In the past, heat waves have increased the risk of death to the elderly and children. In 2003, for instance, various estimates showed the Paris heat wave took between 7,000 and 15,000 lives. Some 700 people died prematurely in a 1995 heat wave in Chicago.

In a paper published in the journal Science this month, NCAR senior scientist Gerald Meehl and colleague Claudia Tebaldi reported the results of global climate model runs that show "there is a distinct geographic pattern to future changes in heat waves.

Model results for areas of Europe and North America, associated with the severe heat waves in Chicago in 1995 and Paris in 2003, show that future heat waves in these areas will become more intense, more frequent and longer lasting in the second half of the 21st century, the authors wrote.

The prediction is based on model results but there is a meteorological explanation that fits, Meehl told United Press International. Heat waves usually are associated with stationary domes of high pressure over an area.

In the future, Meehl told UPI, the average climate change gives you these high pressure anomalies over North America and Europe. In a given heat wave, you get a dome of high pressure, then the climate change gives you an additive effect. ... It is more conducive for those events to be severe.

Changes in tropical ocean temperatures and deep convection result in changes in atmospheric circulation over North America and Europe that are more conducive to the formation of these stationary high pressure systems. The effects are analogous to the way that El Nino Pacific sea surface temperature warming near the cost of Chile has a wide-ranging impact on climate and weather in North and South America, and sometimes as far away as western Europe.

One of the problems associated with studying the issue, Meehl said, is there is no universally accepted definition of what a heat wave is -- but like pornography, we all know it when we see it.

Meehl and Tebaldi adopted two definitions, both of which have been associated with affects on health and the economy. The first one evolved from a study of the Chicago event, which found several consecutive nights with no relief from warm nighttime temperatures may have the greatest impact on human health. The second definition used three criteria relying on location specific thresholds for maximum temperatures.

While the NCAR work indicates the frequency and intensity of heat waves will increase in Europe and North America, they do not do so uniformly.

In Meehl's simulation, the Great Basin, Rocky Mountains and Southern United States show the greatest impact. In Europe, the Mediterranean Basin is hardest hit. In other words, for the most part, the hot get hotter.

Heat waves have been associated with considerable human health consequences. This has been in part from their suddenness and relative rarity in the regions they hit.

People seem to have a remarkable adaptability to this kind of event, however, and may be able to adjust.

The British Medical Journal concluded in a study in 2000, Populations in Europe have adjusted successfully to mean summer temperatures ranging from 13.5 degrees C to 24.1 degrees C, and can be expected to adjust to global warming predicted for the next half century with little sustained increase in heat related mortality.

W. Larry Kenney, Pennsylvania State University professor of physiology and kinesiology, said people can be quite resilient to heat.

As a long as humans can sweat and evaporate the sweat and get enough fluid, they can tolerate temperatures up to 200 degrees Fahrenheit for a short time, he said.

A famous study conducted in London put a man, a dog, a piece of raw meat and some eggs in an oven for 20 minutes. The eggs and meat were cooked, but the man and dog emerged unharmed, because they were able to sweat -- or to pant, in the case of the dog, he noted.

As humans, we all have some variability in response to those sorts of temperatures, Kenney said. In my opinion, the ability to withstand hot temperatures is much more a function of short-term adjustment.

Cardiovascular and aerobic fitness, as well hydration, are critical factors in adjusting to heat waves. At most risk of death are the elderly and young children.

Kenney said the elderly who die in heat waves do so primarily from cardiovascular complications.

When an older person is exposed to hot temperatures for a long period of time, the heart has to work very hard to pump blood to the skin. They are losing the fluid and causing the heart to work even harder, he said. In the elderly, if they have had heart problems in the past, the additional strain by the need to regulate their body temperature is what causes the problem.

As the BMJ somewhat heartlessly noted, Some of those who died in the heat may not have lived long if a heat wave had not occurred.

Infants and children, Kenney said, are smaller, and so they heat up faster, with less surface area on which to distribute the increase. They also often are at the mercy of someone else when it comes to fluid intake.

Meehl said in addition to human health effects, heat waves have economic and ecological impacts. The effects on the biosphere are especially difficult to estimate.

Average changes in climate are interesting and can help explain some of these patterns, he said. But it is the extreme events that really have the impacts. They can have a lot of effects.

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