The Health Effects of Global Warming

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3. The Health Effects of Global Warming
Many researchers, environmentalists, and politicians forecast that
rising world temperatures in the next century will have devastating
effects on human health (NRC 1991; Mitchell 1991; Cline 1992; Gore
1992; IPCC 1992). Referring to the world as a whole, Working Group
II of the Intergovernmental Panel on Climate Change (1995b, SPM-
10) asserted: ‘‘Climate change is likely to have wide-ranging and
mostly adverse impacts on human health, with signi?cant loss of
life.’’ The authors of the IPCC report feared that increases in heat
waves would cause a rise in deaths from cardio-respiratory compli-
cations. They also foresaw a rise in vector-borne diseases, such as
malaria and dengue and yellow fevers. The report did acknowledge
brie?y that, in colder regions, there would be fewer cold-related
Most of the causes of premature death have nothing to do with
climate. Worldwide the leading causes are chronic diseases—
accounting for 24 million deaths in 1996—such as maladies of the
circulatory system, cancers, mental disorders, chronic respiratory
conditions, and musculoskeletal disorders, none of which has any-
thing to do with climate but everything to do with aging (World
Health Report
1997, vol. 2, no. 1.). Another 17 million, most of them
in poor countries, succumbed in the same year to disorders caused
by infections or parasites, such as diarrhea, tuberculosis, measles,
and malaria. Many of those diseases are unrelated to climate; most
have to do with poverty.
Diarrheal diseases, such as cholera and dysentery, killed 2.5 mil-
lion of the 52 million people who died worldwide in 1996. Through
the provision of fresh water and proper sanitation, those diseases
are easily preventable. Although a warmer climate might make the
environment more hospitable for such af?ictions as cholera, dysen-
tery, and typhoid in areas without good sanitation or clean water,
chlorination and ?ltration could halt their spread.

Both the scienti?c community and the medical establishment
assert that the frightful forecasts of an upsurge in disease and early
mortality stemming from climate change are unfounded, exagger-
ated, or misleading and do not require action to reduce greenhouse
gas emissions. Science magazine reported that ‘‘predictions that
global warming will spark epidemics have little basis, say infectious-
disease specialists, who argue that public health measures will inevi-
tably outweigh effects of climate’’ (Taubes 1997). It added: ‘‘Many
of the researchers behind the dire predictions concede that the sce-
narios are speculative.’’ The American Council on Science and
Health has recommended that spending to reduce greenhouse gas
emissions will make societies poorer and that any additional outlays
should go instead to such public health measures in developing
countries as improving drinking water and sanitation, vector control,
medical infrastructures, and systems of emergency response to
extreme weather events (Shindell and Raso 1997).
This chapter examines the effect of climate and, in particular,
temperatures on mortality in the United States. Anecdotal evidence
suggests that warmer temperatures may actually promote health.
Folklore alleges that physicians sometimes recommend that patients
escape to a more clement climate, never to a colder one.
The few studies that have examined in depth the relation between
warming and human health or mortality have focused either on
increases in the number of days of very hot weather, which can
increase mortality, or on the spread of infectious diseases by such
vectors as mosquitoes, ?ies, and snails (Smith and Tirpak 1989;
Kalkstein 1991; Stone 1995). Nevertheless, several major studies of
the implications of global warming for the United States have
neglected or claimed a lack of data on its effects on health or human
welfare (NRC 1978; Nordhaus 1991; Cline 1992).
Other studies of the in?uence of climate change on human health
have examined a rather narrow set of potential medical problems.
The underlying research has generally referred to Lyme disease,
malaria, dengue and yellow fevers, and encephalitis, none of which
is a major health problem in the United States. The IPCC (1995b, p.
SPM-10) has emphasized that the ‘‘geographical zone of potential
malaria transmission in response to world temperature increases at
the upper part of the IPCC-projected range (5° to 9°F by 2100) would
increase from approximately 45 percent of the world population to
approximately 60 percent by the latter half of the next century.’’

The Health Effects of Global Warming
Concern about tropical and insect-spread diseases seems over-
blown. Inhabitants of Singapore, which lies almost on the equator,
and of Hong Kong and Hawaii, which are also in the tropics, enjoy
life spans as long as or longer than those of people living in Western
Europe, Japan, and North America. Both Singapore and Hong Kong
are free of malaria, but that mosquito-spread disease ravages nearby
regions. Modern sanitation in advanced countries prevents the
spread of many scourges found in hot climates. Such low-technology
and relatively cheap devices as window screens can slow the spread
of insect vectors. The World Health Organization (1990, 21) notes:
until recent times, endemic malaria was widespread in
Europe and parts of North America and . . . yellow fever
occasionally caused epidemics in Portugal, Spain and the
USA. Stringent control measures . . . and certain changes
in life-style following economic progress, have led to the
eradication of malaria and yellow fever in these areas.
Under the stimulus of a warmer climate, insect-spread diseases
might or might not increase. Many of the hosts or the insects them-
selves ?ourish within a relatively small temperature or climatic
range. Plague, for example, spreads when the temperature is
between 66° and 79°F with relatively high humidity but decreases
during periods of high rainfall (White and Hertz-Picciotto 1995, 7-
7-3). Higher temperatures and more rainfall are conducive to an
increase in encephalitis. Malaria-bearing mosquitoes ?ourish under
humid conditions with temperatures above 61° and below 95°F.
Relative humidity below 25 percent causes either death or dormancy.
Parasitic diseases, such as AIDS, Lyme disease, yellow fever,
malaria, and cholera, can usually be controlled through technology,
good sanitary practices, and education of the public. Even without
warming, it is certainly possible that dengue fever or malaria could
invade North America. Unfortunately, some of the government’s
well-meaning environmental policies may make the vector more
likely. The preservation of wetlands, although useful in conserving
species diversity, also provides prime breeding grounds for mosqui-
toes that can carry the diseases. If the United States does in the
future suffer from such insect-borne scourges, the infestation may
have less to do with global warming than with the preservation of
swampy areas.

Torrid Summers
Recent summers have sizzled. Newspapers have reported the
tragic deaths of the poor and the aged on days when the mercury
reached torrid levels. Prophets of doom forecast that rising tempera-
tures in the next century portend a future of calamitous mortality.
Scenes of men, women, and children collapsing on hot streets haunt
our imaginations.
Happily the evidence refutes that dire scenario. First, however,
let us review the documentation supporting the supposition that
human mortality will rise with rising temperatures. Death rates
during periods of very hot weather have jumped in certain cities,
but above-normal mortality has not been recorded during all hot
spells or in all cities. Moreover, research concerned with ‘‘killer’’
heat waves has generally ignored or downplayed the reduction in
fatalities that warmer winter months would bring.
In a 1991 paper, Laurence Kalkstein, one of the most respected
and careful scholars examining the health effects of climate change,
?nds that deaths are related to the length of the hot spell. He suggests
that it takes an extended heat wave to raise the death rate. In a later
work, he reports that heat spells early in the summer or quick rises
in temperature trigger deaths; in other words, unseasonal or rapid
warming produces mortality (Kalkstein 1992). But if rapid warming
causes deaths, we should ?nd that most of the mortality during heat
spells occurs on the ?rst day or so and that fatalities then taper off,
rather than increase with the length of the warm spell. As indicated,
Kalkstein ?nds the opposite: deaths go up after a long spell of
hot weather.
Kalkstein also ?nds that a particular weather pattern—character-
ized by high temperatures, strong southeast winds, moderate
humidity, and relatively clear skies with little cloud cover—is corre-
lated with increased mortality in St. Louis. For other cities either no
weather pattern was related to mortality or the patterns that corre-
lated with extra deaths differed. Even in St. Louis, many of the days
that exhibited the suspect weather showed no unusual number of
fatalities. Moreover, very hot days, those with temperatures over
100°F, failed to show death rates higher than the rates on those days
when the thermometer made it only to 95°F. In fact, the number of
recorded deaths in St. Louis during that particular weather pattern
varied considerably more than during other periods, which reduces
our con?dence in the results.

The Health Effects of Global Warming
Researchers analyzing hot days and deaths have found no constant
relationship; even when extremes in weather and mortality are corre-
lated, the relationship is inconsistent. Cities with the highest average
number of summer deaths are found in the Midwest or Northeast
while those with the lowest number are in the South (Kalkstein and
Davis 1989, 56). Typically analysts have failed to ?nd any relation-
ship between excess mortality and temperature in southern cities,
which experience the most heat (Kalkstein 1992, 372). Other studies
have found that people who move from a cold to a subtropical
climate adjust within a very short period (Rotton 1983). Moreover,
Kalkstein and others have reported without explanation that the
‘‘threshold’’ between temperatures that lead to excess deaths and
those that have no effect varies signi?cantly among the cities. In
some, such as Los Angeles, San Francisco, Boston, and Pittsburgh,
the threshold was below 85° while in Phoenix and Las Vegas, it
exceeded 110°F.
Scholars have also reported contradictory and implausible results.
According to several analyses, air pollution is not correlated with
premature deaths (Kalkstein 1991). Some studies have found that
during hot spells mortality goes up sharply in females; other
researchers have measured increased deaths among men (Kalkstein
1992 using Applegate et al. 1981, Bridger et al. 1976, Ellis 1972).
Blacks are apparently more susceptible in St. Louis; whites, in New
York. The lack of agreement on the effects of weather and on prema-
ture deaths again raises suspicions about the robustness of the
Measurement error may also foul up daily ?gures. In 1995, for
example, Chicago suffered through an extraordinarily hot July that
the press characterized as a harbinger of global warming. The coro-
ner reported a marked increase in deaths. What was very curious
was that on Friday, Saturday, and Sunday, July 14, 15, and 16, the
reported deaths were way below the normal of 78 per day—only
14 people were reported to have died on Saturday—but on the two
following days, Monday and Tuesday, fatalities were well above
normal. The previous record low body count for any day in the
last 30 years had been 46! Given that on Friday, July 14, a record
temperature of 106°F was measured at Midway Airport, those num-
bers are not only remarkable but suspicious. Could it have been that
most people in the coroner’s of?ce took the hot weekend off and
counted bodies on Monday and Tuesday?

Researchers have attributed the absence of heat-related deaths in
southern cities to acclimatization and the prevalence of housing that
shields residents from high temperatures. In the North, the housing
of the elderly and the poor is usually old and dilapidated. Over the
next hundred years, if not sooner, most of those buildings will be
torn down and replaced. Should the climate warm, builders will
move toward structures that protect the inhabitants from extreme
heat, as housing in the South allegedly does now.
These ?ndings may imply simply that out-of-the-ordinary high
temperatures increase the mortality of those in a weakened state.
Studies have found that those most likely to die during heat spells
are elderly (Kalkstein and Davis 1989, 62; Kalkstein 1992). Little
attention has focused on the question of whether excess deaths repre-
sented only premature mortality of a few days among the old or sick
or whether the excess deaths shortened lives signi?cantly. Studies
examining excess deaths by months fail to ?nd any positive correla-
tion with high temperatures, indicating that any daily excess is offset
by a reduction in fatalities over the next few days. In the South,
where temperatures are routinely very high during the summer,
even the elderly adjust. Consequently, if the climate becomes war-
mer, no excess deaths can be expected.
Fear of killer heat waves appears exaggerated. If temperatures
rise slowly over the next century, possibly by the 2° to 6°F currently
predicted, people will become acclimated while housing can and,
in the normal cycle, will be replaced. After all, half the housing
stock in the United States has been built during the last 25 years.
Consequently, if warming takes place, people and housing will
adapt; even if extended warm spells occur, mortality should not rise
sharply. Moreover, the models and the evidence to date suggest that
most of the warming will take place in the winter and at night.
Consequently extreme heat events are unlikely to become much
more common.
Heat-stress does increase mortality; but it typically affects only
the old and in?rm, whose lives may be shortened by a few days or
perhaps a week. There is no evidence, however, that general mortal-
ity rises signi?cantly. The numbers of heat-stress-related deaths are
very small; in the United States they are exceeded by the number
of deaths due to weather-related cold. During the latest 10-year

The Health Effects of Global Warming
Figure 3-1
6.00 –
5.00 –
4.00 –
3.00 – 2.5
2.00 –
1.00 –
0.00 – –

SOURCE: Vital Statistics of the United States (1983–1992).
period for which we have data (?gure 3-1), which includes the very
hot summer of 1988, the average number of weather-connected heat
deaths was 132, compared with 385 for those who died from cold.
Even during 1988, more than double the number of Americans died
from the cold than passed on from the heat of summer. A somewhat
warmer climate would clearly reduce more deaths in the winter
than it would add in the summer.
Mosquito-Borne Diseases
A growing chorus has been chanting that global climate change
will spread insect-borne diseases, such as malaria, dengue fever,
and yellow fever, to temperate latitudes. In 1996, the health effects
of global warming have been the subject of lengthy journal articles
in the Journal of the American Medical Association (1996), and Lancet
(1996), an international journal of medical science and practice. In
September 1996, the Australian Medical Association sponsored a
major conference on the subject. Professor Paul Epstein of the Har-
vard School of Public Health claimed that in the past few years
mosquitoes carrying malaria and dengue fever had been found at
higher altitudes in Africa, Asia, and Latin America. In North
America, David Danzig (1995), in a Sierra Club publication, has

Figure 3-2
2,500 –
2,000 –
1,500 –
1,000 –
500 –
0 – –

SOURCE: Centers for Disease Control and the Statistical Abstract of the United
contended that only the tip of Florida is currently warm enough to
support malaria-carrying mosquitoes but that global warming could
make most of us vulnerable. He should check his history.
Before the Second World War, malaria was widespread in the
United States. The Centers for Disease Control and the Statistical
Abstract of the United States
for the relevant years reported that over
120,000 cases were reported in 1934; as late as 1940, the number of
new sufferers totaled 78,000. After the war, reported malaria cases
in the United States plunged from 63,000 in 1945 to a little over
2,000 in 1950 to only 522 in 1955. By 1960, DDT had almost eliminated
the disease; only 72 cases were recorded in the whole country. In
1969 and 1970, the CDC reported a resurgence to around 3,000 cases
annually, brought in by service personnel returning from Vietnam.
Subsequently, immigrants from tropical areas have spawned small
upticks in new cases.
In the 1980s and 1990s, as Figure 3-2 shows, the number of reported
cases has averaged around 1,200 to 1,300 annually. The CDC reports
that since 1985 approximately 1,000 of those cases have been
imported every year, with visitors and recent immigrants accounting
for about half. The rest come from travelers arriving from tropical
countries, service personnel returning from infested areas, and a
handful of individuals, typically those living near international air-
ports, bitten by a mosquito that hitched a ride from a poor country.

The Health Effects of Global Warming
More stringent efforts to keep out the unwanted ‘‘immigrants’’ may
be called for if the problem worsens.
Yellow and dengue fevers were both common in the United States
from the 17th century onward. Epidemics of yellow fever ravaged
New Yorkers and killed tens of thousands of people. In one year,
1878, of 100,000 cases reported along the East Coast, 20,000 people
died. Between 1827 and 1946, eight major pandemics of dengue
fever overran the United States. In 1922, the disease spread from
Texas, with half a million cases, through Louisiana, Georgia, and
Florida. Savannah suffered with 30,000 cases, of which nearly 10,000
had hemorrhagic symptoms, a very serious form of the disease. In
contrast, in 1996 the CDC listed 86 imported cases of dengue and
dengue hemorrhagic fever and eight local transmissions, all in Texas.
There were no reported cases of yellow fever.
As a public health issue, those diseases, which did plague the
United States in the reputedly colder 19th and early 20th centuries,
have been largely exterminated. There is no evidence that a resur-
gence is imminent. Certainly the climate is not keeping the spread
of the diseases in check. If it was warm enough in the cold 19th
century for the mosquitoes to thrive, it is warm enough now!
Is there any basis at all for these scare-mongering prophecies?
Is malaria rising worldwide? Not according to the World Health
Organization. As Figure 3-3 shows, from 1983 to the latest year for
which data exist, 1992, the number of cases of malaria reported in
Africa, the most heavily infested section of the world, has fallen
sharply, especially in the most recent years. For the rest of the world,
reports are somewhat less encouraging. Malaria continues to be a
problem, but there has been no increase in cases reported even
though the world’s population has climbed. The good news is that
the rate of malaria per 100,000 people has fallen for the whole world.
What brought an end to the scourges? The introduction of DDT
clearly played a major role. From the end of World War II until it
was banned in 1972, the pesticide worked wonders to eliminate
harmful insects, espcially mosquitoes. But it was not just insecticides
that did the trick. Simple steps, such as screens on windows, the
elimination of standing water, and the movement to the suburbs,
which reduced population density and thus the risk of transmission,
have played a critical role in eliminating mosquito-borne diseases.
In 1995, however, a dengue pandemic af?icted the Caribbean,
Central America, and Mexico, generating around 74,000 cases. Over

Figure 3-3
25 –
20 –
Total excluding Africa
15 –
10 –
In Millions
5 –
0 –

SOURCE: World Health Organization, Malaria Control Program, Geneva,
4,000 Mexicans living in the Tamaulipas state, which borders Texas,
came down with the disease. Yet Americans living a short distance
away remained unaffected. The contrast between the twin cities of
Reynosa, Mexico, which suffered 2,361 cases, and Hidalgo, Texas,
just across the border, is striking. Including the border towns, Texas
reported only 8 nonimported cases for the whole state.
The only reasonable explanation for the difference between the
spread of dengue in Tamaulipas and its absence in Texas is living
standards. Where people enjoy good sanitation and public educa-
tion, have the knowledge and willingness to manage standing water
around households, implement programs to control mosquitoes,
and employ screens and air-conditioning, mosquito-borne diseases
cannot spread. If the climate does warm, those factors will remain.
In short, Americans need not fear an epidemic of tropical diseases.
A recent manifestation of fear-mongering about the health effects
of global warming is a curious article in Science, taken from a modi-
?ed text of Rita Colwell’s presidential address to the American