Extreme Heat Events

Although the mean climate temperature increase is a slow, global phenomenon, the effects are not equally distributed in time or space. Local climate and temperature remain highly variable; it is the average of temperatures that will grow higher. This also means that the extreme ranges of temperatures will also increase, resulting in fewer cold days but more frequent and severe heat extremes. Furthermore, these effects have been most profound in the high-population areas between 40° N and 70° N, and are further amplified by heat re-radiating from land mass, as well as from urban concrete and asphalt.⁴⁰

The net result of these changes is increasing frequency and severity of extreme heat events, particularly in northern urban centers, resulting in acute human injury. Current models predict a 50% increase in the annual likelihood of severe heat events over the next five decades, with the Chicago Heat Wave of 1995 and the European Heat Wave of 2003 cited as examples of the injury potential of these future events. Both events were marked by multiple days of oppressive temperatures above 100° F (37.8° C), with atypically high levels of humidity resulting in less heat dissipation during night. Respectively, 739 and 27,000 people died as a result of heat-related illnesses, with the bedridden, elders, and urban poor persons being particularly at risk.²² In Chicago, another 3300 people presented to emergency departments for heat-related care. In France alone, costs related to these injuries and preparing for further similar events exceeded $8 billion.²³

These injuries and costs were likely grossly underestimated, as heat-related exacerbations of cardiac and pulmonary events were not necessarily categorized as direct heat injuries. With the associated increase on cardiopulmonary demand, extreme temperatures increase cardiac events, as evidenced by data from other heat waves. During an Australian heat wave in 2009, the number of patients presenting to emergency departments with myocardial infarction was three times higher than that during the same periods in prior years.⁵

Heat waves are associated with increased air stagnation, which worsens urban pollution as well as causes increased pollution resulting from air conditioning use and other energy consumption. As a result, persons with chronic respiratory illnesses such as asthma and COPD are at increased risk of acute events.²²

Mean Temperature Increases

Even excluding the extreme heat events that result from climate warming, the increased average of mean temperatures alone has a marked longitudinal impact on human health. These impacts include agricultural yields, freshwater demand, and energy consumption.

Within the United States, much research has focused on the impact of temperature increases on California agriculture. This state is already operating under severe water constraints because of population expansion and forced irrigation of desert areas. Mean temperature increases will result in increased water consumption for almost all plants (and farm animals), although this will be partially offset by increased plant productivity due to increased atmospheric CO₂. Most models predict a net increased need for water.

At the same time, this warming is reducing the snowpack and spring run-off that supplies much of California’s groundwater. Although the majority of California’s water is consumed south of Sacramento, 75% of precipitation falls north of that city. As a result, the Sierra snowpack and resultant run-off are critical for meeting Southern California’s water needs. Conservative estimates predict a 40% reduction in this snow mass over the next 100 years, resulting in approximately 25% less water available to mid- and southern California.²⁶ Loss of available water is further exacerbated by the rise in sea level associated with climate warming. Rivers supplying mid-California will become increasingly brackish with sea level rise, and near-shore ground water sources will experience similar salination.³⁰

In addition to increased frequency of warm days, reduction of cold days creates further problems. Although less frequent cold days may favor the stability and yields of certain crops, it is detrimental to others. For example, fruit and nut trees require a minimum number of hours below 45° F (7.2° C) to properly germinate. Conservative projections show that within 100 years, there will be insufficient cold hours for these plants to thrive, leading to reduced crop yields of citrus and nut products and reduced biodiversity.²⁵

Although California serves as a well-researched example of the longitudinal effects of mean temperature increase, it is certainly not an isolated case. Developed and developing countries around the world will face similar issues. Developed countries will likely be able to mitigate the impacts on human health, at least initially, through aggressive resource management as well as importation of food and other agricultural products. Although these measures will increase costs, the low percentage of income per capita spent on meeting basic needs in developed countries will allow them to absorb these changes in the short run.

In contrast, developing countries are likely to reach crisis points in the near future. With less geographic diversity and fewer resources to mitigate disruptions of water and agriculture, smaller developing countries are at greater risk to experience abrupt changes, with potential famine, conflict, and population displacement. Projections estimate that African countries will likely be among the first to reach crisis. By 2020, 75 to 250 million Africans are expected to be water stressed, as well as face nutritional consequences due to 50% reduction in water-based agricultural yields.²⁰

Extreme Weather Events

Climate warming impacts the frequency and severity of many weather events, including precipitation, drought, and cyclones. Some of the mechanisms by which climate warming increases extreme weather events are understood. One such mechanism occurs through warmer ambient temperatures increasing evaporation and absolute humidity. This results in more water carried as vapor in the atmosphere. Rain is therefore less frequent, but the rain storms become more intense as the amount of water in the storm clouds is greater. Longer periods of drought interspersed with more intense rain events are predicted by most models. Evidence suggests that this may already be occurring. Net increases in total annual precipitation of 10% to 15% have been documented for much of North America over the last half century. This has not been accompanied by an increase in storm frequency, so the increase is accounted for mainly by increased intensity of storms.⁴⁰

These temperatures are also likely to exacerbate recognized weather systems as a whole, such as the El Ninõ Southern Oscillation (ENSO). By further warming seawater in the tropical Pacific, climate warming will likely intensify the precipitation variability associated with the ENSO phenomenon, as well as increase cyclone intensity. Many models project a decrease in North American cyclonic events. However, these same models also predict lower central pressures in the eyes of these cyclones, and thus greater storm intensity, as well as increased ability of such storms to retain power as they move toward North American landmass.⁴⁰

To predict the impact of future events on human health, it is only necessary to study recent extreme weather events. One such event is the landfall of Hurricane Katrina in the southern United States in 2005. Although it is not possible to calculate the degree to which global warming contributed to Hurricane Katrina, it is possible to say that similar events are predicted to increase in frequency as a result of global warming. Hurricane Katrina was unique in that it retained its intensity farther north than do most cyclones, and was particularly intense when landfall occurred (a phenomenon predicted as more likely in fature events). This is believed to be partly driven by warmer waters in the Loop Current, which allowed the storm to gain intensity as it moved over ocean. By landfall, it was one of the most intense storms ever recorded in the Gulf of Mexico. As 53 levees were overwhelmed by the sea surges, 80% of the city of New Orleans was flooded. The direct death toll resulting from winds, flooding, and resulting structural collapse were estimated at almost 2000 persons, with thousands more displaced from their homes. Direct accounts exist of suicides, homicides, and assaults occurring in the wake of this disaster, although official statistics have been very difficult to create. Also difficult to calculate have been the causalty figures resulting from poor sanitary conditions faced by survivors. For example, much of New Orleans’s groundwater was contaminated with feces, dead livestock, and chemicals leeching from flooded industrial facilities. Soon after the event, the CDC recorded unsafe levels of Escherichia coli and other contaminants in the floodwaters throughout New Orleans, with at least seven deaths directly attributed to consumption of water contaminated with Vibrio vulnificus.²

The human health consequences of this devastating event manifested in unique, unpredictable ways. One sampling study found that in the months after the hurricane, 20% of persons with chronic medical conditions living in the New Orleans area before the hurricane had reduced or stopped treating these conditions. This held true for many people who were successfully relocated. Reasons given included lack of physician access (>40%), financial/insurance disruption (23%), and transportation or time constraints (>25%).¹². Another unique example lies in the homicide rate in Houston for the year following the event. Houston absorbed many evacuees following the event. In November and December after the hurricane, Houston’s homicide rate increased by 70%, far disproportional to the population increase. Many sources further cited that a large portion of these crimes had evacuees as convicted or suspected perpetrators in many of these crimes.^15,41

The total human health toll, and the challenges facing physicians and public health, will likely never by fully elucidated for this event. However, Hurricane Katrina serves as an example of the wide range of human health and societal challenges likely to accompany increasingly extreme weather over the next century (Figures 111-2 and 111-3).

FIGURE 111-2 Satellite imagery of Hurricane Katrina making landfall over the United States, 2005.

(Reproduced from the National Oceanic and Atmospheric Administration website. http://www.katrina.noaa.gov/satellite/satellite.html.)

FIGURE 111-3 Train overturned by the force of Hurricane Katrina, Louisiana, 2005.

(Courtesy Jay Lemery, MD.)

Infectious Agents and Their Vectors

Global warming has the potential to impact the viability of pathogens. This may be through direct effects on the organisms, or indirectly, such as by altering vectors and reservoirs. Much research in this area has focused on malaria, and the impact that global warming has begun to have on vector mosquitoes.

Links between malaria, temperature, and weather have been noted for millennia. Populations affected by malaria have long been aware that warm, wet conditions favor mosquito survival and reproduction, and correlate with epidemics. An Argentine study found that the Aedes mosquitoes in Buenos Aires had the highest abundances in breeding following periods of several months with mean temperatures above 20° C (68° F) and accumulated rainfalls above 150 mm.¹³ Mean increases in planetary temperature and associated increases in precipitation are likely to favor malaria transmission.

Current models predict that the 2° to 3° C (3.6° to 5.4° F) annual temperature increase predicted over the next century will increase the number of people at annual risk for malaria by 3% to 5% (greater than 200 million people). Specifically, these conditions will prolong malaria seasons and favor mosquito migration to higher altitudes, infecting populations not traditionally at risk.¹³ Using the same type of models, similar trends are predicted for many other insect vector–based diseases. Both dengue and other types of encephalitis are predicted to increase in incidence in North America as a result of climate change.¹⁷ Malaria epidemics are more frequent and severe following the El Ninõ weather cycle, which will become more frequent and severe as a result of climate warming.

Public Health Conditions Favoring Disease

The impact of climate change on infectious disease is not just limited to its direct impact on organisms and vectors. Living conditions, sanitation, and public health measures all play critical roles in the incidence of many infectious diseases. The potential for climate change to impact these variables is significant.

One marked example of the importance of public health measures is the stark contrast in incidence of dengue on opposite sides of the U.S.–Mexican border.¹³ Despite having identical climates, areas just south of the border have a 500-fold increase in incidence versus areas just north of the border. Much of this disparity is attributable to differences in sanitation, water supply, and other public health measures. In an exhaustive study of developed and developing countries over the past century, it was found that improved water supply and sanitation resulted in “substantial reductions in morbidity from diarrhea (26%), ascariasis (29%), guinea worm infection (78%), schistosomiasis (77%), and trachoma (27%), and median reduction of 65% in diarrhea-specific mortality and 55% in general child mortality.”⁷

Given the likely increase in extreme weather events, such as cyclonic activity and flooding, it is likely that sanitation and other public health measures will be at increased risk for disruption. Much of any reduction in infectious disease burden made in developed countries is at threat during acute events. In addition to acute periods of threat, sanitation and water supply may erode in countries as climate change stresses displace populations.

For many reasons, the IPCC predicts a net increase in certain infectious diseases over the coming century. Perhaps most worrisome is the projection that by 2030 the incidence of diarrheal illness will be 10% higher in certain regions than if no climate warming had occurred.¹⁷ Given that diarrheal illness is one of the leading causes of infant and child mortality worldwide, this increase alone will have a profound effect on the average life expectancy within affected countries (Figure 111-4).

FIGURE 111-4 Threats to human health in Africa as a result of environmental change.

(Reproduced from United Nations Environment Programme, GRID-Arendal website. http://www.grida.no/publications/et/ep4/page/2638.aspx.)

Dermatologic and Ocular Effects

The relationship between UVB exposure and the incidence of skin cancers has been understood for decades. It has therefore been argued that as ozone depletion increases ground-level exposure to UVB, depletion of the ozone layer is likely to result in increased skin cancer incidence (assuming no behavioral changes). Models for nonmelanoma skin cancers predict a 2.3% increase in skin cancer for every 1% depletion in stratospheric ozone. Although more difficult to predict because of lower baseline incidence as well as a weaker correlation with sun exposure, increases in melanomic skin cancer have also been postulated.²⁷

Because the eye is exposed to sunlight, ocular diseases are also predicted to increase. Acute UVB radiation exposure causes photokeratits (snowblindess). Chronic exposure leads to the formation of cataracts, a major cause of adult blindness worldwide. Current models predict that for every 1% drop in stratospheric ozone, there will be 3400 additional annual cases of blindness due to cataracts.²⁴

Immune Systems and Infectious Disease

UVB radiation appears to also have systemic effects on human immunity. Study of the intersection between UVB radiation and immunology is known as photoimmunology.

It has long been observed by physicians that active pulmonary tuberculosis appears to be worsened in many patients by direct sunlight exposure. Nineteenth-century physicians also noted a clear correlation between sunlight exposure and increased virulence and mortality of smallpox infections. However, sunlight has proved an effective therapy for treating other diseases. It was from observations made between sunlight exposure and disease improvement that led dermatologists to use UVB treatment to effectively treat psoriasis and certain types of skin hypersensitivities and lesions.³⁹

At the laboratory level, data are extensive that direct UVB exposure at levels present on Earth suppresses the response of certain immune pathways in the skin of mice and humans. This suppression is likely responsible for improvement in some skin diseases and increased virulence in others. For instance, when human subjects with and without recent UVB exposure were afflicted with leishmaniasis, those with UVB exposure showed less initial skin reaction to the infection⁸ but subsequently demonstrated more rapid and virulent systemic infection. Similar observations show increased infection and virulence with regard to many viruses, such as herpes simplex virus and papilloma. Although the full effects of increased UVB light on immune systems and infectious diseases have yet to be elucidated, our current limited understanding causes concern that ozone depletion may have a deleterious effect on the incidence and prevalance of certian human diseases.³⁹

Biodiversity and Epidemics

Epidemic infections are related to the number of species and the genetic diversity within and between species. A complex web of millions of species interacting with each other is a more stable system than a simpler web of fewer species. In a complex ecosystem, there are fewer epidemics because each species of plant or animal is present in realitively small number, which ultimately makes it more difficult for a disease-causing bacteria or virus to find a suitable host. Contagious disease epidemics of humans or other species depend on the population density of the individuals susceptible to that parasite.

To further understand this principle, one must recall that bacteria and viruses do not intrinsically have a mechanism to seek out and select potential hosts. They must rely on air or water movement, gravity, or the movements and direct contacts of their hosts with other hosts to spread. Random movement of air molecules or other molecules in the surrounding environment predicts the number of bacteria, fungal spores, or viruses at a given distance from the original source.

Plant pathologists, epidemiologists, mathematicians, and other scholars have studied these relationships to understand how epidemics of diseases will spread and wane through populations of animals, humans, or crop plants. These models have predicted that the density of susceptible individuals must exceed a density threshold before a disease can become epidemic. Thus, climate change may favor epidemics not only from the loss of biodiversity but through the forcing of human populations into greater proximity and density.

Biodiversity and Medicines

Organisms evolve their own enzymes, cell structures, and receptors to obtain the nutrients they need from the environment and to protect themselves from predation. Toxins have evolved to disrupt certain pathways in animals, bacteria, or fungi but may remain harmless or even beneficial to other organisms, such as humans.

Most of the antibiotics and other drugs are analogs of compounds originally isolated from bacteria, fungi, plants, animals, or marine invertebrates. Loss of biodiversity will mean potential loss of natural toxins not yet known to humans.

Hepatitis A virus

Hepatitis A is found in water supplies contaminated with human waste. This virus causes self-limited liver infection, leading to fever, fatigue, lethargy, abdominal pain with vomiting and diarrhea, and in some cases, jaundice. Unlike other forms of viral hepatitis (hepatitis B, hepatitis C), these infections are self-limited and do not have a chronic disease carrier state. Hepatitis E has a similar disease course as does hepatitis A, although it holds a particular risk for pregnant women in their third trimester, who have a 20% likelihood of death when afflicted.