EXTREME WEATHER
Most of the potentially 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 cause loss of life and property, it is important to understand what impact global warming may have on their occurrence.
Global climate change has different effects on different regions of the Earth. Although regional climate forecasts are improving, they are still uncertain. However, we know that a warmer atmosphere will result in a greater number of tropical storms, extreme heat waves, floods and droughts.
Intense tropical cyclone activity has increased since 1970 in the North Atlantic and is projected to worsen, because tropical storms are powered by factors affected by climate change. In order to occur, tropical storms need warm ocean temperatures, no strong changes in wind speed or direction, and high humidity. Two of these factors have increased as a result of global warming; oceans have become warmer, and humidity and water vapor have increased 4% since 1970 because warm air holds more vapor than cold air.
Hurricanes occur when surface temperatures exceed 79° F (26°C). As moist, hot air rises, the lower air pressure at sea level pulls the surrounding air into a rotating pattern. Then, the water-vapor laden air spirals and rises to the higher altitudes that cool it and releases heat as it condenses into rain. Hurricanes are fed by evaporation and condensation, which bring the ocean’s heat energy into the vortex.
Hurricanes have become more destructive in both the Atlantic and the Pacific. In 2005, a study showed that tropical cyclone wind speed and duration over the last 30 years had increased by 70%. Another study conducted in 2006 showed a 60% increase in the destructiveness of tropical storms from 1958-2001. The study correlated the rise in mean annual tropical sea surface temperatures (0.45° F / 0.25° C) to the intensification of tropical storms. Deeper levels of warmer water have also intensified tropical storms because dragging up colder water from beneath the surface can be a stopping factor. However, when Hurricane Katrina hit the deep pools of warm water in the Gulf of Mexico, it became more powerful, when it might have been stopped by a pool of cold water.
Recent events have shown the disastrous effects of extreme weather. Hurricane Katrina caused a loss of 338 square kilometers (210 square miles) of coastal wetlands, levees and islands. 1,800 people died, 300,000 homes were destroyed, and total economic losses are estimated at $100 billion. The December 26, 2004 Tsunami resulted in more than 150,000 people dead or missing, and left millions homeless in eleven different countries.
Greater numbers of heat waves but fewer periods of extreme cold are likely consequences of a warmer atmosphere. The climatological 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. 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, so 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.
Figure 1: Typology of Hazards. (2005). In UNEP/GRID-Arendal Maps and Graphics Library. Retrieved 15:56, March 26, 2008 from http://maps.grida.no/go/graphic/typology_of_hazards
Additional Resources
Hurricane Dean: Mexico Shows How to Do It Right (September 2007)
