And Human Health
By Paul R. Epstein
08 October, 2005
New England Journal of Medicine
1998, Hurricane Mitch dropped six feet of rain on Central America in
three days. In its wake, the incidence of malaria, dengue fever, cholera,
and leptospirosis soared. In 2000, rain and three cyclones inundated
Mozambique for six weeks, and the incidence of malaria rose fivefold.
In 2003, a summer heat wave in Europe killed tens of thousands of people,
wilted crops, set forests ablaze, and melted 10 percent of the Alpine
This summer's blistering
heat wave was unprecedented with regard to intensity, duration, and
geographic extent. More than 200 U.S. cities registered new record high
temperatures. In Phoenix, Arizona, sustained temperatures above 100°F
(38°C) for 39 consecutive days, including a week above 110°F
(43°C), took a harsh toll on the homeless. Then came Hurricane Katrina,
gathering steam from the heated Gulf of Mexico and causing devastation
in coastal communities.
These sorts of extreme
weather events reflect massive and ongoing changes in our climate to
which biologic systems on all continents are reacting. So concluded
the United Nations Intergovernmental Panel on Climate Change,1 a collaboration
of more than 2000 scientists from 100 countries. In 2001, the panel
concluded that humans are playing a major role in causing these changes,
largely through deforestation and the combustion of fossil fuels that
produce heat-trapping gases such as carbon dioxide.
Since 2001, we've
learned substantially more. The pace of atmospheric warming and the
accumulation of carbon dioxide are quickening; polar and alpine ice
is melting at rates not thought possible several years ago2; the deep
ocean is heating up, and circumpolar winds are accelerating; and warming
in the lower atmosphere is retarding the repair of the protective "ozone
shield" in the stratosphere. Moreover, ice cores that are drilled
in Greenland indicate that the climate can change abruptly. Given the
current rate of carbon dioxide buildup and the projected degree of global
warming, we are entering uncharted seas.
As we survey these
seas, we can see some of the health effects that may lie ahead if the
increase in very extreme weather events continues.3 Heat waves like
the one that hit Chicago in 1995, killing some 750 people and hospitalizing
thousands, have become more common.1 Hot, humid nights, which have become
more frequent with global warming, magnify the effects. The 2003 European
heat wave - involving temperatures that were 18°F (10°C) above
the 30-year average, with no relief at night - killed 21,000 to 35,000
people in five countries.
But even more subtle,
gradual climatic changes can damage human health. During the past two
decades, the prevalence of asthma in the United States has quadrupled,
in part because of climate-related factors. For Caribbean islanders,
respiratory irritants come in dust clouds that emanate from Africa's
expanding deserts and are then swept across the Atlantic by trade winds
accelerated by the widening pressure gradients over warming oceans.
Increased levels of plant pollen and soil fungi may also be involved.
When ragweed is grown in conditions with twice the ambient level of
carbon dioxide, the stalks sprout 10 percent taller than controls but
produce 60 percent more pollen. Elevated carbon dioxide levels also
promote the growth and sporulation of some soil fungi, and diesel particles
help to deliver these aeroallergens deep into our alveoli and present
them to immune cells along the way.
The melting of the
earth's ice cover has already become a source of physical trauma. In
Alaska, Inuits report an increase in accidents caused by walking on
thin ice.2 Ocean warming and Arctic thawing are also spawning severe
winter storms and hazardous travel conditions in the continental United
States. Although tropical sea surfaces are warming and becoming saltier,
parts of the North Atlantic are freshening from melting polar ice and
increased amounts of rain falling at high latitudes. Contrasting barometric
pressures over changing oceans increase winds and propel storms.
Meanwhile, in the
past three decades, widening social inequities and changes in biodiversity
- which alter the balance among predators, competitors, and prey that
help keep pests and pathogens in check - have apparently contributed
to the resurgence of infectious diseases. Global warming and wider fluctuations
in weather help to spread these diseases: temperature constrains the
range of microbes and vectors, and weather affects the timing and intensity
of disease outbreaks.4 Disease-bearing ticks in Sweden are moving northward
as winters become warmer, and models project a similar shift in the
United States and Canada. The encroachment of human housing on wilderness
and reductions in the populations of predators of deer and competitors
of mice are largely responsible for the current spread of Lyme disease.
can carry many diseases, are very sensitive to temperature changes.
Warming of their environment - within their viable range - boosts their
rates of reproduction and the number of blood meals they take, prolongs
their breeding season, and shortens the maturation period for the microbes
they disperse. In highland regions, as permafrost thaws and glaciers
retreat, mosquitoes and plant communities are migrating to higher ground.5
The increased weather
variability that accompanies climate instability contributed to the
emergence of both the hantavirus pulmonary syndrome and West Nile virus
in the United States. Six years of drought in the Southwest apparently
reduced the populations of predators, and early heavy rainfall in 1993
produced a bounty of piñon nuts and grasshoppers for rodents
to eat. The resulting legions of white-footed mice heralded the appearance
of hantavirus in the Americas. The origin of the 1999 outbreak of West
Nile virus in New York City remains a mystery, but city-dwelling, bird-biting
Culex pipiens mosquitoes thrive in shallow pools of foul water that
remain in drains during droughts. When dry springs yield to sweltering
summers, viral development accelerates and, with it, the cycle of mosquito-to-bird
transmission. During the hot, arid summer of 2002, West Nile virus traveled
across the country, infecting 230 species of animals, including 138
species of birds, along the way. Many of the affected birds of prey
normally help to rein in rodent populations that can spread hantaviruses,
arenaviruses, and yersinia and leptospira bacteria, as well as ticks
infected with Borrelia burgdorferi.
Extremely wet weather
may bring its own share of ills. Floods are frequently followed by disease
clusters: downpours can drive rodents from burrows, deposit mosquito-breeding
sites, foster fungus growth in houses, and flush pathogens, nutrients,
and chemicals into waterways. Milwaukee's cryptosporidium outbreak,
for instance, accompanied the 1993 floods of the Mississippi River,
and norovirus and toxins spread in Katrina's wake. Major coastal storms
can also trigger harmful algal blooms ("red tides"), which
can be toxic, help to create hypoxic "dead zones" in gulfs
and bays, and harbor pathogens.
for their part, can weaken trees' defenses against infestations and
promote wildfires, which can cause injuries, burns, respiratory illness,
and deaths. Shifting weather patterns are jeopardizing water quality
and quantity in many countries, where groundwater systems are already
being overdrawn and underfed. Most montane ice fields are predicted
to disappear during this century - removing a primary source of water
for humans, livestock, and agriculture in some parts of the world.
A still greater
threat to human health comes from illnesses affecting wildlife, livestock,
crops, forests, and marine organisms. The Millennium Ecosystem Assessment
of 2005 revealed that 60 percent of the resources and life-support systems
examined - from fisheries to fresh water - are already in decline or
are being used in unsustainable ways. The resulting biologic impoverishment
may have important consequences for our air, food, and water.
Crops are being
confronted with more volatile weather, vanishing pollinators, and the
proliferation of pests and pathogens. One fungal disease, soybean rust,
is thought to have been ushered into the United States by Hurricane
Ivan last fall. Warmth and moisture will favor its propagation.
And many habitats
are not faring well. Our coastal zones, for example, are in trouble:
coral reefs are suffering from warming-induced "bleaching,"
excess waste, physical damage, overfishing, and fungal and bacterial
diseases. Reefs provide a buffer against storms and groundwater salinization
and offer protection for fish, the primary protein source for many inhabitants
of island nations. One reef resident, the cone snail, produces a peptide
that is 1000 times as potent as morphine and that is not addictive.
We may never know what other potential treatments will be lost as reefs
All in all, it would
appear that we may be underestimating the breadth of biologic responses
to changes in climate. Treating climate-related ills will require preparation,
and early-warning systems forecasting extreme weather can help to reduce
casualties and curtail the spread of disease. But primary prevention
would require halting the extraction, mining, transport, refining, and
combustion of fossil fuels - a transformation that many experts believe
would have innumerable health and environmental benefits and would help
to stabilize the climate.
The good news is
that we may also be underestimating the economic benefits of the clean-energy
transition. When the financial incentives are adequate, renewable energy,
energy- efficient and hybrid technologies, "green buildings,"
and expanded public transportation systems can constitute an engine
of growth for the 21st century.
Dr. Epstein is the
associate director of the Center for Health and the Global Environment,
Harvard Medical School, Boston.
An interview with
Dr. Epstein can be heard at www.nejm.org.
1. Houghton JT,
Ding Y, Griggs DJ, et al., eds. Climate change 2001: the scientific
basis: contribution of the Working Group I to the third assessment report
of the Intergovernmental Panel on Climate Change. Cambridge, England:
Cambridge University Press, 2001.
2. Hassol SJ. ACIA,
Impacts of a warming Arctic: arctic climate impact assessment. Cambridge,
England: Cambridge University Press, 2004.
3. Leaf A. Potential
health effects of global climatic and environmental changes. N Engl
J Med 1989;321:1577-1583. [ISI] [Medline]
4. McMichael AJ,
Campbell-Lendrum DH, Corvalán CF, et al., eds. Climate change
and human health: risks and responses.
Geneva: World Health Organization, 2003:250.
5. Epstein PR, Diaz
HF, Elias S, et al. Biological and physical signs of climate change:
focus on mosquito-borne diseases. Bull Am Meteorol Soc 1998;78:409-17.