HISTORICAL VIEW OF RISING SEA LEVELS & GLOBAL WARMING
THIS JOURNAL considers the historical position of environmental issues such as rising sea levels and global warming. Writings within this article should supplement further understanding of issues that, at times, are often misunderstood.
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RISING SEA LEVELS
13,500 BC
As the huge ice sheet covering Canada collapsed and its edges melted rapidly back, large quantities of water were returned to the oceans. The global sea level started to rise dramatically, flooding huge areas of lowland. In 16,000 BC, the ‘Long Island’ coastline lay about seventy-five miles to the south-east of its present position. Starting in 13,500 BC it moved rapidly north-westwards as the lowlands were inundated, and by 9,000 BC it was forty-five miles closer to present day Long Island. In the Persian Gulf, the coastline was displaced sideways by as much as 300 miles in 5,000 years. After that the submergence halted.
This spectacular submergence of lowlands, with all its ecological implications, was repeated all round the world. Only Africa was relatively unchanged by it, because of its narrow continental shelves. The impact on wildlife was enormous. These often extensive lowlands were important grazing lands; submerged, they were put out of use. The seabed on the continental shelf east of New York is scattered with mammoth teeth, which tell their own sad story.
The process of submergence is sometimes described as the Flandrian transgression, because it marked the beginning of the Flandrian interglacial. The transgression was rapid across gently sloping lowlands, though even at a speed of four feet in every 100 years still much slower than an incoming tide. Along the Lancashire coast of England, the sea rose vertically twenty-one feet in 200 years, the equivalent of the daily tidal range in Sussex today.
The rise in sea level was not smooth, because of small-scale variations in climate. As a result the sea rose in jerks, often making low cliffs that now lie under the water. In the Mediterranean, erosional notches have been found showing that the sea hovered for a time at 180 feet, ninety feet and thirty-three feet below its present level.
The flooding of the continental shelves had lots of implications. Some see it as the most important geographical event of recent times. Certainly, it changed the shapes of coastlines from the unfamiliar to the familiar. The British Isles, France, the Irish Sea and the North Sea were all joined together by rather featureless lowlands; it was only after the submergence that the distinctive rugged outline of the half-drowned British Isles appeared. Similarly, between south-east Asia and Australia a huge landmass, the Sundra shelf, was turned by the invasion of the sea into the Malay Peninsula and a complex cluster of islands that includes the islands of Sumatra, Java and Borneo.
The inundation initiated the building of many modern deltas, such as the Mississippi and Nile deltas. It did this by invading and obstructing the lower courses of the rivers. By checking their flow, the mass of sea water induced the rivers to drop their loads of silt and sand. The Nile channel became choked with sand bars, causing the water to divert around their sides, and this led to the river dividing in two, and then in two again. In this way the delta was produced.
The inundation also stimulated the development of coral reefs; it drowned the lower courses of river valleys and stimulated the development of flood plains such as the River Thames; it drowned the lower parts of glaciated valleys and formed fjords such as the Sogne Fjord in Norway or Glacier Bay in Alaska. The advancing sea bulldozed seabed sediment forward to create all our modern beaches and the formation of barrier islands, like Cape Hatteras.
By reshaping the outlines of the continents, it strongly influenced the migration patterns of a wide range of organisms – including human beings. While the sea level was low, it was possible for people to walk into North America from Asia across what is now the Bering Strait and was then a broad plain called Beringia. This is how North America became populated, near the end of the last cold stage. It was possible, too, for people to walk from France to Britain across the dried-out floor of the southern North Sea and English Channel. Once the rising sea flooded the land bridges those routes were no longer passable.
The flooding and permanent loss of huge areas of useful lowland stuck on the folk memories of many communities round the world. The transgression, for instance, formed the basis of the flood myths that many communities have at the core of their religious texts. The earliest known myth, written down for the first time in about 3000 BC, was Gilgamesh. It tells of a great flood, in which early people were saved by building an ark. A version of the same flood story has been handed down to us as the biblical story of Noah’s flood. So, this highly significant event, or process, played its part in myth-building, in helping people to assemble stories that explained or illuminated their place in the world.
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GLOBAL WARMING
8300 BC
TODAY, of course, the media is full of talk about global warming, the increase in the temperature of about half a degree Celsius that took place during the twentieth century. Rightly, journalists, environmentalists and politicians of every political party and persuasion are working hard in conditioning us to see this current episode of global warming as a catastrophic episode. Some scientists suggest that the process of global warming has now reached such a stage as being irrevocable. But, if we look back to the much larger-scale, much more dramatic global warming episodes of 10,000 years ago, a time when the temperature actually rose several degrees, we see something different. We invariably see that global warming episode in an entirely positive light. It was the end of the last cold stage, the start of the Flandrian interglacial. It was a springboard for human progress, a real beginning for Homo sapiens sapiens that would shortly make agriculture and settled life in villages and towns possible. It was therefore, as everyone agrees a very positive benchmark in human prehistory. With this in mind, does it not seem that we are rather inconsistent in our responses to global warming and our evaluation of it?
The warming that started in 8,300 BC brought with it some spectacular environmental changes, especially in middle latitudes. The ice sheet covering Scotland and northern England melted away completely, though not continuously. There were zigzags in the temperature graph, times when the climate was cooling, times when the warming resumed. The warming episode also generated profound changes in southern Britain. In Norfolk, as we know from pollen preserved in pond sediments, the treeless tundra gave way to birch woodland for a short time, and that in turn gave way to a woodland of Scots pine; that developed into a mixed woodland dominated by elm, oak and hazel trees. All of this change happened within the space of 1,500 years.
In the context of human history and prehistory, these things happened not so long ago. The first totem poles were raised at Stonehenge (on the site of the visitor car park) in a clearing in the early pine woodland, and they were put up, one after another, not long after the interglacial started. The stones at Stonehenge were erected about halfway between then and now.
A continuation of this article, which will be updated in essay form, can be found by clicking:
Global warming: ‘What the numbers show’
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© Mark Dowe 2008: all rights protected
Filed under: Environment, climate change, global warming | Tagged: africa, barrier islands, beringia, climate change, climate variations, continental shelves, coral reefs, deltas, ecological implications, fjords, flandrian interglacial, flandrian transgression, flood plains, gilgamesh, global warming, human progress, long island, migration patterns, noah's flood, persian gulf, rising sea levels, sundra shelf
