As our understanding of potential global warming trends continues to rise, questions about their effects become more pronounced. Since the late 1980s, the influence of climate change on human health has become a major concern. Warnings have been heard increasingly from researchers as well as from international organizations such as the World Health Organization. Their message has been clear: while the magnitude of impact of global warming on human nealth remains vague, with the number of people affected varying from site to site, global warming is expected to affect human health in multiple ways.

Two such ways were described in the spring issue of IoWatch. Global warming is expected to alter the distribution and ecology of infectious agents (thus leading to a resurgence and spread of certain infectious diseases) and to alter crop production, agricultural pests, and other factors that reflect on the human food supply. Iowa is not immune to such problems. This past July, for example, Iowa recorded its first known case of the hantavirus infection that was described in our last IoWatch, becoming the 26th state to report this emerging and often deadly disease. In August, an article in The Atlantic Monthly described the return and resurgence of locally transmitted malaria in many areas of the U.S.

The following paragraphs describe two other health-related concerns: the rising heat stress and increasing numbers of extreme climatic events that are predicted to accompany global warming.

Human Health and Rising Temperatures

We've grown up with stories of the Dust Bowl days here in Iowa, of those times when the spirits of Midwesterners shriveled along with their crops and the sky was darkened by parched earth lifted skyward. The average July temperature in Iowa that landmark decade was a mere 4°F warmer than the mean 50 years later, about in the middle of the range of increase predicted for a century from now with global warming. But that was the average, not the extreme. The first of the great heat waves swept into Iowa in July, 1930, killing thousands of field horses. By Sunday, August 3, the average high for the state was 106.4°F, hotter than any previous day on record. That was but a harbinger of things to come. Intense heat continued to attack the state, with Keokuk reaching 118° in the summer of 1934, and the state's average maximum climbing to 108.7° on July 14, 1936. While the July-August average for 1930 was 76.1°, the average for 1934 crept up to 76.5°, and in 1936--the most torrid summer of the decade--the state's average temperature spiked at 81.3°. In a typical year, about 175 people in the U.S. die from excessive summer heat and sunshine, their bodies losing the ability to maintain homeostasis and prevent core temperatures from rising to meet those of the outer world. Between 1930 and 1936, the heat killed nearly 15,000 people -- 4768 in 1936 alone.

Humans are in many ways frail creatures, dependent on protection both from the cold and from heat in order to survive. This need for shelter from the elements would be modulated by the changing climate. Researchers believe taht warming of our global climate could significantly increase the number of heat-related deaths and illnesses, an increase that would be only partially matched by the decrease in cold-related deaths.

Because of their vulnerability, people have developed social, behavioral, and technological mechanisms for isolating themselves from the weather--for example certain ways of dressing and of heating and air conditioning their homes. In addition, given time, people acclimatize to temperature extremes to some degree. But extended hot temperatures can overtax the body's natural cooling mechanisms, especially when humidity is high and winds are low. Marked short-term fluctuations--such as the very hot days tha tmost global climate models predict will increase--are especially difficult to withstand because they allow little time for physical or physiological adjustment. As temperatures rise, the body struggles to maintain a constant internal temperature. At some point, however, its natural cooling mechanisms no longer suffice. The cardiovascular system no longer responds properly, resulting in heat exhaustion with faintness, rapid heartbeat, low blood pressure, cold clammy skin, and nausea. Or the body's core termperature may begin to rise, producing heat stroke and in extreme cases leadint to confusion or delirium, seizures, coma, and death. Both heat stroke and heat exhaustion are more probably among the very young, the elderly, persons with existing conditions, and those with impaired mobility. The urban poor also have fewer recourses to escape from the extremes. High temperatures also raise the incidence of hospital admissions and mortality from cardiovascular and respiratory disorders and certain types of accidents.

Mortality rates increase dramatically during heat waves, a fact demonstrated in this depiction of daily mortality during a 1966 heat wave in New York. (Source: Kalkstein LS, WJ Maunder, G Jendritzky, 1966. Climage and Human Health, 2nd edition. World Meteorological organization, No. 843.)

Global warming may stimulate illness and death in less direct manners also. Higher temperatures accelerate the chemical reactions that produce secondary air pollutants, such as tropospheric ozone, from primary air pollutants. Thus more frequent and intense periods of very hot weather could be coupled with increasing air pollutant concentrations. While ozone is beneficial in the upper atmosphere, where it filters out the sun's UV rays, when inhaled it damages cells of the respiratory tract and impairs lung functioning. It can increase the effects of other pollutants (such as SO₂) in the lungs, and decrease defenses against infections. It appears that higher temperatures also act synergistically with air pollution: the effects of the two together induce more health problems than the sum of the two would imply. And lastly, warmer weather could affect water supply, food production, and other biological systems (such as plant production of spores and pollen that in turn could alter human health.

Human Health and Extreme Climatic Events

Bangladesh is washed from the north by runoff from the Himalayans and from the south by the storms sweeping in from the Bay of Bengal. If people were scarce and abundant land offered multiple sanctuaries, an occasional drenching of this tiny nation would bring little hardship. But Bangladesh's flatlands offer few safehouses for the country's densely packed millions, especially those who inhabit the gigantic delta of the Ganges and Brahmaputra rivers. Monssons can bring catastrophic floods cascading down from the highlands. And storm surges from the frequent cyclones sometimes sweep over 100 miles inland. These cyclones killed 300,000 people in 1970, and another 140,000 in 1991. Continues population growth will push more people onto the vulnerable coastal lands, where the masses will be exposed to increasingly frequent and severe storm surges. Or they may be forced to fight a more sinister fate: losing their landbase permantly to a slowly rising sea.

Changing climate is likely to increase weather variability. Heavy precipitation events are predicted to become more frequent regionally, while models show otgher regions already prone to drought experiencing longer and more severe dry periods. These events may be exacerbated by human activities, overgazing and forest clearing for example, multiplying the liklihood of both drought and of flooding. The effects of the resulting regional droughts and floods will depend on the vulnerability of each natural environmant and human society.

Apart from the obvious dangers of drowning and acute injury, flooded areas experience health problems as sanitation systems break down, food supplies are destroyed, croplands are eroded or contaminated, and dangerous chemicals or wastes are washed into floodwaters. Drought affects health primarily through impacting the production of food and its distribution. Malnutrition increases. Severe diarrhea and diseases associated with poor hygiene (such as scabies and conjunctivitis) increase as water quantity and quality decline. Death strikes not only through starvation, but also through diseases that take hold because of decreased immunity caused by starvation.

Both floods and droughts stimulate infectious disease as sanitation and public health infrastructures degenerate and as populations become more crowded. These disasters further affect health by increasing stress-related psychological disorders and provoking migration of displaced persons (producing "environmental refugees").

The human impacts of climate-related didasters have clearly and considerably risen during the past 25 years. However this growth is due in part to the soaring size of the human population and to its expanding vulnerability, with marginal populations increasingly being forced into "disaster risk zones." Persons with the fewest technical and social resources are affected most strongly. Projections indicate that Earth's poorest populations will increasingly congregate on land most vulnerable to disasters, and will have the fewest resources to recover from such disasters. Asia experiences the highest number of natural disasters and the greatest number of affected people. Developing countries in general experience the most deaths from natural disasters and the lartgest relative economic impact. However the total price tag of such disasters is far heavier in rich countries.

The various costs of disasters--both to life and to the pocketbook--are shown here for the decade 1986 to 1995. As can be seen, natural disasters are many times for costly than accidents and fires. Other data reveal that nearly half of all insured losses from natural catastrophes during the past 40 years have occurred since 1990. This fact has increased the concern of the world's 1.4-trillion-dollar insurance industry about potential climate change, and has drawn the reinsurance leaders in Europe into advocating limits for greenhouse gas emissions at the United Nations. (Source: "The Geosciences and the UN system, in the Service of Humankind," A discussion paper produced by the World Meteorological Organization, Sept. 1996)

The predicted rise of the Earth's seas is anothe rdistince but extreme process that would threaten human health. Models indicate that the oceans' rise--stimulated by thermal expansion of sea water and the melting of glaciers--would equal on the average a half meter by the year 2100, with the actual rise varying regionally. Since more than half of the world's population now lives within 60 km of the sea, and this population is growing faster than the world's average, rising seas could affect the health of most future humans. Small islands and heavily populated deltas would be among the most intensely impacted. Bangladesh, for example, could lose 17% of its land surface if the sea level rises a meter.

The health effects of rising seas would include the obvious displacement of large populations and the physical and mental stresses associated with forced migration. Food production and thus nutrition could decline as agricultural lands are flodded; large populations in Southeast Asia, for example, would lose some of the world's major rice-growing paddies. Increasing coastal erosion and injuries to coastal wetlands could impact the 70% of commercial fish species that complete their life cycles in these habitats. Saltwater flooding and intrusion of aquifers would reduce the amount of fresh water available both for humans and for agriculture, and could alter the distribution of disease vectors such as malaria-transmitting mosquitoes. And populations remaining in coastal areas would be increasingly susceptible to the impacts of floods and ocean-generated storm surges.

Extreme weather events are rising in frequency and in the annual costs of damage--a fact demonstrated here by the worldwide impact of major windstorms (those with over $500 million in total damage) between 1960 and 1992. Other data show that nearly half of all insured losses from natural catastrophes during the past 40 years have occurred since 1990, and that extreme weather events have become increasingly frequent in the past six years. Anthropogenic climate change is expected to increase the variability of weather, with extreme weather events becoming more frequent and affecting human health in multiple ways. (Source: "Energy Efficiency Strategies for Insurance Companies," reprint from Center for Building Science News, by Evan Mills.)

Although Iowans need not personally fear threats from rising oceans, the prospect of increasing heat stress is quite real. Recognizing this, CGRER is commencing a project to compute risk assessments for increasing heat stress in Iowa. Greg Carmichael and his students will be evaluating deaths correlated with periods of high heat and humidity in the past. Then, with the help of climate models, they will compute changes in the number of heat stress-related deaths that could be expected from warming climates.

Other CGRER members are working on ways to reduce damages and deaths from extreme climatic events. Witold Krajewski, Allen Bradley, and Kosta Georgakakos, for example, are involved in projects that will improve flood forecasting and thus increase time to prepare or flee from dangerous exposures.

Three additional CGRER grants that relate the burning of fossil fuels to agricultural production and to human health are described in the column, "What's Up at CGRER?"   Implementation of Phase III of the Iowa Greenhouse Gas Action Plan also is discussed there. Iowa can take pride in being one of the first states to reach the third phase of this planning process, which involves putting into action specific proposals to decrease Iowa's greenhouse gas emissions.

While such efforts are critically important, global prevention remains the best medicine. That means taking steps to decrease the emission of CO₂ and other greenhouse gases into the atmosphere. Neither scientists nor policy makers dispute the human-induced increases in atmospheric carbon dioxide or the rise in temperature over the hundred years. The majority of scientists now link the two, tracing the temperature rise directly to increasing greenhouse gas concentrations.

Even those skeptics who deny human-induced climate warming cannot argue about today's actions producing changes tomorrow. As CGRER co-director Jerry Schnoor says, "We simply cannot spend a few hundred years burning fossil fuels that took many millions of years to accumulate with climatic consequences. The concentration of trace gases in the atmosphere is rising rapidly. We absolutely must stabilize atmospheric concentrations of these gases."

Governmental leaders from around the world will meet in Kyoto, Japan, in December, once again to negotiate a global treaty for reducing greenhouse gas emissions. The discussion will focus on enforceable guidelines for emissions between the years 2000 and 2010. Proposals such as a carbon tax or binding limitations to emissions would be major steps forward that could lead to significant declines in the production of greenhouse gases.

Although the U.S. produces about 22% of the world's greenhouse gases, the Clinton administration to date has refrained from committing itself to any specific proposals or decreases in emissions. However, hopes remain high that our government will invoke meaningful actions. Certain businesses have formed strong coalitions to lobby against any mandated decrease in emissions, arguing that this would precipitate massive economic declines and rising unemployment. But economic analyses have shown that many potential emission-reducing steps could be taken for which total benefits outweigh total costs. Additionally, the costs of failing to act on the economy, as well as human health, could be tremendous.

The Kyoto conference could signal a turning point for our atmosphere, marking the time when we choose to significantly limit the emission of greenhouse gases and safeguard the health of future life. Whether or not that will happen now remains in the hands of our policy makers.

Note: The majority of information in this article was taken from the book Climate Change and Human Health (AJ McMichael, A Haines, R Slooff, and S Kovats editors; published in 1996 by the World Health Organization, Geneva), a detailed assessment prepared by an international task group on behalf of the World Health Organization, the World Meteorological Organization, and the United Nations Environmental Programme. Details of Iowa's Dust Bowl days came from The Greenhouse Effect by Harold Bernard, published in 1980 by Ballinger Publishing, Cambridge MA.