Climate Changes in Prehistory and History
By Ken Hsu <firstname.lastname@example.org>
The press has been reporting an impending catastrophe of global warming as an inevitable consequence of the greenhouse effect of the industrially produced carbon dioxide. The scientific community has, however, not reached a consensus. The orthodox opinion, based mainly on computer-modeling, relates the current global warming to the atmospheric greenhouse. Their opponents suggest, on the contrary, that the greenhouse-effect leads to increased cloud cover, more snowfall in polar regions, and global cooling. That there has been a global-warming trend since the last century is, however, a well-established fact. What I would like to explore is not so much the reason why, but to present the record of climatic changes during the prehistorical and historical times.
The earth underwent episodes of continental glaciation during the last two million years. North America, Scandinavia, and Europe were covered by ice sheets during the last Ice Age. The ice caps started their retreat after they reached their greatest extent, when Switzerland was entirely covered under ice. The post-glacial warming began some 16,000 or 15,000 years ago, but there was one last glacial advance some 11,000 years ago, The climate of Zurich then was as cool as that of the Engadine now, and Lake Zurich was frozen for several months every year. The finest suspension from the "glacial milk" could then settle to the lake bottom, to form a clay lamina above the melt-water deposit of the spring and summer. Accumulating annually, the laminated sediments are called varves.
A wholly new epoch, called the Holocene, had its beginning after the last glacial retreat, some 10,000 years ago; it has been warm since. The uninformed have the impression that the global climate has not changed until the recent global warming. In fact, the average temperatures on earth have varied within the range of 10 to 20 C. The average masks, however, the significance of regional variations. Besides, not so much the temperature but the precipitation changes are more important to the well being of the society. A cold spring in northern Europe, for example, does not make a big difference in the temperature record, but late planting could result in crop failures. Colder years in China may not seem significant in the terms of global average, but the cooling could bring forth a long drought with dire consequences. Scientists studying sediments and ice cores have found only faint signals of alternating of cold and warm millennia. Historians have recorded, however, all too obvious impact of climatic changes on the history of civilization.
The first half of the Holocene was warmer, and the Climatic Optimum ended several thousand years ago. The global average may have changed a degree or two, but lakes freeze when subzero temperatures prevail in the winter. Zurichsee has not been frozen for more than 30 years, but Alpine lakes above 1600 or 1800 m freeze every year. Studying the varves of Silvaplana, my student Andreas Lehmann found no Holocene varves older than 4000 years, when there was no "glacial-milk" sediment.. The conclusion is inescapable: There were no varves because the Engadine lakes were not frozen every year. There were no "glacial milk" deposits when there were no Alpine glaciers!
I was excited by Lehmann's discovery and called my former student Dr. Kerry Kelts at Minnesota. He headed our Limnology Laborary at ETH-Z before accepting a professorship at University of Minnesota. Kelts was not surprised. He told me dryly: "I have been telling you all those years of the 4000 BP event, and you did not listen. There was a global cooling when the Climatic Optimum came to an end."