March 2007 | Back to Table of Contents

Clinical and Health Affairs

Global Climate Change and Human Health

By Jonathan Sellman, M.D., and J. Drake Hamilton

Abstract
Global climate change is a vexing economic, environmental, and public health problem. Climatologists have documented a rise in global average temperature that is unprecedented since the Industrial Revolution. This temperature change correlates with increasing concentrations of atmospheric carbon dioxide (CO2) and other greenhouse gases, which largely are of human origin. Global climate change is likely to profoundly affect global health. This article summarizes the science of climatology and global climate change caused by human actions and reviews potential health effects.

In December 2006, Howard Frumkin, M.D., Dr.P.H., director of the Centers for Disease Control and Prevention’s National Center for Environmental Health, warned that climate change is perhaps the largest looming public health challenge we face. And he said that the increased scientific certainty about climate change should mobilize health professionals to take action.¹

Why is global climate change relevant to health care providers? The World Health Organization (WHO) has estimated that changes in the earth’s climate caused by humans now lead to at least 5 million cases of illness each year and that global climate change causes more than 150,000 deaths every year. The WHO projects that the human toll from climate change could double by the year 2030.² Broad agreement exists among scientists who study climate that greenhouse gas emissions generated by human activity are causing most of the warming observed over the last 50 years.³ This article summarizes the science of climatology and anthropogenic (human-caused) global climate change and the potential health effects of climate change.

The Science of Climate Change
The global climate is influenced by interactions between incoming solar radiation, terrestrial radiation (long-wave radiation emitted by the earth), and the composition of the earth’s atmosphere.⁴ Although some solar radiation is reflected by the upper atmosphere, most is transmitted through our atmosphere to the earth’s surface. The earth is warmed by the solar rays and emits infrared radiation. A set of naturally occurring greenhouse gases trap some of the outgoing infrared radiation, which in turn traps heat near the earth and warms the lower atmosphere. This phenomenon is responsible for maintaining the temperature of the earth’s surface at a global average of 15 degrees C (59 degrees F).

Scientists who study climate have documented rapid increases in the concentrations of greenhouse gases in recent decades and, particularly, since the Industrial Revolution. Carbon dioxide (CO2) and methane are the most important greenhouse gases, as measured by their concentrations in the atmosphere and potential to trap heat.

Since the dawn of the industrial age, the atmospheric CO2 concentration has increased from 280 ppm to 380 ppm. This concentration of CO2 far exceeds the natural range of CO2 over the past 650,000 years as determined by ice core sampling.³ During the past decade, the average annual increase has been 1.9 ppm.³ This unprecedented increase in CO2 correlates with the documented increased combustion of fossil fuels (coal, oil, and natural gas).³

Methane (CH4) concentrations in the atmosphere have increased nearly 150% since 1750. Current atmospheric CH4 concentrations are higher than at any time in the past 420,000 years.⁵ It is very likely that the observed increase in methane concentrations is anthropogenic. The documented increase in the atmospheric concentration of methane correlates with the increase in agricultural intensity worldwide.³

Greenhouse gases absorb some of the long-wave, terrestrial radiation emitted from the earth, trapping more heat in the lower atmosphere and increasing temperatures. Because the heat-trapping effects of atmospheric CO2 persist over centuries, climate scientists point to the inevitability of a future rise in global temperatures and highlight the need to begin to reduce significantly greenhouse gas emissions within 10 years.⁶

Troublesome Temperature Changes
Since the late 1800s, average global temperatures have increased by about 0.7 degrees C.³ In the 1990s, they reached their highest levels since recordkeeping began in 1861. Eleven of the last 12 years rank among the warmest in recorded history.³ The year 2006 was the hottest on record for the continental United States. Minnesota has followed this pattern, with many meteorological stations documenting increases in annual average temperatures of 2 degrees to 4 degrees F since the late 1800s.

These temperature changes are significant and worrisome. Environmental effects of increased global temperatures have been documented. Since the 1960s, global snow cover has decreased in extent by about 10%, mountain glaciers have retreated worldwide, and sea levels have risen 1 cm to 2 cm per decade.^3,7 Across the Northern Hemisphere, there has been a decrease in the duration of lake ice cover of between 2 weeks and 1 month during the past 100 years of record; many of the lakes studied are in Minnesota and Wisconsin. Researchers who study butterflies, birds, and plant-blooming periods in North America and Europe have documented spring arriving about 1 week earlier and autumn 1 week later than 100 years ago.^7,8

Global warming is expected to increase the atmosphere’s capacity for moisture.⁹ As a result, heat emergencies, severe storms, extreme precipitation events, and prolonged droughts are expected to increase. Rising sea levels and storm surges associated with hurricanes are expected to inundate coastal areas.

Possible Health Effects
Effects of global climate change on human health may be direct or indirect. Historically, research investigating the link between weather and human health has focused on the direct effects of heat waves and extreme weather events, for which empirical data are readily available and correlations are easily demonstrable. In addition, secondary effects such as changes in the distribution and incidence of infectious diseases may result.

Often lost in the discussion of health effects is consideration of potential displacement of people from their homes and communities. Global climate change may trigger large-scale migrations of people fleeing areas made uninhabitable by changing weather patterns. The following sections discuss the health effects of each of these types of change.

♦ Heat Emergencies/Droughts
Each year in the United States, approximately 400 people die from heat-related illness (heat exhaustion and heat stroke).¹⁰ Prolonged periods of elevated temperatures can result in a marked increase in mortality rates. For example, the 2003 European heat wave caused particularly high mortality (more than 22,000 people were killed).¹¹ Risk for heat-related illness and increased mortality are greatest for 2 groups: the elderly and the very young.

Effects of increased temperatures are exacerbated by other factors such as excessive exercise, reduced fluid intake, alcohol and/or other drug use, cardiac disease, mental illness, prolonged outdoor activity, low income, social isolation, and lack of access to air conditioning.¹⁰ Residence in an urban area is also a primary risk factor for mortality during heat waves because city temperatures are increased by the urban heat island effect, which raises temperatures 3 degrees to 5 degrees F or more compared with nonurban areas. The homeless are at increased risk of morbidity because of multiple risk factors including lack of access to air-conditioned shelter.

Comparative studies have documented that heat-related mortality occurs at a lower temperature in countries with cooler summers, suggesting that people exposed to warmer summer temperatures adapt to those temperatures.^12,13 Physiological acclimatization is a partial explanation for such differences in heat tolerance, but social adaptations such as use of air conditioning are also important.¹³

♦ Extreme Precipitation
Increased drought and increased precipitation are both predicted by global climate change models.⁹ This may seem counter-intuitive, but both extremes are projected because of the way water in the atmosphere behaves at warmer temperatures. Warmer air is capable of holding more moisture. As a result, droughts may become more common. But once the rain begins, the warmer atmosphere is likely to produce heavier amounts of precipitation.

Flooding is the most immediate health risk associated with heavy precipitation. Of all causes of storm-related deaths, flooding kills the most people. An average of 100 people die in floods annually in the United States.¹⁴ Worldwide, an estimated 25,000 people die in floods annually. Risks of increased flooding from climate change place tremendous numbers of people in potential jeopardy, as 1 billion people are estimated to live in the flood-prone areas. This number is expected to double by 2050.¹⁵

♦ Infectious Disease
Climate change has an undeniable impact on infectious disease. Shifts in the world’s temperature zones will likely affect populations of a range of insect vectors and hosts of vector-borne and zoonotic diseases. Seasonal weather change has been associated with outbreaks of neisserial meningitis in the meningococcal belt of sub-Saharan Africa, Rift Valley Fever in East Africa, melioidosis in Australia, and leptospirosis during the rainy season in the tropics.^16-19 The greatest impact of climate change on human infectious disease is likely to occur where diseases such as malaria, dengue, Rift Valley Fever, and trypanosomiasis are endemic and where an underfunded and limited public health infrastructure is unable to control insect vector populations. The influence of shifts in local weather patterns on outbreaks of water-borne diseases such as cholera, cryptosporidiosis, and giardiasis should be considered as well.

Vector-borne disease may spread through 2 mechanisms. Epidemic spread may occur after introduction of a pathogen into a new locale inhabited by susceptible hosts and vectors that are capable of transmission of the pathogen. Malaria in the United States provides an illustration. In the late 19th and early 20th centuries, after arriving in North America with European settlers and African slaves, malaria became endemic in the United States, and in Minnesota. In 1934, 125,556 cases were reported in the United States.²⁰ Since 1992, local transmission of malaria in the United States has been documented in 10 outbreaks involving 17 cases.²¹ The most recent outbreak was of vivax malaria in northern Virginia in 2002.²¹ Mosquito vectors able to transmit malaria, yellow fever, dengue fever, and a variety of arboviruses already exist in the United States, and one does not need to invoke global climate change for the presence of appropriate conditions for possible sustained transmission of these diseases.²²

The second mechanism of vector-borne disease spread is expansion of the range of the vector into areas that were previously inhospitable because of temperature or humidity. The range for mosquitoes, for example, is largely constrained by temperature. Theoretically, with warming temperatures, mosquitoes may be able to live in higher elevations or higher latitudes. Data have been conflicting, but recent work in the east African highlands suggests climate warming has led to an increased incidence of malaria.^23,24 Further epidemiologic studies need to address this issue.

Warmer temperatures and increased humidity may also increase tick populations. In Minnesota, the deer tick, Ixodes scapularis, is the vector for 3 diseases: Lyme disease, anaplasmosis, and babesiosis. These diseases have been emerging over the past 20 years for a variety of reasons. Climate change models predict that milder winters tend to increase deer, field mouse, and tick populations, and this could further increase the incidence of tick-borne diseases in humans. In recent years, the Minnesota Department of Health has reported a record number of cases of tick-borne disease.²⁵ In Sweden, a substantial increase in tick-borne encephalitis has been linked to climate changes.²⁶

Pathogens can be transmitted by runoff from heavy rainfall, which can contaminate surface water and groundwater reserves and also deliver large volumes of sediment capable of compromising municipal water plants.²⁷ In 1 study, 68% of 548 outbreaks of waterborne illness in the United States from 1948 through 1994 were preceded by precipitation events rated above the 80th percentile.²⁷ Excessive precipitation triggered an epidemic of giardiasis in Montana in 1980.²⁸ The largest outbreak ever documented of gastrointestinal disease, the cryptosporidiosis crisis in Milwaukee in 1993, followed a period of heavy rainfall.²⁹ Increased waterborne disease outbreaks are likely if scientists are correct in predicting more extreme precipitation events.

Climate change is also expected to influence the appearance and incidence of emerging infectious diseases. Three-quarters of newly described emerging infectious diseases are zoonotic in origin.³⁰ Rodent-borne diseases are 1 group of zoonoses that may increase in incidence with climate change. Rodents have a very high reproductive capacity, and their populations generally benefit from ecological disruption.²²

Hantavirus pulmonary syndrome, caused by the Sin Nombre virus carried by rodents is an example. The first recognized outbreak of Hantavirus pulmonary syndrome was described in the Four Corners region of the southwestern United States after intense precipitation.³¹ The emergence of this disease has been attributed to a pattern of drought followed by intense rains.³² Drought lasting from 1987 to 1992 reduced predator populations. Intense rains in 1993 led to a bumper crop of piñon nuts and a population explosion of grasshoppers, prime food sources of the native rodents. A rebound in the rodent host population increased the likelihood of human exposure to rodents. When a cluster of human cases of Hantavirus disease was recognized by health care workers, the previously unknown virus was identified.

♦ Human Migration
Many discussions about the potential health effects of global climate change overlook an important outcome of the extreme weather patterns predicted by the models: local and global human migration. Families and whole communities may be displaced as catastrophic storms, worsening weather patterns, or oceanic inundation result in the desertification of cropland, salination of drinking water, flooding of homes, and destruction of low-lying coastal areas. Many of these environmental refugees may immigrate to the United States.²² An estimated 1 million people fled the ravages of Hurricane Katrina, the largest number of Americans to be displaced in at least 150 years.³³

Climate change models suggest a 1 m rise in sea level may occur by the end of this century. Such a change would have a devastating effect on millions of people. In Bangladesh alone, about 17 million people live less than 1 m above sea level. Some residents of the Polynesian country, Tuvalu, which has atolls that peak just above the waterline, have already decided to immigrate to New Zealand because of the threat of rising waters. Similarly, the Alaskan village of Shishmaref elected to move their entire community inland because of the receding coastline.

The magnitude and pace of global warming over the next century will dictate the extent of these migrations and the severity of the resulting social upheaval. Conflict, malnutrition, famine, and disease may accompany such social disruption. Infectious diseases including typhus, typhoid, melioidosis, tuberculosis, malaria, HIV infection, and parasitic diseases may present immediately following immigration or potentially remain quiescent for years.

What You Can Do
Global climate change is a thorny and complex problem. Scientists who study the atmosphere and climate agree that average global temperatures are increasing and that the primary cause is burning of fossil fuels. Warming trends are expected to persist for decades to centuries even after we begin to reduce greenhouse gas emissions. Because of this, the environmental and health impacts of climate change are to be expected long after interventions are implemented.

The extent and pace of future climate change will largely be determined by decisions made today about burning fossil fuels worldwide. Health care practitioners may wish to join the call to action from professionals in other fields, who are urging individuals, businesses, and elected leaders to move quickly to slow and then sharply reduce our emissions of greenhouse gases. Most climate scientists now warn that there is a very short window of time to begin serious emissions reductions if we are to avoid dangerous climate impacts.⁶ Delay means it will be much more difficult and expensive to tackle this problem. MM

Jonathan Sellman is an infectious disease consultant with HealthPartners Medical Group and a clinical scholar assistant professor in the division of infectious diseases at the University of Minnesota. J. Drake Hamilton is science policy director at Fresh Energy in St. Paul.

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