Scientists continue to debate the effects of global warming upon both the intensity and the frequency of extreme weather events. It is possible that one or both of these factors may increase as global temperature increases, causing increased loss of life and property.

Extreme weather events are weather systems that become abnormally severe and have high impacts on human life, property, and the environment. In the last hundred years, especially in the latter half of the twentieth century, global surface temperature have experienced a rapid increase. This rapid warming may cause significant changes in Earth's weather patterns, including affecting the frequency and intensity of extreme weather events.

One of the possible consequences of global warming is that it will enhance evaporation and transpiration of water vapor from oceans, rivers, lakes, and vegetated lands. Temperature is the main factor determining the moisture-holding capacity of air. The higher the temperature, the more moisture an air parcel can hold. Although global warming may not occur uniformly across the globe, the averaged temperature increase in the atmosphere should give rise to a corresponding increase of average humidity. More water content in the atmosphere will increase the probability of heavy precipitation, which can lead to floods.

Another global warming-related factor that influences heavy precipitation and flooding is a convection-related increase in the severity of storms. In addition to increased temperatures, global warming may involve a higher extension of Earth's troposphere. Both of these processes can lead to more and stronger convection, which can in turn produce more violent convective storms and heavier precipitation.

Since global warming is not uniform, a strong warming can occur in some parts of the world while other parts of the world cool. In response to such non-uniform conditions, different patterns of atmospheric circulation can be realized in different parts of the globe. As a result, in some areas air may be enriched by moisture, and in other areas the air may lose moisture, depending on the general atmospheric circulation patterns and moisture transport in each region. Therefore, while global warming may increase the intensity and frequency of heavy precipitation and floods in some locations, it may also increase the severity and frequency of drought conditions in others.

Global warming increases air temperature in both average and extreme contexts. That is, it results in an increase not only in average temperature but also in daily, monthly, and yearly maximum temperatures. The increase in temperature extremes suggests an increase in the number of hot days as well. Therefore, global warming will most likely increase both the severity and frequency of heat waves. Such heat waves, along with drought, may increase the occurrence of wildfires.

Although wildfire is not a weather phenomenon, its occurrence is closely related to weather conditions. In particular, warm temperatures and low humidity are the two necessary conditions for wildfires. Because global warming can generate warm surface temperatures and frequent drought conditions, it will increase the likelihood of wildfires.

One of the necessary conditions for tropical-storm formation is high sea surface temperature (SST). Tropical storms typically develop over the ocean when SSTs exceed 26œ-27œ Celsius. Based on this criterion, recent global warming trends seem to suggest an increase in the number of tropical storms. Furthermore, tropical storms derive energy from latent heat brought by water vapor evaporated from oceans. Higher SSTs promote greater evaporation of water into the atmosphere. This factor suggests that future warm climates may also produce more powerful hurricanes and typhoons. However, the question of whether global warming would cause an increase in storm frequency is unresolved, and different studies have produced conflicting results.

Unlike tropical storms, extratropical cyclones derive energy from a non-uniform temperature distribution, or a temperature contrast between locations in the northern and southern latitudes. In meteorology, such a condition is called a "temperature gradient." Strong temperature gradients will generate unstable atmospheric conditions, which will initiate large-scale cyclones. These cyclones typically occur in the cool season, and they are enforced by upper-level jet streams and also characterized by surface fronts.

A general consensus exists that global warming will decrease temperature gradients and also decrease the intensity of jet streams. The combined effect of these changes would tend to decrease the intensity and frequency of extratropical cyclones. However, some scientists argue that, because global warming tends to increase humidity, extratropical cyclones may gain extra energy from latent heat flux due to water vapor condensation.

Severe thunderstorms and tornadoes are convectivescale and microscale weather systems. They are different from extratropical cyclones, which are forced by large-scale temperature gradients. Thunderstorm development strongly depends on convection, which is influenced by surface radiative heating, convergence of surface flows, and topographic forcing. In a future warm climate, increase of global surface temperatures may provide favorable conditions for convection to occur. The number of both annual tornado sightings and annual tornado warning days increased over the second half of the twentieth century, but the number of the most severe tornadoes (F2-F5) exhibited a slight decrease.

A 2007 report by the Intergovernmental Panel on Climate Change (IPCC) predicted that global warming would likely increase the number and frequency of extreme weather events. These increased extreme weather events would generate profound impacts on global socioeconomic development. Such impacts include, among many other things, increased risks to human life and health, increased property and infrastructure losses, increased cost and pressure on government's disaster relief and mitigation resources, and increased costs of private insurance.

Bibliography:

1)         Ahrens, C. Donald. Essentials of Meteorology: An Invitation to the Atmosphere. 5th ed. Belmont, Calif.: Thomson Brooks/Cole, 2008.

2)         Diffenbaugh, N. S., R. J. Trapp, and H. Brooks. "Does Global Warming Influence Tornado Activity?" Eos 89, no. 53 (2008).

3)         Intergovernmental Panel on Climate Change. Climate Change, 2007--Synthesis Report: Contribution of Working Groups I, II, and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Edited by the Core Writing Team, Rajendra K. Pachauri, and Andy Reisinger. Geneva, Switzerland: Author, 2008.

4)         Lutgens, Frederick K., and Edward J. Tarbuck. The Atmosphere. 10th ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2007.

5)         Tebaldi, C., J. M. Arblaster, K. Hayhoe, and G. A. Meehl. "Going to the Extremes: An Intercomparison of Model-Simulated Historical and Future Changes in Extreme Events." Climate Change 79: 185-211 (2006).

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