the topic of sea level rise, see these previous posts:

New PNAS papers
The current buzz surrounding sea level rise is associated with two papers just published in PNAS:
Temperature driven global sea level variability in the Common Era by Kopp et al. of Rutgers University, published in PNAS.
Future sea level rise constrained by observations and long-term commitment by Mengel et al., published in PNAS
From the phys.org summary:
Global sea level rose faster in the 20th century than in any of the 27 previous centuries, according to a Rutgers University-led study published today.
Moreover, without global warming, global sea level would have risen by less than half the observed 20th century increase and might even have fallen.
Instead, global sea level rose by about 14 centimeters, or 5.5 inches, from 1900 to 2000. That’s a substantial increase, especially for vulnerable, low-lying coastal areas. “The 20th century rise was extraordinary in the context of the last three millennia – and the rise over the last two decades has been even faster,” said Robert Kopp, the lead author.
The study used a new statistical approach developed over the last two and a half years by Kopp, his postdoctoral associates Carling Hay and Eric Morrow, and Jerry Mitrovica, a professor at Harvard University.
Notably, the study found that global sea level declined by about 8 centimeters [3 inches] from 1000 to 1400, a period when the planet cooled by about 0.2 degrees Celsius [0.4 degrees Fahrenheit].
A statistical analysis can only be as good as the data it’s built upon. For this study, a team led by Andrew Kemp compiled a new database of geological sea-level indicators from marshes, coral atolls and archaeological sites that spanned the last 3,000 years.
The database included records from 24 locations around the world. The analysis also tapped 66 tide-gauge records from the last 300 years.
Kopp’s collaborators Klaus Bittermann and Stefan Rahmstorf at the Potsdam Institute for Climate Impact Research in Germany used the study’s global sea-level reconstruction to calculate how temperatures relate to the rate of sea-level change. Based on this relationship, the study found that, without global warming, 20th century global sea-level change would very likely have been between a decrease of 3 centimeters [1.2 inches] and a rise of 7 centimeters [2.8 inches].
A companion report finds that, without the global warming-induced component of sea-level rise, more than half of the 8,000 coastal nuisance floods observed at studied U.S. tide gauge sites since 1950 would not have occurred.
Stefan Rahmstorf has a RealClimate post discussing the new papers.
Additional media reports:

An interesting angle from the NYTimes article:
One of the authors of the new paper, Dr. Rahmstorf, had previously published estimates suggesting the sea could rise as much as five or six feet by 2100. But with the improved calculations from the new paper, his latest upper estimate is three to four feet.
That means Dr. Rahmstorf’s forecast is now more consistent with calculations issued in 2013 by the Intergovernmental Panel on Climate Change, a United Nations body that periodically reviews and summarizes climate research. That body found that continued high emissions might produce a rise in the sea of 1.7 to 3.2 feet over the 21st century.
Roger Pielke Jr tweets:  Sans spin: Climate scientist known for being wrong admits error, reduces estimate of sea level rise by 50%. IPCC was right
NASA
Last August, NASA  issued two new press releases:  Warming Seas, Melting Ice Sheets and NASA Science Zeros in on Ocean Rise. How Much?  How Soon?  It seems that these press releases were triggered by a new interdisciplinary Sea Level Change Team.  Excerpts:
For thousands of years, sea level has remained relatively stable and human communities have settled along the planet’s coastlines. But now Earth’s seas are rising. Globally, sea level has risen about eight inches since the beginning of the 20th century and more than two inches in the last 20 years alone.
All signs suggest that this rise is accelerating.
NASA has been recording the height of the ocean surface from space since 1992. That year, NASA and the French space agency, CNES, launched the first of a series of spaceborne altimeters that have been making continuous measurements ever since. The first instrument, Topex/Poseidon, and its successors, Jason-1 and -2, have recorded about 2.9 inches (7.4 centimeters) of rise in sea level averaged over the globe.
Ok, some simple arithmetic.  7.4 cm over a period of 23 years equals 3.2 mm/yr.  No surprise – this is the number cited by the AR5 for satellite based sea level rise estimates.
“Given what we know now about how the ocean expands as it warms and how ice sheets and glaciers are adding water to the seas, it’s pretty certain we are locked into at least 3 feet of sea level rise, and probably more,” said Steve Nerem of the University of Colorado, Boulder, and lead of the Sea Level Change Team. “But we don’t know whether it will happen within a century or somewhat longer.”
Here is the last sea level rise plot from the NASA altimeters:

IPCC AR5
I don’t really see anything new here, but the new PNAS papers and the NASA press release serve to highlight an issue that has been nagging me.
The IPCC AR5 includes the following figure of sea level rise over the last century (Figure 3.14)

Figure 3.14 18-year trends of global mean sea level rise estimated at 1-year intervals. The time is the start date of the 18-year period, and the shading represents the 90% confidence. The estimate from satellite altimetry is also given, with the 90% confidence given as an error bar. [AR5 WGI Figure 3.14]
Global sea level has been rising for the past several thousand years, owing to the retreat of glaciers from the last ice age. The key issue is whether the rate of sea level rise is accelerating owing to anthropogenic global warming. It is seen from the figure above that the rate of sea level rise during 1930-1950 was comparable to, if not larger than, the value in recent years. The challenges to determining global sea level rise, particularly over the past 100 or 1000 years, are substantial.
Somehow, the IPCC AR5 drew this conclusion, in context of Figure 3.14:
“Since the early 1970’s, glacier mass loss and ocean thermal expansion from warming together explain about 75% of the observed global sea level rise (high confidence)”
Go figure.
Other recent papers
Other recent papers that I’ve collected on the topic:
Modeling sea-level change using errors-in-variable integrated Gaussian processes, by Cahill et al. (published in Journal of Applied Statistics)
“Our analysis of the global tide-gauge record shows that the rate of GMSL rise increased (accelerated) continuously from 1.13 mm/yr in 1880 AD to 1.92 mm/yr in 2009 AD.” While Cahill et all find an acceleration in sea level rise over the past century, their inferred rate of current sea level rise (< 2 mm/yr) is significantly lower than the value of 3.2 mm/yr derived from satellite altimetry measurements.
Robust reef growth in Indian Ocean kept up with sea level rise > 20X faster in past [link] …
New paper shows sea levels dropped along west coast of North & South America from 1993-2013 [link] …
Coastal planning should be based on proven sea level data, by Parker and Ollier
Highlights:

  • The network of tide gauges provides the only information of value for costal planning.
  • The worldwide naïve average of sea level is +0.24 mm/year with no acceleration.
  •  The climate models have crucial flaws making them useless.
  • Planning schemes must only reflect the proven local and global historical data.

JC reflections
So, what to make of all this?
Sea level rise is the main ‘danger’ from human caused climate change (any increase in extreme weather events is hypothesized rather demonstrated using historical data, with possible exception of heat waves in a few regions).
At a presentation that I made earlier this year to CEOs of small electric cooperatives, one participant was surprised by what I had to say about sea level rise – he hadn’t realized that there had been sea level rise prior to 1950.  I.e., like ‘climate change’, all sea level rise has been sold as caused by humans.
Sea level has overall been rising for thousands of years; however, as the Kopp et al. paper points out, there have been century scale periods of lowering sea level in the recent millennia.  It is not clear from my cursory reading as to whether meaningful decadal and multi-decadal variations in sea level can be discerned from their data.
The key issue is whether the sea level rise during the past 50 years reflect an acceleration in sea level rise.  The IPCC figure 3.14 suggests that there is no acceleration, given the large rates of sea level rise in the first half of the 20th century.  Until we have an understanding of variations in decadal and multi-decadal sea level rise, we can’t make a convincing argument as to acceleration.
With regards to coastal planning, I absolutely agree with the paper linked to above.  Locations where sea level rise is a problem invariably have rates of sea level rise that are much greater than even the altimeter values of 3.2 mm/yr are caused by local geologic processes, land use, and or coastal/river engineering.  Global values of sea level rise have essentially no use in coastal planning; rather they seem mainly relevant in terms of motivating ‘action’ on carbon mitigation policy.
Sea level will continue to rise, no matter what we do about CO2 emissions.  We need creative solutions – one of my favorites remains the garbage solution.
 Filed under: Oceans

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https://judithcurry.com/2016/02/23/is-sea-level-rise-accelerating/

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Judith Curry