Originally published in the New England
Journal of Medicine, Oct. 6, 2005, Vol 353:1433-1436
(disclaimer: this is a powerful and important
article, and cannot remain locked behind copyright. The full
text with graphics is republished here with credit to the
author, the journal, and for educational purposes only.)
In 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 glacial mass.
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.
Hurricane Katrina Passing over the Gulf of Mexico
The map shows the three-day average
of sea-surface temperatures from August 25, 2005, through
August 27, 2005, and Hurricane Katrina growing in strength
and breadth as it passes over the unusually warm Gulf
of Mexico. Yellow, orange, and red areas at or above
82°F (27.8°C, the temperature required for hurricanes
to strengthen). Since the 1970s, the number of category
4 and 5 hurricanes has increased as sea temperatures
have risen.
From the Scientific Visualization Studio
of the National Aeronautics and Space Administration.
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.
Increase from 1992 (Left) to 2002 (Right) in the
Amount of the Greenland Ice Sheet Melted in the Summer.
The extent of seasonal melting on the
Greenland ice sheet has been observed by satellite since
1979. The melt zone (orange), where summer warmth turns
snow and ice around the edges of the ice sheet into
slush and water, has been expanding inland and to record-high
elevations in recent years. When the meltwater seeps
through cracks in the ice sheet, it may accelerate melting
and allow ice to slide more easily over bedrock, speeding
its movement to the sea. In addition to contributing
to a rising sea level, this process adds freshwater
to the ocean, with potential effects on ocean circulation
and regional climate.
Map by Clifford Grabhorn, from the Arctic
Climate Impact Assessment.
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.
Mosquitoes, which 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.
Prolonged droughts, 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 deteriorate.
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.
Source Information
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.
References
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.
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