Sunday, February 8, 2009

Advancing and Retreating Glaciers in the Holocene

I was musing as to how to start this post, and so I Googled “Medieval Warming Period”, and found that there is a post at Gristmill about talking to an AGW sceptic, that states
There is no good evidence that the MWP was a globally warm period comparable to today. Regionally, there may have been places that exhibited notable warmth -- Europe, for example -- but all global proxy reconstructions agree it is warmer now, and the temperature is rising faster now, than at any time in the last one or even two thousand years.
This is actually a set of incorrect statements. And it epitomizes one of the problems facing those of us that would prefer to have this as a scientific debate about the facts. The problem is the blanket refusal of those who consider that greenhouse gases are the cause of global warming to recognize the considerable body of scientific evidence that contradicts their case.

I am going to be quoting from Jean Grove’s “Little Ice Ages – Ancient and Modern” again in this post, but from volume 2 this time. As with the first volume the book is copiously referenced with detailed scientific studies, the Bibliography runs 70 pages of 2-columns, with about 16 references/column/page, which works out to over 2,000 references on the subject. The subject is as the title says it is, the ice ages of the past millennia, with evidence of their extent and intensity, and their global nature. Between the periods of increased cold, there are periods where it is warmer, even than today. These also are documented, with evidence. The evidence is debated, and weighed as to credibility. It does not require a proxy to show that where a farm existed and there is now ice, that it was warmer when the farm was there than it is now, as an example, or when the advance of a glacier sheared off the trees, to surmise that it was getting colder.

The scope of the work, and recognize that this is a summation of the work of the hundreds of scientists writing the papers that she has combined and integrated, covers largely the advance and retreat of glaciers. In the first volume, which I covered earlier, the main focus was on looking at the last Little Ice Age, and its global impact with glaciers from Alaska to New Zealand growing to new lengths, and then in the early 1800’s starting to contract.

In the second volume the emphasis switches to the Holocene as a whole, namely roughly the past 10,000 years. And, while glacier advance and retreat cannot by itself be taken as a sole indicator of global temperature, some of the concurrent events (the rise and fall of the tree line in the region surrounding the glaciers, and the formation of lakes or peat bogs where glaciers stood, for example) serve as proxy thermometers. As an example, writing about the Engebreen and the main Svartisen glacier
A warm period about 1500 BP (before present) (cal AD 500) is recorded by the growth of pine forest above the present pine limit. Then came ice advances marked by sheared off trees in the period AD 360 -900 (Worsley & Alexander 1975). Although the exact years in which these trees were killed has not been identified by comparing their ring sequences with the long dendrochronological series now available, their radiocarbon dates can be taken as more precise than those derived from soil and peat samples. They demonstrate that Engabreen advanced and retreated several times in this period to form a complex moraine. Karlen (1991) pointed out that the tree ring sequence indicates several temperature fluctuations with cold summers around 550 AD, and again between 780 and 970 AD, though with 20-30 years of summer warmth around AD 930.
In such a manner and going from country to country is the evidence amassed. Karlen, for example, shows that the altitude of pine forests were 100 m above the current limit in the Medieval Warming Period around 1,000 years ago, and that the height of the tree line had varied up and down over 200 m in the Holocene. He concluded that this represented a change of about 1.5 degC.

I have used Greenland as an example of historic climate change in the past, and it is interesting to quote
By about 6,000 calBP the inland ice margin had retreated to a position close to the present (Weidick et al 1990) . . . .Retreat of the ice continued and from at least 4700 to 2700 calPB the inland ice margin was inland of its present position by as much as 15 km. The scale of the retreat is indicated b subfossil shells, reindeer bones and a walrus tusk, brought to the surface by the ice of Jakobshavn Isbrae and deposited in moraines. The reindeer bones which have been dated to 3040 +/- 60 BP must have come from above the snow line over 50 km from the ice margin, and moved westwards in the ice at a rate of about 20 -50 m per year.

The mean annual temperature at Jakobshavn on Disco Bay in the Holocene Climatic otimum is estimated have been about -1 degC, as opposed to -3 degC currently. The great fjord stretching 60 km to the east where the tongue of Jakobshavn Isbrae is now more than 700 m thick, was then free of ice. With readvances probably around 2500 and 1200 BP the ice haltingly returned. Finally, between 1850 and 1880, the Little Ice Age advance culminated, since when the ice has continuously thinned (Weidick 1996).
The front recession has been plotted over the past 150 years, and is shown here.
Retreat of the Jakobshavn glacier

The book continues, going from ice field to ice field as it moves from the Arctic to the Antarctic. Then it plots a curve showing the coincidence of the periods of glacial advance around the world, over the Holocene. Obviously in some parts the work has yet to be done to fully define the periods and details of the glacier movements. Yet the one thing that stands out is that over the past 3,000 years there has been a remarkable consistency in the periods, around the world, when glaciers advanced, and when in the presence of a warming period they retreated.
Regional summaries of glacier expansion (Grove Fig. 15.25)

The book notes the existence of the Dansgaard-Oeschger cycles
The Dansgaard-Oeschger cycles turn out to he bundled into longer cooling cylces, each of which ends with an abrupt shift from cold to warm temperatures. These bundles are known as Bond cycles. It appears that during the abrupt cold-to-warm shifts at the end of Bond cycles temperatures must have changed by several degrees within a matter of decades, both at the ocean surface, and in the atmosphere over Greenland.
Though not understood, the book suggests that the climate change may have been induced by changes in atmospheric dust.

Well, as you can see, the real story is quite different to that which Gristmill would have you believe. Sadly at present there are just not that many folk that will question the statements that they blandly make. I will add the correlative plot from the book in the morning (or rather later in the morning).
Addenda The graphic is added above - fig 15.25.


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