Heat Waves

Introduction

A heat wave is an extended period of unusually high temperatures and often high humidity that causes temporary modifications in lifestyle and may have adverse health effects on affected human populations. There is no universal temperature threshold for a heat wave, as social and cultural practices largely define perceptions of and responses to heat. In parts of the world where summer conditions often exceed the physiologic threshold—the point at which adequate removal of heat from the body is impeded—behavioral changes such as taking siestas and

decreasing activity levels throughout the day and housing modifications that increase air movement allow people to withstand higher temperatures for longer periods than people living in cooler climates. Thus, what might be considered a heat wave in a normally cool region would not be considered extreme weather in a warmer region.

The World Meteorological Organization (WMO) defines a heat wave as a period during which the daily maximum temperature exceeds for more than five consecutive days the maximum normal temperature by 9°F (5°C), the “normal” period being defined as 1961–1990. Because of global warming, the frequency, duration, and severity of heat waves are predicted to increase in most parts of the world. The impacts on human health, regional economies, and ecosystems may be significant.

Historical Background and Scientific Foundations

Heat waves almost always occur during the summer months. An area of high pressure with little to no cloud cover causes the ground (and, secondarily, the ambient air) to absorb high levels of heat from the sun. Urban areas are particularly susceptible, as asphalt and concrete store heat longer than vegetation-covered ground and release this heat at night, causing the increased nighttime temperatures known as the “urban heat island effect.” Heat released by urban power consumption (including the increased usage of air conditioning) exacerbates the problem. A study of the 1995 Chicago heat wave found that several consecutive nights with very warm nighttime minimum temperatures may be the most important indicator of human health impacts.

Two recent heat waves of note were the Chicago heat wave of 1995 and the European heat wave of 2003. Both of these events had particularly severe impacts on human morbidity (illness) and mortality. During the five-day Chicago heat wave, deaths increased by 85% for all age groups. In the European heat wave of 2003, approximately 22,000 people died in Europe. Paris experienced the highest mortality, which was partially attributed to air pollution caused by the heat and lack of wind leaving a blanket of smog over the city.

WORDS TO KNOW

GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.

HYPERTHERMIA: A condition in which internal body temperatures rise to dangerous and even lethal levels. These temperatures can reach anywhere between 104 and 115°F (40 and 46°C). Hyperthermia affects many people during hot summer seasons. It is also a condition that occurs during some surgeries. If not cared for immediately, patients who suffer hyperthermia may be unaware of the symptoms and allow themselves to attain dangerous heat levels.

INTERGOVERNMENTAL PANEL ON CLIMATE CHANGE (IPCC): Panel of scientists established by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) in 1988 to assess the science, technology, and socioeconomic information needed to understand the risk of human-induced climate change.

MORBIDITY: The existence of disease or illness.

THERMOREGULATORY: Concerned with maintaining a constant internal temperature, as by many animals (or, rarely, plants). Thermoregulatory ability limits the climate adaptability of an organism: penguins, for example, cannot live in warm climates because they cannot discard enough heat to keep their internal temperature down. Decline in Galápagos penguins has been linked to anthropogenic climate warming.

URBAN HEAT ISLAND EFFECT: Warming of atmosphere in and immediately around a built-up area. Occurs because pavement and buildings absorb solar energy while being little cooled by evaporation compared to vegetation-covered ground. Skeptics of global climate change at one time argued that the expansion of urban heat islands near and around weather stations has caused an illusion of global warming by biasing temperature measurements. Although urban heat islands do exist, the argument that they produce an illusion of global warming has been discredited.

Impacts and Issues

Periods of excessive heat impact human health. In the body, thermoregulatory mechanisms work to maintain a constant core temperature by allowing the heat generated by metabolism to be released through the skin (and, to a lesser degree, the lungs) to the surrounding air. When the air is so hot and saturated with moisture that release of heat from the body is impeded, core temperature begins to rise and health problems, possibly resulting in death, will occur. The age, sex, and fitness level of a person determine the threshold level of heat and humidity at which core temperature will begin to rise. Prior conditioning to warmer climates and recent exposure to extreme events will also have an influence.

The health problems caused by heat waves are primarily heat exhaustion and heat stroke (or hyperthermia). When these two forms of heat illness are not countered in a timely manner, death may result. Heat exhaustion occurs when a person is exercising or working excessively in a very hot and humid environment, resulting in heavy sweating and thus a large fluid loss. As blood flow to the skin increases, blood flow in the deep, vital organs decreases. This results in a form of mild shock which, left untreated, will then lead to heat stroke.

Heat stroke occurs when in order to maintain blood flow the body shuts down the temperature control mechanisms that allow sweating. The core body temperature then rises to such a level that brain damage and death can result. The National Weather Service states that heat waves are the major source of weather-related deaths in the United States in most years. Heat waves are responsible for more deaths annually in the United States than tornadoes, hurricanes, and floods.

The psychological impacts of excessive heat are also significant. As part of the body's response to extreme heat, adrenaline is generated in order to keep the body within normal temperature limits. A side effect of this stress hormone is an increase in aggressive behavior. Scientists have documented rises in violent crime and impulsive behavior at work (such as sudden job quitting) during periods of extreme heat.

A heat wave can influence the economy and infrastructure of an affected region. The large draw on electric power sources due to increased air conditioning causes electricity demand to rise sharply. Power outages then result, creating blackouts that leave many homes and businesses without power. Ruptured water lines and buckled roads are also caused by excessive heat. Finally, heat waves that occur during periods of drought can contribute to wildfires that destroy homes, businesses, and forests.

The heat stress associated with heat waves also causes mortality in animal populations, such as those in aquatic systems. Heat waves have been known to produce large die-offs of salmon in Canadian streams. Seventy-six percent of fish species were severely affected by a single hot day in a Michigan lake, in which water temperatures reached 100°F (38°C).

Animals suffer other detrimental effects of heat waves beside death. For example, in many reptiles, an individual's sex is determined by the maximum temperature it experiences during a key period of embryonic development. For Map turtles, only males are produced when maximum incubation temperatures are below 82°F (28°C) and only females are produced when maximum incubation temperatures are above 86°F (30°C). Thus, changes in the incidence of extreme heat can result in a highly skewed sex ratio in a population.

According to the Intergovernmental Panel on Climate Change (IPCC), heat waves have increased in duration since 1950. Global climate models created by scientists at the National Center for Atmospheric Research and by a number of other groups agree that heat waves in much of the world, including most of Europe and North America, will become more intense, frequent, and of longer duration in the second half of the twenty-first century. Observations coupled with models indicate that recent heat waves in these areas coincide with a specific atmospheric circulation pattern that is intensified by ongoing rises in greenhouse gases.

The IPCC has cited studies of the potential impacts of hypothesized changes in climate variability and/or extreme events. Results of these studies, along with observations of impacts from extreme events in the past, suggest that changes in climate variability and extremes (which include heat waves) are likely to be at least as important as changes in mean (average) climate conditions in determining the impacts of climate change in the future.

See Also Extreme Weather; Temperature and Temperature Scales; Temperature Record.

BIBLIOGRAPHY

Periodicals

Easterling, David R., et al. “Climate Extremes: Observations, Modeling, and Impacts.” Science 289, no. 5487 (September 22, 2000): 2068–2074.

Meehl, Gerald A., and Claudia Tebaldi. “More Intense, More Frequent, and Longer Lasting Heat Waves in the 21st Century.” Science 305, no. 5686 (August 13, 2004): 994–997.

Parmesan, Camille, Terry L. Root, and Michael R. Willig. “Impacts of Extreme Weather and Climate on Terrestrial Biota.” Bulletin of the American Meteorological Society 81, no. 3 (2000): 443–449.

Robinson, Peter J. “On the Definition of a Heat Wave.” Journal of Applied Meteorology 40, no. 4 (2001): 762–775.

Simister, John, and Cary Cooper. “Thermal Stress in the U.S.A.: Effects on Violence and on Employee Behaviour.” Stress and Health 21, no. 1 (February 2005): 3–15.

Web Sites

“Are You Ready? Extreme Heat.” Federal Emergency Management Agency. <http://www.fema.gov/areyouready/heat.shtm> (accessed October 28, 2007).

“Chapter 1: Overview of Impacts, Adaptation, and Vulnerability to Climate Change.” IPCC Climate Change 2001: Working Group II: Impacts, Adaptation and Vulnerability. <http://www.grida.no/climate/ipcc_tar/wg2/061.htm#1434> (accessed October 30, 2007).

Michele Chapman