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In his book The End of Roman Britain, Michael Jones has a chapter on the changing climate of fifth-century Britain. Tacitus writes in Agricola that the soil in Britain is fertile and that crops grow quickly there, but also that they ripen slowly because of the heavy rainfall and dampness of the soil. During the Roman era, the climate in Britain generally was favorable for agriculture, although much of the land, especially in the highlands, was only marginally so.
From the second through the fourth centuries, the population of Roman Britain has been estimated to be approximately 3.5 million people. Jones contends that feeding this number would have put considerable strain on the ecology. Fifty or sixty thousand cultivated acres, alone, would have been required to feed the approximately 50,000 soldiers stationed in Britain in the first century AD. There were other environmental demands, as well, including fodder for military animals and quantities of iron, wood, and leather for construction, roads, and tents.
The native Britons would have been hard pressed to pay the cost of sustaining such a presence. A growing population, urbanization, and the demands of the Roman army and government would all have intensified agriculture and grazing in previously undeveloped areas. In time, he argues, this over-utilization of the land would prove to be disastrous.
About AD 400, there was a shift to wetter, colder weather in Britain, a deterioration that intensified after AD 450. By the late Roman period, there may have been as much as a 10% increase in rainfall. Together with deforestation and expanded agriculture and grazing, heavier rains would have aggravated soil erosion and flooding. Soil would be leached of its nutrients and fertility; and heaths and bogs would have claimed arable soil and lessened productivity.
Annual average temperature also dropped during this time, perhaps as much as 2.5 degrees F (1.5° C). This would have lowered the elevation at which grains could grow by 650 feet and shortened the growing season by almost one whole month. Colder weather also would have reduced the hay crop and made it more difficult to sustain animals through the winter. Weakened animals, too, would have been more susceptible to disease.
Marginal land for agriculture would have become sub-marginal, and much of the highland zone, with its longer winters, heavier rainfall, and lower average temperature, no longer as productive. Shortfalls in the north, in turn, would likely have increased demand for exportation of grains and increased taxation in the south.
He feels that declining agricultural production from land that already was fully exploited must have had profound demographic consequences. It even is possible that raids by Picts and Scots were influenced by climatic change, since worsening weather would have been most intensely felt by those tribes in the north and west. The Picts beyond Hadrian's Wall may have experienced conditions twice as severe as in the south, and the Scots, too, in Ireland would have had more inclement weather.
Although the literary evidence is meager, a deterioration in climate very likely worsened Britain's social and economic problems, and made it vulnerable to attack. Certainly, this is the impression conveyed by Gildas, who, writing a century later, records an oral tradition of invasion, famine, brigandage, and social revolt during the mid-fifth century.
Reference: The End of Roman Britain (1996) by Michael E. Jones.
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