Most agricultural systems can cope with a reasonable range of weather conditions, but destructive weather occurs in all climates. For example, a region with a rainy climate may face a small probability in any given year that rainfall will be less than the minimum required for a particular crop. In most of Africa, rainfall in the 1950s and 1960s was fairly plentiful and regular - but variability doubled in the 1970s and 1980s in some regions. By altering temperature and precipitation patterns, greenhouse gas-induced climate change may create new climate conditions that in many regions would currently be considered extreme. Climatic conditions often interact with socio-economic conditions to undermine food security.
Climate variability tends to have the greatest impact on people who are landless, poor, or isolated.
Changes in socio-economic conditions can lead to dramatic changes in food security. Disadvantageous terms of trade, poor infrastructure, and armed conflict also make it more difficult for people to cope with the effects of climatic extremes. For example, while food security would anyway have been a problem in Sudan during the severe drought of the 1980s and 1990s, economic recession and civil strife were major causes of the widespread famine. In China, on the other hand, land reform and economic growth during the same period substantially reduced food poverty. Some one billion people may be at risk of hunger, although estimates vary widely.