Comments on this topic are closed. There is a new article on sea level where comments are welcome.
Sou 24 October 2014
Anthony Watts has seen fit to post a rather silly comment from Eric Worrall about a new PNAS paper on sea level and ice. Anthony also adds his tuppence-worth. (Archived here.)
The paper is from a team led by Professor Kurt Lambeck of The Australian National University (ANU).
A fascinating journey up and down the seas
What the Kurt Lambeck and his co-authors have done is paint a wonderfully vivid 35,000 year history of changes in sea level and major ice sheets.
It's taken me a while to read the paper. In part because the subject matter is provided in rich detail, and in part because I found it so fascinating. It's extraordinarily well written. The authors have managed to cram a huge amount of information into the few words allowed by PNAS, while writing in a manner that a lay person like myself could understand.
1,000 observations of sea level
The paper has a text box on the first page headed "Significance", which explains the value of the research to earth science of documenting changes in sea level going back 35,000 years. It also highlights their finding that, over the past 6,000 years and up until around 150 years ago, there was no evidence for global oscillations in sea level lasting longer than around 200 years or being larger than around 15 to 20 cm in amplitude. It states:
Several areas of earth science require knowledge of the fluctuations in sea level and ice volume through glacial cycles. These include understanding past ice sheets and providing boundary conditions for paleoclimate models, calibrating marine-sediment isotopic records, and providing the background signal for evaluating anthropogenic contributions to sea level. From ∼1,000 observations of sea level, allowing for isostatic and tectonic contributions, we have quantified the rise and fall in global ocean and ice volumes for the past 35,000 years. Of particular note is that during the ∼6,000 y up to the start of the recent rise ∼100−150 y ago, there is no evidence for global oscillations in sea level on time scales exceeding ∼200 y duration or 15−20 cm amplitude.
In other words, the rise in sea level observed over the past 150 years looks to be unusual in the context of the past 6,000 years. Particularly as the sea level rise isn't showing any sign of stopping. Quite the contrary, sea levels will continue to rise and by quite a lot as the ice sheets in Antarctica and Greenland continue to melt.
Yet to me that's not the most fascinating feature of the paper, probably because I'd pretty well figured that out (as you have too, no doubt). If you have the chance to read the paper, you'll get an account of what happened to sea level and ice going back way before the past 6,000 years - right back to 35,000 years ago, and you'll learn the basis of that account. The paper is still available here. I don't know for how long.
Climate and sea level: A life's work
The authors used data from many, many locations to create the history. Where there were gaps in the data, they used information from models to work out what probably happened. It has probably taken years of research. I notice that Professor Lambeck has been publishing papers on the subject for quite some time. David Mark interviewed him on ABC PM the other day, and opened with:
David Mark: "Several decades ago Kurt Lambeck, a Professor of geophysics at the ANU, set out to answer a question".
Kurt Lambeck: "What is the relation between climate and sea level, and what can we learn about this? So it's been a long, long study that has taken us around the world."
Here is Figure 4 from the paper, showing what they term the ice-volume equivalent sea level plus more. Click to enlarge:
|Fig 4. Solution for the ice-volume esl function and change in ice volume. (A) Individual esl estimates (blue) and the objective estimate of the denoised time series (red line). The Inset gives an expanded scale for the last 9,000 y. (B) The same esl estimate and its 95% probability limiting values. Also shown are the major climate events in the interval [the Last Glacial Maximum (LGM), Heinrich events H1 to H3, the Bølling-Allerød warm period (B-A), and the Younger Dryas cold period (Y-D)] as well as the timing of MWP-1A, 1B, and the 8.2 ka BP cooling event. (C) The 95% probability estimates of the esl estimates. (D) Estimates of sea-level rate of change. Source: Lambeck14|
Near-field and far-field
In the above, you can get some idea of the how much data underpins the estimates of different periods. Fig. 1 in the paper shows the distribution of what the authors refer to as far-field sea-level data, which comes from lots of different places and lots of different sources. To explain near-field and far-field, I'll quote from the paper:
The sea-level response within, or close to, the former ice margins (near-field) is primarily a function of the underlying rheology and ice thickness while, far from the former ice margins (far-field), it is mainly a function of earth rheology and the change in total ice volume through time.
The data is much and varied in type and location. The paper goes into a lot of detail to explain how it was interpreted and how periods of sparse data were dealt with.
35,000 years of sea level history
Here is a bit of a summary of the changes in sea level and ice cover, but I urge you to read the paper itself if you are interested in the subject. The authors start their history at the beginning of the period they looked at. I've added my own commentary about what was happening to human society - and what might have happened had the world been like it is today.
Imagine if today's world existed 35,000 years ago. For around four thousand years, we'd barely have noticed the slight decline in sea level. We'd have plodded along - on less land, and being quite a bit colder than today, for many generations. Then suddenly we'd be faced with a rapid cooling and we'd find the sea level dropping by around 25 metres over the next 1,000 years.
For the next 8,000 years or so, from 29,000 to 21,000 years BP (before present = before 1950), we'd struggle on through cool climates. If we lived in Scandinavia we'd have built houses from ice and snow, or emigrated south to get away from eastward and southward expansion of the Scandinavian ice sheet. At the end of this period, the equivalent sea level was at its lowest point of the past 35,000 years, at 134 metres, "corresponding to ∼52 × 106 km3 more grounded ice—including on shelves—at the LGM [last glacial maximum] than today".
At last it would start to warm up again. The ice would start to retreat. Over a period of four thousand years or so, starting around 21,000 to 20,000 years BP, seas would rise - a lot. By around 10 to 15 metres before 18,000 years ago.
Things would settle down for a few generations. There'd be relative stability for about 1500 years from 18,000 to 16,500 years ago.
The next 10,000 or so years would see a major retreat of ice, from 16,500 years ago to around 7,000 years BP. The equivalent sea level would rise by around 12 metres each thousand years, with a total rise of around 120 metres. That's a lot of coastline disappearing. Had it been a modern society with today's world population, people would have had to keep relocating towns and cities inland further and further. But it wasn't. Human society was still in its infancy. It was during the latter part of this period that people moved out of the Stone Age and into farming.
During this period, about 14,500 years BP there was a very rapid rise in sea level lasting up to 500 years (maybe less). Seas rose at around 40mm a year. The paper states: "the globally averaged rise in sea level of ∼20 m occurs at a rate of ∼40 mm•y−1 or greater." That would have made life a bit difficult if people hadn't been fairly mobile and if there'd been the same population density as today. Which, of course, there wasn't.
Then there were some periods where sea levels rose, then steadied then rose again. After which there was a period from around 11,400 to 8,200 years BP "of near-uniform global rise". It rose a lot. Around 1.5 metres a century, and seems to have been fairly steady. This would have been during the time when civilisations began. People started making tools out of metal, and farming.
The final phase of the North American deglaciation occurred around 7,000 years ago. It showed up in this research as a period of slower sea level rise - over the period 8,200 to 6,700 years BP.
From 6,700 years ago until recently (around 150 years ago), the seas rose but at a progressively declining rate. From the paper (my paras):
A progressive decrease in rate of rise from 6.7 ka to recent time. This interval comprises nearly 60% of the database (Fig. 1). The total global rise for the past 6.7 ka was ∼4 m (∼1.2 × 106 km3 of grounded ice), of which ∼3 m occurred in the interval 6.7–4.2 ka BP with a further rise of ≤1 m up to the time of onset of recent sea-level rise ∼100–150 y ago.
In this interval of 4.2 ka to ∼0.15 ka, there is no evidence for oscillations in global-mean sea level of amplitudes exceeding 15–20 cm on time scales of ∼200 y (about equal to the accuracy of radiocarbon ages for this period, taking into consideration reservoir uncertainties; also, bins of 200 y contain an average of ∼15 observations/bin). This absence of oscillations in sea level for this period is consistent with the most complete record of microatoll data from Kiritimati (23).
The record for the past 1,000 y is sparse compared with that from 1 to 6.7 ka BP, but there is no evidence in this data set to indicate that regional climate fluctuations, such as the Medieval warm period followed by the Little Ice Age, are associated with significant global sea-level oscillations.So, there you have it. A potted history of the seas and ice of the past 35,000 years. But do go and read the paper if you can. It's a lot richer in content than I was able to portray here. For example, the paper describes how the mantle changes as ice melts and forms. How the scientists were able to distinguish sea level rise and fall from land rising and falling (eg coral atolls). The paper gives an inkling of the painstaking research that must have gone into preparing this record. It reads like an adventure story - or as close as you'll get from a PNAS paper about sea level and glaciation :)
The denier protest
Of course the paper got a dumb protest from WUWT. What did you expect? Anthony Watts often claims the equivalent of saying that ice doesn't melt when it gets hot. It's pretty clear from his knee jerk reaction every time sea level is mentioned, that one thing that frightens him, probably more than anything else about climate change, is the prospect of rising seas.
This article was no exception. Of course he didn't read the paper. I doubt he even read his copy and paste of an article. First he copied some of what Eric "eugenics" Worrall wrote (archived here):
Claim: No change in sea level until modern times – but that change is dwarfed by sea levels of the past
Eric Worrall writes of a new paper trying to blame sea level rise on The Industrial Revolution, which started about 150 years ago:
The Australian National University has published a startling claim that sea level change has been more or less steady for the last 6000 years – until 150 years ago, when the sea started rising more rapidly.
I don't know where Anthony got the notion that the paper claimed no change in sea level until modern times. As you can see from the above, it said nothing of the sort. It was a detailed documentation of lots and lots of change in sea level over the past 35,000 years. In fact, his WUWT headline is contradicted by his second paragraph, where he writes that sea level change had been more or less steady. Maybe he can't tell the difference between steady change and no change. (How dumb is that!)
Nor do I know why they are startled to learn that the past 6,000 years showed fairly steady change. Maybe they figured that with temperatures dropping a bit for most of that time, that seas would have dropped to. I figure it's because ice and ocean currents can take some time to act in response to a gradual warming after the last glacial maximum. If anyone wants to chip in here, feel free.
Anthony pops in to his own article and adds his tuppence worth, writing:
But, other science suggests even higher sea levels during interglacials.
A paper published April 17th 2014 in Nature reconstructs sea levels over the past 5.3 million years and shows that sea levels were higher than the present during almost every interglacial period over the past 5.3 million years...
You'll recall how often people at WUWT claim that the world has stopped warming - based on the last (variously) 16, 17, 18, 19 or even 20 years of global surface temperature - what they call the "pause", though the hottest year on record was a mere four years ago. Suddenly Anthony Watts turns to a very different world, a world of five million years ago, as if to say "nothing to see here, move along".
Sea levels during the last interglacial ~120,000 years ago were up to 5 meters higher than the present in this location and up to 9.5 meters higher at other locations (h/t to The Hockey Shtick)
So, if we had sea levels of 16-31 feet higher than the present 100,000 years ago, well before the dawn of the industrial revolution, what caused that? Inquiring minds want to know.
Anthony Watts doesn't have an enquiring mind or he would have looked it up. He's telling a fib there. He is also quite careless with dates, isn't he. Does he mean 120,000 years ago or 100,000 years ago? That's a 20,000 year difference, longer than the entire Eemian!
Okay, so let's assume he is referring to the Eemian, which is generally considered to be 130,000 to 114,000 years ago. The Earth as a whole was probably not much warmer than it is today, if it was any warmer. The difference between then and now, according to this paper in Nature Geoscience, is insolation. (Think Milankovitch, like Anthony Watts should have done.) The ice sheet in Greenland was thought to be something between 30% and 60% smaller than it is today. The authors worked out that probably about 55% of the ice loss on Greenland at that time was because of a global warming and 45% could be attributed to the much higher Northern Hemisphere insolation than is the case today.
There's another thing that Anthony Watts neglects. If the global surface temperature back then rose as high as it is these days, then the chances are that seas will also rise as high in the future. Most likely higher, even accounting for the fact that insolation is different, because temperatures these days are going up and up and up. Remember, the Eemian lasted around 14,000 years. That's long time. It wouldn't have got warm all at once. Almost certainly the warming was a lot more gradual than is happening now. Who really knows how the speed of change we're causing will affect ice melt? We do know that the West Antarctic Ice Sheet collapse has started and is considered to be irreversible. Greenland - the jury is still out on that. If we cut emissions enough soon enough, we may be able to delay things there or even prevent them altogether (though that's getting less and less likely as time goes by).
Anyway, even though scientists caution against it, because of the differences - if Anthony Watts keeps looking back at the Eemian, he could be drawing parallels as much as differences.
From the WUWT comments
There's a real lack of awareness in the comments. A lot of them don't stop to wonder that if seas could get a lot higher in the past, then what is stopping them from getting higher in the future, the way we're heating up the planet.
milodonharlani takes a different tune to most WUWT-ers
October 14, 2014 at 4:37 pm
The whole premise of this laughable paper is wrong. The Industrial Revolution did not begin 150 years ago. It was already in full swing long before 1864.
It began with harnessing water power in the middle of the 18th century (eg, Kay’s flying shuttle, 1733, leading to Hargreaves’ spinning jenny, 1764), then increasingly switching to coal from 1763-75, with the development of Watt’s improvements on Newcomen’s 1712 steam engine.
Already in 1820-70 progress had advanced to the “Second Industrial Revolution”.
Contrast milodonharlani's comment with that from Frank (excerpt)
October 14, 2014 at 4:04 pm
...Even the IPCC doesn’t think GHG’s perturbed climate much before 1950. If so, sea level rise from 1850-1950 was caused by the end of the LIA. However, sea level rise over 1850-1950 is believed to be about 10 cm (4 inches). ...
Farmer Gez - really? Maybe Farmer Gez meant he heard Professor Lambeck interviewed on the ABC.
October 14, 2014 at 2:17 pm
I heard Eric Worrall interviewed on the paper. His main message was that the sea rise was due to expansion from temperature increase. The spin from our warmist Aussie ABC did not really fit Worrell’s rather unexciting observations.
O H Dahlsveen
October 14, 2014 at 2:40 pm
So, I wonder, what is the sea or “ocean floor” doing while the “sea level” is increasing? – Is anybody checking on that?
richardguy72 (no comment!)
October 14, 2014 at 3:45 pm
Isostatic Rebound is false as the Ice age myth, It is time that we realize that sea llevels were once much higher and covered the earth, Darwin was wrong and as a result the theory of Isostatic Rebound came to us through Agassiz who was also wrong by basing his Isostatic Rebound on Darwins mistake, Woe is us!! is anybody out there listening?
gabrianga - I don't know if this is fact or fantasy, I haven't checked.
October 14, 2014 at 7:24 pm
The sea level in Kakadu National Park used to reach the top of the Arnhem Escarpment, between 400/600 feet where many of the Aboriginal residential and art gallery caves can still be found..
Now Kakadu is mainly floodplain at sea level with the Escarpment a sharp reminder of where the seas once reached.
Perhaps the IPCC can explain?
Kurt Lambeck, Hélène Roubya, Anthony Purcell, Yiying Sun, and Malcolm Sambridge. "Sea level and global ice volumes from the Last Glacial Maximum to the Holocene." Proceedings of the National Academy of Sciences (2014): doi: 10.1073/pnas.1411762111
Rohling, E. J., G. L. Foster, K. M. Grant, G. Marino, A. P. Roberts, M. E. Tamisiea, and F. Williams. "Sea-level and deep-sea-temperature variability over the past 5.3 million years." Nature 508, no. 7497 (2014): 477-482. doi:10.1038/nature13230
van de Berg, Willem Jan, Michiel van den Broeke, Janneke Ettema, Erik van Meijgaard, and Frank Kaspar. "Significant contribution of insolation to Eemian melting of the Greenland ice sheet." Nature Geoscience 4, no. 10 (2011): 679-683. DOI: 10.1038/NGEO1245