Famine
Climate and population pressure

Thomas Malthus's Essay on the Principle of Population has made popular the theory of the Malthusian catastrophe —that many famines are caused by imbalance of food production compared to the large populations of countries whose population exceeds the regional carrying capacity. However, anthropologist Alex de Waal, executive director of the World Peace Foundation, refutes the Malthus theory, looking instead to political factors as major causes of recent (over the last 150 years) famines. Historically, famines have occurred from agricultural problems such as drought, crop failure, or pestilence. Changing weather patterns, the ineffectiveness of medieval governments in dealing with crises, wars, and epidemic diseases such as the Black Death helped to cause hundreds of famines in Europe during the Middle Ages, including 95 in Britain and 75 in France. In France, the Hundred Years' War, crop failures and epidemics reduced the population by two-thirds. Economist Amartya Sen has also argued that sustained famines historically have been caused by political instability and repressive political regimes rather than overpopulation.

The failure of a harvest or change in conditions, such as drought, can create a situation whereby large numbers of people continue to live where the carrying capacity of the land has temporarily dropped radically. Famine is often associated with subsistence agriculture. The total absence of agriculture in an economically strong area does not cause famine; Arizona and other wealthy regions import the vast majority of their food, since such regions produce sufficient economic goods for trade.

Famines have also been caused by volcanism. The 1815 eruption of the Mount Tambora volcano in Indonesia caused crop failures and famines worldwide and caused the worst famine of the 19th century. The current consensus of the scientific community is that the aerosols and dust released into the upper atmosphere causes cooler temperatures by preventing the sun's energy from reaching the ground. The same mechanism is theorized to be caused by very large meteorite impacts to the extent of causing mass extinctions.
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