Ice Age Denoument

As my long time readers are aware, I have taken a particular interest in the climatic history of the Holocene generally in order to establish historic drivers to the observed climatic variation. The millennia long Bronze Age climatic optimum has been of particular interest. During this period, it was observed that the Sahara was created by the destruction of a well vegetated ecosystem. This also coincides with a similar decline throughout the Middle East.

I pointed out that this was a heat capture system that explains the warmth of the Northern Hemisphere during this time and its loss explains the less optimal climate since.

It is also noted that the principal rise in sea levels from melt waters ended about seven thousand years ago, having lasted a total of perhaps seven thousand years. Other sources had suggested a much shorter melting time frame, so perhaps we should be a bit wary as yet in accepting the source that I am appending here. Those curves are way too smooth.

What I have not paid attention to yet is the last several thousands of years simply because it appears to be a minor variation as compared to the previous collapse. The fact is that there was plenty of land based ice up to perhaps four thousand years ago and this was reflected in the still substantial as compared to today, increase in sea level. It was about twice the current level of sea level increase.

This suggests that a lot of the original ice sheet lingered on the ground near sea level into the late Bronze Age. This surely kept the Arctic cold and possibly a large part of the present boreal forest at bay. In other words, present historical conditions in the high arctic are actually very new and only properly established in the past three thousand years or so.

This also suggests that the Bronze Age had an extra cooling engine in the arctic to offset the warmth coming out of the south. All this is much closer in time in terms of effect than I had anticipated and negates facile comparisons to those climatic conditions. It appears way more robust than anticipated.

It would be useful to do carbon 14 dating on bog trapped wood throughout the boreal forest to get an idea of just how long the forests have been present. I see no evidence that anyone has launched such a program anywhere, let alone in the more northerly regions. It would give us a time line on were and for how long surface ice lingered representing necessary input for the generation of climate models.

A derivative of this result is that during the climatic optimum of the Bronze Age, the Greenland ice sheet would have been far less vulnerable to change and that the sea ice would have remained largely intact. The medieval optimum was possibly the first real look at an ice free summer Arctic and that lasted for many decades. We can never prove it of course.

Image:Holocene Sea Level.png
From Global Warming

Holocene_Sea_Level.png; Other sizes: 100, 200, 300, 450

Sea level rise since the last glacial episode

Sea level rise from direct measurements during the last 120 years

This figure shows changes in
sea level during the Holocene, the time following the end of the most recent glacial period, based on data from Fleming et al. 1998, Fleming 2000, & Milne et al. 2005. These papers collected data from various reports and adjusted them for subsequent vertical geologic motions, primarily those associated with post-glacial continental and hydroisostatic rebound. The first refers to deformations caused by the weight of continental ice sheets pressing down on the land, the latter refers to uplift in coastal areas resulting from the increased weight of water associated with rising sea levels. It should be noted that because of the latter effect and associated uplift, many islands, especially in the Pacific, exhibited higher local sea levels in the mid Holocene than they do today. Uncertainty about the magnitude of these corrections is the dominant uncertainty in many measurements of Holocene scale sea level change.

The black curve is based on minimizing the sum of squares error weighted distance between this curve and the plotted data. It was constructed by adjusting a number of specified tie points, typically placed every 1 kyr and forced to go to 0 at the modern day. A small number of extreme outliers were dropped. It should be noted that some authors propose the existence of significant short-term fluctuations in sea level such that the sea level curve might oscillate up and down about this ~1 kyr mean state. Others dispute this and argue that sea level change has been a smooth and gradual process for essentially the entire length of the Holocene. Regardless of such putative fluctuations, evidence such as presented by Morhange et al. (2001) suggests that in the last 10 kyr sea level has never been higher than it is at present.

Copyright This figure was prepared by Robert A. Rohde from published data.

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[abstract] [DOI] Fleming, Kevin, Paul Johnston, Dan Zwartz, Yusuke Yokoyama, Kurt Lambeck and John Chappell (1998). "Refining the eustatic sea-level curve since the Last Glacial Maximum using far- and intermediate-field sites". Earth and Planetary Science Letters 163 (1-4): 327-342.
Fleming, Kevin Michael (2000). Glacial Rebound and Sea-level Change Constraints on the Greenland Ice Sheet. Australian National University. PhD Thesis.

abstract] [DOI] Milne, Glenn A., Antony J. Long and Sophie E. Bassett (2005). "Modelling Holocene relative sea-level observations from the Caribbean and South America". Quaternary Science Reviews 24 (10-11): 1183-1202.

abstract] [DOI] Morhange, C., J. Laborel, and A. Hesnard (2001). "Changes of relative sea level during the past 5000 years in the ancient harbor of Marseilles, Southern France". Palaeogeography, Palaeoclimatology, Palaeoecology 166: 319-329.

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