EARLY HOLOCENE WARMING

Wednesday, May 9, 2012




One of the lessons from the present plethora of research into climatic history is that change is not necessarily gradual. In the case of Europe the transition from the tail end of the ice ages to a much more temperate climate was quite rapid. About 9500 B.C. amelioration started to produce warm surface waters (above 14°C [57.2°F]) around the coasts of western Europe, and warming rates may have reached about 1°C (1.8°F) per century in these waters. On land, rates of 3 to 4°C (5.4 to 7.2°F) per 500 years have been postulated for France and even 1.7 to 2.8°C (3.06 to 5.04°F) per century in not yet insular Britain. Overall the climates of Europe may have reached levels similar to those of the twentieth century or even a little warmer by 7000 B.C.

The consequences for the natural world and hence for human habitats were profound. The vegetation belts and their associated fauna shifted northward, so most of Europe was a cool temperate forest zone with dominance by broad-leaved trees. There were montane variants in the Alps, and over much of Scandinavia and eastern Russia the overwhelming dominance of conifers meant that a taiga, or open forest, was the land cover. A taiga biome also penetrated some of the loess lands of the northern European plain, and the Black Sea had a broad penumbra of moist steppe, which was in essence treeless grassland. Within all these biomes, the better conditions encouraged rapid plant growth, so many lakes left in glaciated regions began to fill with organic debris and the area of open water shrank when colonized by marginal vegetation.

A major result of the warming was more free water in the oceans as the polar, mountain, and Laurentide ice sheets melted, producing what are termed “eustatic” rises in sea level. Such increments, however, often were in opposition to isostatic rises in land levels as land surfaces rose when freed from the weight of the ice that had depressed them. The northern part of the Gulf of Bothnia has risen about 850 meters during the Holocene and is still rising at 9 millimeters per year. Northern Britain is still rising, too, though at less than 3 millimeters per year, and the south is sinking at up to 2 millimeters per year. Thus many European coasts during the era of barbarism were the outcomes of competition between eustasy and isostasy, with the latter winning easily to the north. The shorelines and harbors from which the Vikings launched their ships were almost 8 meters above the modern sea level.

The largest-scale physical consequence of sea-level change is found in the Baltic. The region underwent a four-stage evolution in which there was an interaction of ice retreat, eustatic rises of sea level, and isostatic rebound. During the Terminal Pleistocene the Baltic essentially was an ice-dammed freshwater lake, but the retreat of ice in central Sweden led this lake to fall by about 28 meters and become connected to the Atlantic, thus turning brackish. By 7000 B.C. this outlet was closed, and the new but narrow outlet that developed in the region of the Great Belt allowed the Baltic to become a freshwater lake again. After 6500 B.C. more saltwater penetrated, since increased eustasy was accompanied by decreasing isostasy, bringing about the twenty-first-century salinity gradients of the Baltic–Lake Ladoga region.

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