Professor Mitrovica has a fascinating lesson about what has been learned regarding our global sea levels over these past few decades.
I include notes for reference and I've also added a few links to sources that Prof. Mitrovica shared, and to others that I felt were appropriate supporting and background information.
In Search of Lost Time:
Ancient Eclipses, Roman Fish Tanks and
the Enigma of Global Sea Level Rise
the Enigma of Global Sea Level Rise
What do ancient eclipse records kept by Babylonian, Chinese, Arabic and Greek scholars, and fish tanks, built by wealthy Romans during100BC-100AD, contribute to our understanding of modern climate change? Dr. Jerry X. Mitrovica will describe the important role these archaeological treasures have played in the understanding of sea-level rise and how they help scientists both "fingerprint" sources of recent sea level changes and make more accurate projections of future sea levels.
Jerry X. Mitrovica, Ph.D., is a Professor of Geophysics in the Department of Earth and Planetary Sciences at Harvard University, a Fellow of the American Geophysical Union, and the Director of the Earth Systems Evolution Program of the Canadian Institute for Advanced Research. A 2007 Guggenheim Foundation Fellow, he is also the recipient of the Rutherford Memorial Medal from the Royal Society of Canada and the Augustus Love Medal from the European Geosciences Union.
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In Search of Lost Time:
Ancient Eclipses, Roman Fish Tanks and the Enigma of Global Sea Level Rise -
personal lecture notes
Professor Jerry X. Mitrovica - June 28, 2010
Introduction
2:20 - Small talk
5:40 - Prof. Mitrovica starts off reviewing the core of the "climate problem".
6:15 - Graph of CO2 level over past million years
7:30 - "I'll be talking about sea level" as a very sensitive lens on the climate system.
8:15 - Tide gauges, global placement, etc.
10:10 - Globally averaged tide gauge data graph - conclusion sea level is rising, trend is about 2/2.5 mm per year. During twentieth century sea level has risen on average an inch a decade.
11:00 - Here's where the skeptics come in.
20th Century Sea Level Rise: The Skeptics
* 2mm/yr is not anomalous - sea level has been rising at this rate for a thousand years.
* Sea level change varies dramatically from place to place - melting ice sheets cannot be the culprit.
* Regardless 2mm/yr is small and stable ... it won't change.
13:00 - Examining the claim: "2mm/yr is not anomalous - sea level has been rising at this rate for a thousand years."
And these things are endemic throughout the equatorial regions.
But 2mm/yr rise would put those corals 10 meters under sea level today.
A very strong indicator sea level hasn't been rising past many thousands of years.
15:00 - We even know why those corals have dropped by a couple meters in the past 5± thousand years. Isostatic rebound from last ice age (5± thousand years ago)
15:50 - Archeological evidence.
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"Sea level in Roman time in the Central Mediterranean and implications for recent change"
Lambeck et al. | Planetary Science Letters 224 (2004) 563–575 http://people.rses.anu.edu.au/lambeck_k/pdf/242.pdf
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Many Roman fish tanks along the west coast of Italy built during 2nd and 1st century BC. These are built at a precise level in relation to the high tide... sluice gate at 20cm below high tide.
20:45 - Lambeck measured around 15 tanks all along the west coast of Italy...
... on average these tanks are now a meter under sea level.
21:15 - Segues into glacial melt and isostatic rebound considerations.
21:50 - Taking these considerations into account...
22:00 - What it tells us is that from what we know, that is the ice age (glacial weight) effect - these fish tanks are pretty much at current sea level.
Lambeck et al. concluded that in the past 2,000 years sea level hasn't been doing much at all.
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Abstract
Instrumental records indicate that ocean volumes during the 20th century have increased so as to raise eustatic sea level by f 1 – 2 mm/year and the few available records suggest that this is higher than for the previous century. Geological data indicate that ocean volumes have increased since the main phase of deglaciation about 7000 years ago but whether this continued into the recent past remains unclear.
Yet, this is important for establishing whether the recent rise is associated with global warming or is part of a longer duration non-anthropogenic signal. Here, we present results for sea-level change in the central Mediterranean basin for the Roman Period using new archaeological evidence. These data provide a precise measure of local sea level of 1.35 F 0.07 m at 2000 years ago. Part of this change is the result of ongoing glacio-hydro isostatic adjustment of the crust subsequent to the last deglaciation.
When corrected for this, using geologically constrained model predictions, the change in eustatic sea level since the Roman Period is 0.13 F 0.09 m. A comparison with tide-gauge records from nearby locations and with geologically constrained model predictions of the glacio-isostatic contributions establishes that the onset of modern sea-level rise occurred in recent time at f 100 F 53 years before present.
D 2004 Elsevier B.V. All rights reserved.
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22:30 - What does it mean?
The skeptical claim that sea level rise has been rising for past thousands of years at present rate - would mean that these fish tank from two thousand years ago would be found at 3 to 4 meters under sea level, but they are actually about 1 meter under sea level.
What does it mean?
That the sea level rise of the past century is anomalous !
23:00 - "Long Term Fluctuations in Earth Rotation 700 BC to AD 1990"
http://adsabs.harvard.edu/abs/1995RSPTA.351..165S
http://adsabs.harvard.edu/abs/1995RSPTA.351..165S
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Philosophical Transactions: Physical Sciences and Engineering, F.R. Stephenson and L.V. Morrison | Volume 351, Issue 1695, pp. 165-202
"Records of solar and lunar eclipses in the period 700 BC to AD 1600, originating from the ancient and medieval civilizations of Babylon, China, Europe and the Arab world, are amassed and critically appraised for their usefulness in answering questions about the long-term variability of the Earth's rate of rotation. Results from previous analyses of lunar occultations in the period AD 1600-1955.5, and from high-precision data in AD 1955.5-1990, are included in the dataset considered in this paper. Using the ..."{Here it get's complicated}
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23:10 - A quick historic overview of ancient astronomers and their focus on tracking eclipse records, in order to be able to predict future eclipses.
25:20 - Stephenson and Morrison results in graph form...
... explanation of what they were doing... comparing the calculated times for past eclipses (using today's rotation rate) with selected ancient - but precise - observational records from around the world.
25:55 - The mis-fit . . . Earth's rotation is slowing.
26:30 - In two thousand years Earth's rotation slowed by four hours
26:50 - Further details...
27:35 - What do tides matter?
28:35 - How do we know dissipation is occurring?
29:00 - Considering the role of the Conservation of Momentum.
26:00 - The moon... recession rate... mirrors on the moon
30:33 - Tidal Dissipation
30:45 - Can we use this information?
31:10 - ... further thoughts including respect and awe for the skill of ancient astronomers and the accuracy of their observations... science in action...
31:30 - Considering potential observational pitfalls and mistakes.
32:00 - Tidal Regression... BUT, there's a problem...
32:30 - Walter Monk's contribution... first, another physics lesson
geophysical thought experiment: arctic, antarctic ice masses, those masses leave the land and enter the ocean... gravity shifts {a UCTV lecture regarding sea level by Dr. Monk}
33:00 - conservation of momentum...
33:30 - (some lost audio here) If taken for a given that sea level has been rising at current rate for past centuries - than OK lets do a calculation.
33:50 - Bottom-line: There is no way that what we have witnessed this past century has been going on for the past two thousand years.
* Five millennia old corals say no;
* Two millennia old Roman fish tanks say no;
* Even eclipses recorded by ancient astronomers (Babylonians, Chinese, Greeks) say no.
34:00 - Then, the next "skeptical" argument comes along:
"Sea level change varies dramatically from place to place -
melting ice sheets cannot be the culprit"
34:40 - Review of tide gauge records
34:50 - Tide gauge placement - a study
* Sit selection - sites to avoid:
- near cities - water extraction... land subsidence...
- close to convergent plate boundaries
- with less than 50 year record
35:30 - Comparison of tide gauge site across the globe.
~ ~ ~ ~ ~ ~ ~Mitrovica doesn't cite anyone in particular - the following is my selection:~ ~ ~Recent relative sea-level trends: attempt to quantify forcing factors | Hans-Peter Plag~ ~ ~Determination and Characterization of 20th Century Global Sea Level Rise | Chung-Yen Kuo~ ~ ~Monitoring Vertical Land Movements at Tide Gauges in the UKAshkenazi, Bingley, Dodson, Penna~ ~ ~ ~ ~ ~ ~
... ice sheet, just like earth exerts a gravitation pull on water.
~ ~ ~ ~ ~ ~ ~New Projections of 'Uneven' Global Sea-Level RiseSpada, Bamber, Hurkmans | Feb. 19, 2013~ ~ ~ ~ ~ ~ ~
~ ~ ~ ~ ~ ~ ~For a little more on that:NASA-GSFC: Is sea level rising? Do we have to worry about it? By how much?~ ~ ~State of Washington, Department of Ecology - Sea Level Rise and Coastal Hazards~ ~ ~Working Group II: Impacts, Adaptation and VulnerabilityChapter 6. Coastal Zones and Marine Ecosystemshttp://www.ipcc.ch/ipccreports/tar/wg2/index.php?idp=298
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~ ~ ~ ~ ~ ~ ~Visual depictions of Sea Level Rise -Peter Hogarth~ ~ ~ ~ ~ ~ ~
That's already gone...
... the change translates to current rate of SL rise that's one and a half times larger than it was during the twentieth century average.
~ ~ ~ ~ ~ ~ ~Contrary to Contrarian Claims, IPCC Temperature Projections Have Been Exceptionally Accurate~ ~ ~Climate Change 2007: Working Group I: The Physical Science BasisProjections of Future Changes in Climate~ ~ ~20-Year-Old Report Successfully Predicted Warming: Scientists~ ~ ~How the IPCC Underestimated Climate Change~ ~ ~ ~ ~ ~ ~
48:25 - Rahmdorf 2007
~ ~ ~ ~ ~ ~ ~Ups and downs of sea level projectionsBy Stefan Rahmstorf and Martin Vermeer | 31 August 2009~ ~ ~Sea-level rise: Where we stand at the start of 2013By Stefan Rahmstorf | 9 January 2013~ ~ ~ ~ ~ ~ ~
... marine basin and it's implications and evidence.
~ ~ ~ ~ ~ ~ ~Central West Antarctica among the most rapidly warming regions on Earth.
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The Sea Level Fingerprint of West Antarctic Collapse. Science, Feb 6, 2009
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“The typical estimate of the sea-level change is five metres, a value arrived at by taking the total volume of the West Antarctic Ice Sheet, converting it to water and spreading it evenly across the oceans, says Mitrovica. “However, this estimate is far too simplified because it ignores three significant effects:
- when an ice sheet melts, its gravitational pull on the ocean is reduced and water moves away from it. The net effect is that the sea level actually falls within 2,000 km of a melting ice sheet, and rises progressively further away from it. If the West Antarctic Ice Sheet collapses, sea level will fall close to the Antarctic and will rise much more than the expected estimate in the northern hemisphere because of this gravitational effect;
- the depression in the Antarctic bedrock that currently sits under the weight of the ice sheet will become filled with water if the ice sheet collapses. However, the size of this hole will shrink as the region rebounds after the ice disappears, pushing some of the water out into the ocean, and this effect will further contribute to the sea-level rise;
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- the melting of the West Antarctic Ice Sheet will actually cause the Earth’s rotation axis to shift rather dramatically – approximately 500 metres from its present position if the entire ice sheet melts. This shift will move water from the southern Atlantic and Pacific oceans northward toward North America and into the southern Indian Ocean.
* 2mm/yr sea level rise IS anomalous.
* Dramatic geographic sea level differences ARE understood - glacial gravity impact.
* Sea level is NOW rising one and half times faster than 20th century average.
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