Question on historical sea levels

I’m looking for factual info here, guys, not a debate on Global Warming.

What were the sea levels during the Medieval Warm Period and the Little Ice Age? What about adjusting for the continental plates bouncing back after the last ice age?

This might help some:

This should help a little more: http://www2.sunysuffolk.edu/mandias/lia/little_ice_age.html

Good Luck
Jim

Hmmm…

A little too far back for my requirements.

Sorry, that’s with reference to the first of your cites.

No problem, I don’t think there is a simple answer to be looked up. You have to hope an expert stumbles along.
I was hoping the links might help you find the answer you were looking for. I know they didn’t really answer it.

Jim

Global sea levels have not changed appreciably since about 7,000 years ago. On a local/regional scale, changes in relative sea level (which would include things like the amount of post-glacial isostatic rebound) could have been more dramatic, as indicated in What Exit?'s first link, but then you have to be specific as to the locale you have in mind before anyone could give you an answer.

I’d guess that the best records would come from geologically stable places like Britain - which got seperated from the mainland at about that time - and southern India / Sri Lanka from placement of ports etc.

Could you clarify your last post, Quartz?

If, for example, you are interested in relative sea level change for Britain, then Britain is where you need to look. Sri Lanka port locations over the past few thousand years aren’t going to help figure out what happened in Britain.

So what specific location are you interested in?

I don’t know if this should be in its own thread. If so, I apologise for the hijack.

We’ve all heard about the dire consequences to coastal cities should the Antarctic ice cap melt. Assuming it did, how much would sea level rise? How far inland would the ocean reach in Los Angeles? What effects would it have on the climate in the high desert?

Johnny L.A., there are three ice caps to consider with respect to global warming and oastal flooding - the Greenland Ice Sheet (GIS), the West Antarctic Ice Sheet (WAIS), and the East Antarctic Ice Sheet (EAIS). Of these the GIS and WAIS are probably most vulnerable to near-term catastrophic collapse, since they are smaller; each would contribute about 6 meters to global sea level. The EAIS, if it were to melt totally, would add another 66 meters to global sea level.

I don’t know the answer to your question about the climate of the high desert, but I’ll see what I can dig up in terms of a map for sea level rise in the LA basin.

Okay, this is not an especially detailed image, but if you look at the map grid here and then click on any of the grid cells to zoom in, you’ll see that most of the coast around LA would only be significantly flooded if you melt down both the GIS and WAIS, and start in on the EAIS. If you have a topographic map handy, just trace out the 20, 40, and 240-ft contour intervals to get a rough idea of the extent of sea incursion.

For more detailed info, this USGS open file report on coastal vulnerability to sea level rise links to poster-sized PDF documents detailing a variety of factors in assessing risk to coastal environments for the entire U.S.

Sure: I’m interested in worldwide levels, but I recognise that the best data will come from areas that have been continuously civilised and relatively free from seismic upheaval. The first rules out the east coast of North America, the second most of the Ring of Fire, including China. Both Britain and India have a sufficient history of civilisation and both areas are relatively seismically quiet. That is why I gave them as examples. I don’t know about eastern South America or the non-Mediterranean coasts of Africa.

Ah, okay! Well, the seismically quiet bit is important, but civilization is not required. :wink: In fact, an inhabited place that was subject to human alteration of the landscape (through construction, incidental changes in runoff patterns, etc.) is just the place you wouldn’t want to be, if you’re after measurement of global sea level change.

The places singled out for measuring global sea level change are in the tropics, well away from any glacio-tectonic movements, specifically where corals like Acropora palmata have a long history of growth. The corals are needed because: a) they are known to grow up to sea level; b) they have annual growth rings that can be counted, like tree rings; and c) their age can be checked via U/Th radioisotope measurements.

By the way, Britain wouldn’t qualify as sufficiently tectonically quiet for global sea level change measurements, having been buried underneath the Scandinavian ice sheet, it is still undergoing rebound, with varying effects on local relative sea level change.* From the Midlands south, relative sea level is rising by a rate of as much as 1.2 mm/yr (i.e., sea level increase exceeds land rebound); however, northern England and Scotland are experiencing relative sea level fall at a rate of as much as 1.5 mm/yr (land rebound rate exceeds sea level rise).

  • I. Shennon and B. Horton, 2002, Holocene land- and sea-level changes in Great Britain, Journal of Quaternary Science, vol. 17(5-6), p. 511-526.

I was thinking in terms of placement of ports etc.

Right. I remember seeing a TV program which showed coral up a hill showing that the location had once been underwater, but that was due to upthrust.

Ah, in my OP I did specify adjusting for rebound.

Then for the segment of human history you’re thinking of (say, 4000 BP to present), global sea level (eustatic) change will have had essentially no effect on the placement of ports. Since a picture is worth a thousand words… this graph is a compilation of some recent data, and the caption is essentially correct (although it errs in citing corrections for post-glacial continental rebound, which would not have been applicable for any of the data locations in that graph).

I mentioned the corals because I got the impression you thought that global sea levels could be determined from the locations of ports, and that wouldn’t be true - or at least, that’s not how it would be done, if global sea levels had continued to change after humans began building ports. Geologists would go out of their way to try to avoid the “noise” of localized human activity as well as other potential geological complications that weren’t already well measured (like isostatic rebound, changes in sedimentation rate, compaction and subsidence of sediments, subsidence due to earthquake or volcanic activity, etc.) when trying to determine eustatic change.

You did. However, most places undergoing post-glacial rebound don’t have that movement monitored through the use of differential GPS measurements, to the same extent that actively deforming regions (like southern California around the San Andreas Fault) sometimes are. So in order to isolate the rebound component of local relative sea level (RSL) change, you would first have to know eustatic change (as well as changes in sedimentation rate, compaction and subsidence of sediments, subsidence due to earthquake or volcanic activity, etc.)… you see the problem here. Best to avoid rebounding places altogether while trying to measure eustatic change, which is why geologists go to the tropics.

I hope that helps clarify my earlier response.

Thanks - unfortunately, Wiki still doesn’t actually answer my original question. Basically the Wiki articles go from too long a period to too short a period, and they concentrate on temperature. Rising sea levels are a cause of worry to some, but without reference to factual data from the recent past, how do we know whether to say, “That’s a real issue” or “That’s your fault for building there.”? (Note that these are not exclusive).

Quartz, I’m afraid I’m missing something in your line of thinking here.

Your OP asked what global sea level was during the Medieval Warm Period and the Little Ice Age. The answer, as shown in the chart I linked to in my last post, is that it was essentially unchanged from the present. The reason that global sea level is essentially unchanged over the past 4,000 years of human civilization is that the largest ice sheets in Greenland and Antarctica have experienced minimal changes in ice volume over that time. I don’t quite understand your comment about the Wiki articles going from too long to too short a period.

I don’t really see where global sea level during the Little Ice Age, say, should bear on future global sea level rise related to warming. (By the way, the term “Little Ice Age” is a colloquialism referring to a few centuries’ worth of enhanced cold weather in the Northern Hemisphere, and does not at all mean a return to true Pleistocene-style glacial conditions.)

Increasing global average temperatures are already leading to a loss of mass from both the Greenland and Antarctic ice sheets. (There have been a number of papers published in the past few months about this; if you like, I can provide references.) Significant loss of ice mass will inevitably lead to an increase in global sea level. It has to - where else would the water go? Measurements of sea level from 1000 years ago aren’t needed to work that out.

Any human structures - ports, homes, you name it - that currently lie near sea level are vulnerable to increased storm damage and/or permanent flooding with an increase in global sea level. The particular impacts at any given location are going to be highly dependent on all those other factors I mentioned earlier that go into calculating local relative sea level change (e.g., isostatic rebound, changes in sedimentation rate, compaction and subsidence of sediments, subsidence due to earthquake or volcanic activity). That variability in vulnerability is obvious if you look, for example, at the maps in the USGS open file report I linked to before.

So, rising global sea levels are definitely a “real issue,” but the extent of coastal impacts is going to be location-dependent. (Socio-political impacts of course will not be.)

In the case of ports (or estuaries, or shorelines, or islands) that were established/inhabited centuries ago, I would say it’s hardly fair to say to the current inhabitants that it’s “[their] fault for living [someplace vulnerable].” The Venetians didn’t have a crystal ball to tell them, when they established Venice in 568, that one day melting ice sheets would threaten to swamp their city. Let’s be realistic here.

OTOH, I think that people who invest lots of money in summer homes on barrier islands are fools, because barrier islands are geologically transient to begin with… rise in global sea level or not, they are dumb places to plant a home.

Hope that helps.

Actually, it shows that it was within a few metres. That’s a significant difference.

Because this and this only go back a bit over a hundred years - far too short a period - whereas thisgoes back 9000 years - far too long a period.

This Wiki article indicates that current temperatures are about the same as the Medieval Warm Period, but that doesn’t mean that sea levels are the same, and I note that Greenland isn’t as habitable as it is supposed to have been.

Inland? From increased evaporation and rainfall? I don’t know.

But I would like them to provide myself points of reference.

At the other extreme, it is fair to so criticise the builders of much of New Orleans.

I quite agree.

Not really, and especially if you look at the last 2000 years, in which the error bars are pretty small. With respect to ancients building ports and such, the local factors in RSL change will likely have been more important. Witness the fates of Ostia, or the harbors of Alexandria, Caesarea, or Tyre and Sidon. The proximal cause for the loss of these port cities was not global sea level change - it was local factors such as silting and earthquake subsidence.

Okay. The first two links you give - recent temperature records and sea level rise dating back to 1850 and 1880, respectively - reflect the temporal limits of actual instrument measurements. Paleoclimate data from inhabited regions prior to 1850 are all proxy data, i.e., they aren’t direct measurements. Some of what’s available in terms of proxy data is simply plotted as part of a larger data set that covers a longer time span (the Holocene as a whole) in the third link. I don’t mean to sound like a wiseacre here, but if you specifically want to know about global sea level change for the period extending roughly from 1000-1700 AD, just look at that portion of the third graph. You’ll see that the best fit line is a straight line through to the present.

Again, if you look at the relevant portion of the graph in your third link, you’ll see that global sea level has NOT changed since the Medieval Warm Period.

As for the habitability of Greenland then vs. now… your comment highlights a confusion that is quite typical of the general public. Global warming = increase in global average temperature. It doesn’t rule out regional variability of climate at all, and in fact that is to be expected, given the chaotic nature of climate processes, especially at the regional scale. The fact that global average temperature might be similar between the Medieval Warm Period and now doesn’t mean that Greenland has to be one of the warmer spots in both cases. How so? Well, all you’d need to do is shift the jet stream pattern a bit. We’re actually getting a taste of that sort of shift this northern hemisphere winter - the east coast of the US has been unusually balmy, whereas eastern Russia has been brutally cold (moreso than usual).

Re where the water from melting ice sheets goes - In order for water released from melting ice sheets to stay inland, as opposed to entering the sea as runoff, it would have to be precipitated as snow that doesn’t melt. In other words, you’d have to be laying the foundation of the next ice age. That’s not going to happen with a continued increase in global average temperature.

Unfortunately, instrument measurements of RSL at ports, etc. don’t exist for 1000 AD. The best you can do, for a specific location, is to see if someone has calculated the current RSL change at that spot and extrapolate backwards. It’s okay as a first-order estimate if post-glacial rebound is the greatest factor in local RSL change, but won’t help you much if sedimentation (silting) has a strong impact, unless you dig up sedimentation rate changes over time as well.

As annoyed as I am over the handling of the New Orleans-Katrina situation, I still think you can’t really place much blame on the founders of New Orleans. Most of the current problems stem from 20th century activities of the Army Corps of Engineers, and to be fair even to them, much of our knowledge of fluvial/floodplain/coastal processes (as opposed to simple description of sediments) post-dates the ACE’s attempts to control nature on a grand scale. Do rain criticism on them - and those responsible for providing funding! - for not adequately preparing once the hazards were better understood, and if they botch the city’s protection against future damage.

At least we see eye to eye on the beach house issue. :slight_smile: