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Climate Change in Ancient Britain

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 was fertile and that crops such as grain grew quickly there, but also that they ripened slowly because of the heavy rainfall and dampness of the soil (XII.58). During the Roman era, the climate in Britain generally was favorable for agriculture, although much of the land, especially in the highlands (as pictured above), 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.

Declining agricultural production from land that already was fully exploited is presumed to 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 as well, and rendered it vulnerable to attack. Certainly, this is the impression conveyed by the British monk Gildas, whose early sixth-century jeremiad On the Ruin and Conquest of Britain records an oral tradition of invasion, famine, brigandage, and social revolt—or, as Gildas himself recounts the matter of Britain's: her disobedience and subjection, rebellion and slavery, persecution and heresy, tyranny and devastation, famine and pestilence, subversion "and finally, of the peace which, by the will of God, was granted her in these our times" (Book I, §2).

No doubt, these travails were intensified by a shift in weather patterns and cooling temperatures that began in AD 535/536, when volcanic ash obscured the sun and introduced a little ice age. The Byzantine historian Procopius records that "it came about during this year that a most dread portent took place. For the sun gave forth its light without brightness, like the moon, during this whole year, and it seemed exceedingly like the sun in eclipse, for the beams it shed were not clear nor such as it is accustomed to shed. And from this time when this thing happened men were free neither from war nor pestilence nor any other thing leading to death" (The Vandal Wars, II.14.5–6). (It has been conjectured that Krakatoa, an Indonesian island between Java and Sumatra, had erupted—which it did again in 1883, when the exploding volcano produced the loudest sound ever heard.)

In Britain, there was plague and the worst weather that century. The entry in the ten-century Annales Cambriae ("Annals of Wales") for AD 537 chronicled "The battle of Camlann, in which Arthur and Medraut [Mordred] fell; and there was plague [mortalitas] in Britain and Ireland." This is the first mention of Mordred, whom Geoffrey of Monmouth describes in the History of the Kings of Britain (c. 1136) as Arthur's traitorous nephew, who usurped the throne while Arthur was away and abducted Guinevere, his queen (X.13). In Geoffrey's account, Arthur died in AD 542, the very year that the Byzantine emperor Justinian contracted the plague in Constantinople (Procopius, The Persian Wars, II.22–23). Too, in the years following Arthur's death, relates Geoffrey, the native Britons were driven westward by the Saxon invasion into Cornwall and Wales, "shattered by these dreadful disasters; but wherever they went, no haven of safety remained open to them in their flight" (XI.8). As to the site of the battle, it has been associated with the Roman fort of Camboglanna (Castlesteads) guarding an approach to Hadrian's Wall, which marked the boundary between Britannia and Caledonia (Scotland).

The photograph is of the Scottish highlands in early autumn.

Reference: The End of Roman Britain (1996) by Michael E. Jones; The Works of Gildas and Nennius (1841) translated by J. A. Giles; Catastrophe: An Investigation into the Origins of the Modern World (1999) by David Keys (Chap. 13).

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