While we often think of climate in terms of averages, the extremes are at least as important in determining a region’s climate. For example, Richmond, Virginia and San Francisco, California have nearly the same annual average temperature, but the temperature in Richmond exhibits much greater seasonal variability. Consequently the two cities have different climates.
When considering climate change predictions, we also tend to focus on means; particularly the number of degrees average global temperatures are expected to rise. But most of the potential damaging consequences relating to climate change are associated with extremes — the number of heat waves, floods, or severe storms, for example. Since extreme weather events hold great potential for loss of life and property, it is important to understand what impact global warming may have on their occurrence.
It remains very difficult to assess the impact of global warming on extreme weather events, in large part because this analysis depends greatly on regional forecasts for global warming. Global warming will almost certainly have different effects on different regions of the Earth, so areas will not be equally susceptible to increased or more intense extreme weather events. Although regional climate forecasts are improving, they are still uncertain. However, we can be fairly certain that a warmer atmosphere will result in a greater number of extreme heat waves. Additionally, a warmer atmosphere can hold more moisture, so changes in the hydrological cycle could alter flood and drought patterns.
Greater numbers of heat waves but fewer periods of extreme cold are likely consequences of a warmer atmosphere. The climatologically record of the past several decades offers evidence for these trends. While most recent winters in North America and Asia have been milder than average, a number of countries have experienced record heat waves. Over 500 people died in Chicago, Illinois, U.S.A. in 1995 when that city’s temperatures neared 100 F (38 C) for almost a week. A heat wave in May of 2002 claimed over 600 lives in India as temperatures soared to 122 F (50 C). A global rise in temperatures increases the possibility that more deadly heat waves such as these will occur.
One of the most important physical consequences of a warmer atmosphere is an increased capacity to hold moisture. According to the Clausius-Clapeyron relation, the amount of water vapor that can be stored in the atmosphere increases rapidly with temperature. A warmer planet is also most likely a wetter planet, as more evaporation could occur.
An increase in the frequency or intensity of floods would be catastrophic in several places around the world. Perhaps no country is more vulnerable than Bangladesh. Over 17 million people live at an elevation of less than 3 ft (1 m) above sea level, and millions more inhabit the flat banks of the Ganges and Brahmaputra Rivers. Past floods have displaced millions in Bangladesh, and increased flooding there would have tragic results. Other nations, including China and Vietnam, have experienced floods killing thousands and causing billions in property damage within the past few years.
While average global rainfall is predicted to increase under global warming, not every point on the planet would experience greater rainfall. Evaporation and precipitation occur at different places, and while wet regions could receive even more rainfall if the planet warms, drier regions may have even more acute shortages of water as evaporation is accelerated in those areas. The Sahel, for example, has become drier over the past several decades, accelerating desertification and placing an even greater premium on already-stretched water supplies.
As research continues into the effects of global climate change on extreme weather, it is important to consider the human and economic toll of extreme weather events. A potential increase in frequency or intensity of these events is another strong reason why we must take action to counteract global climate change.