Natural variability in the climate often produces extremes in the weather. An important question which scientists are trying to answer is whether mankinds interference with the global climate through the enhancement of the natural greenhouse effect will increase the frequency or magnitude of extreme weather events.
Some definitions of climatic extremes choose to separate the nature of the event from its social and economic consequences. A climate extreme then, is a significant departure from the normal state of the climate, irrespective of its actual impact on life or any other aspect of the Earth's ecology. When a climate extreme has an adverse impact on human welfare, it becomes a climatic disaster.
Extreme weather events include droughts, floods and associated landslides, storms, cyclones and tornadoes, ocean and coastal surges, heat waves and cold snaps. A warmer world should in theory be wetter as well, since the rate of evaporation is increased and the atmosphere will contain more moisture for precipitation. Changes in precipitation however, will not be the same all over the world. Wet areas are likely to become wetter, with more frequent episodes of flooding, whilst dry areas may become drier, with longer periods of drought leading to an increased threat of desertification. In general, as more heat and moisture is put into the atmosphere, the likelihood of storms, hurricanes and tornadoes will increase.
Any shift in average climate will almost inevitably result in a change in the frequency of extreme events. In general, more heat waves and fewer frosts could be expected as the average temperature rises, whilst the return period of severe flooding will be reduced substantially if precipitation increases. A 1 in a 100 year event for example, may become a 1 in 10 year event, whilst 1 in a 10 year event may become a 1 in 3 year event. For less adaptable societies in the developing world a shorter return period of extreme weather events may not allow them to fully recover from the effects of one event before the next event strikes.
Every region of the world experiences record-breaking climate extremes from time to time. In 1989, for example, the "Big Wet" in eastern Australia brought torrential downpours and the worst flooding in two centuries. Many people in England will remember the "Hurricane" of October 1987. More recently, Central America was devastated by Hurricane "Mitch" which wrought havoc for an entire month during the 1998 hurricane season. Droughts are another devastating type of climate extreme. Early this century, a trend towards increased drought in the North American Midwest culminated in the "Dust Bowl" decade of the "dirty thirties", after which conditions eased. During the 1970s and 1980s annual rainfall over the Sahel zone of northern Africa dropped 25% below the average, leading to severe desiccation and famine.
Frequent reports of record-breaking events suggest that climate extremes are becoming more common. There is no scientific evidence however, that this is the case at the global level. Through the 20th century there does not appear to be any discernible trend in extreme weather events. In addition, given the large natural variability in climate and the general rarity of climatic extremes it is hard to determine whether they are now occurring as a result of global warming. A more likely explanation is that increased human vulnerability to climate extremes, particularly in developing countries, is transforming extreme events into climatic disasters. The communications revolution has also made people much more aware of the occurrence of extreme events and of their impact. What can be said with certainty however, is that any change in climate will affect society much more through extreme weather events rather than subtle changes in climate averages.