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Global Warming

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The effect of greenhouse gases on global warming is, in my opinion, the most important environmental issue facing the world today. Our knowledge of the underlying causes of climate change is growing, but the problem brims with uncertainties, raising serious scientific and ethical questions.
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South Korea
e Movement
Satellite image of brown haze flowing across the East China Sea past the Korean Peninsula and northeastward toward Japan. Image courtesy
of the SeaWiFS Project, NASA/Goddard Space Flight Center, and ORBIMAGE.
Global Warming
Veerabhadran Ramanathan
These remarks were given at a meeting of the American Academy, held at the University of California, San Diego on November 21, 2005.
Veerabhadran Ramanathan is Victor C. Alderson
study of global change. He demonstrated
Why should we worry about this blanket of
Professor of Ocean Sciences and Director of the
that the amount of carbon dioxide in the
added co2 and other greenhouse gases?
Center for Atmospheric Sciences at the University
atmosphere increased by about 20 percent
The fundamental energy source for the planet
of California, San Diego. He has been a Fellow of
since the time he began his work in the mid-
is sunlight. However, not all of the solar en-
the American Academy since 1995.
1950s. If you take one million molecules of
ergy is absorbed: about 30 percent is reflect-
air, approximately 375 will be carbon dioxide.
ed back to space by clouds, the atmosphere,
Compare this with the fact that in the last
and land and sea surface, including sea ice,
The effect of greenhouse gases on global
four hundred thousand years, the amount of
ice sheets, and the like. The planet is warmed
warming is, in my opinion, the most impor-
co2 concentration has never been larger
by the remaining 70 percent and, in turn, re-
tant environmental issue facing the world
than 290 parts per million.
radiates the heat as infrared energy (also
today. Our knowledge of the underlying
known as thermal energy or heat radiation).
The rapidity of the increase leaves little doubt
causes of climate change is growing, but the
Over the long term, climate is governed by
that human impact is the cause. What lies
problem brims with uncertainties, raising
the balance between the incoming solar
behind such a signi½cant increase in a rela-
serious scienti½c and ethical questions.
heating and the cooling associated with the
tively short time? The lifetime of carbon di-
outgoing infrared energy. The added co2
In studies of global temperatures, one ques-
oxide is over a century. If today you release a
upsets this balance by absorbing and re-
tion always arises at the outset: When did
can of co2, roughly 25–35 percent of it will
emitting the infrared energy. In this process,
humans become a major force in modifying
still be with us a hundred years from now.
the blanket of co2 acts just like a wool
the climate system? My own timetable begins
What happens to this co
blanket on a cold night by trapping the
2? The air carries it
in the 1950s, when the world’s population in-
around until it covers the entire planet like a
outgoing infrared heat within the surface-
creased by over 60 percent, resulting in a per-
blanket of gases. In fact, no matter where we
atmosphere system and causing the planet
ceptible impact on many indices of change.
measure–the Arctic, the Antarctic, the sur-
to become warmer.
With high-precision observations, my late
face, or 20 miles above the surface–we detect
colleague, Dave Keeling, produced the single
The fact that added co
the increase in
2 can lead to a large
co2. Fossil-fuel combustion
most important times-series data set for the
global warming was estimated more than
and biomass burning are the major sources
for this increase.
36 Bulletin of the American Academy Spring 2006

periment (with P. J. Crutzen), which de-
high-precision radiometers to discover that
The effect of greenhouse
ployed six aircraft and two ships with several
black carbon and other absorbing particles
tens of instruments, I had the opportunity to
in the brown haze over the Indian Ocean and
gases on global warming
observe from the C-130 aircraft the brown
the Arabian Sea reduced sunlight by as much
is, in my opinion, the most
clouds spreading from South Asia and
as 10–15 percent. The sunlight-reduction
blanketing most of the North Indian Ocean.
effect at the surface was larger by a factor of
important environmental
We then used satellite data to show that the
two or more than estimated by climate
South Asian brown haze occurs every year
models. In terms of the ocean surface, black
issue facing the world
generally between November and May. It
carbon in the brown haze reduces the average
consists of a 3 km mixture of anthropogenic
radiative heating by as much as 10 percent
(human-produced) sulfates, nitrates, organics,
and enhances atmospheric solar radiative
black carbon, dust and fly ash particles, and
heating by as much as 50–100 percent.
110 years ago by the Swedish Nobelist
Svante Arrhenius. In the mid-1970s a series
Composition of a brown cloud
of complications began to unfold, leading to
the realization that carbon dioxide was not
the only cause of global warming. Mario
Molina and Sherwood Rowland’s research
Black Carbon
on the impact of chlorofluorocarbons
(cfcs) on the ozone hole led to my inves-
tigations on the potential greenhouse effect
of cfcs. Our research revealed the unex-
(includes all other
pected result that adding one molecule of
ions in the br
cfc to the atmosphere would have the
same greenhouse effect as adding more
than ten thousand molecules of co2. The
fact that cfcs, which are relatively rare in
the atmosphere, could be such a powerful
Sea-salt &
ate ion)
force in global warming was initially met
with disbelief. But as other researchers
NH +
4 (Ammonium ion)
reproduced our ½ndings, a Pandora’s box
of greenhouse gases, including methane,
ozone, halocarbons used as refrigerants
+ (Potassium ion)
and propellants, and nitrous oxide from
SO (Sulf
ate ion)
fertilizer, began to open.
We could no longer ignore the effect of
greenhouse gases and our sense that global
natural aerosols such as sea salt and mineral
Aerosols also produce more cloud drops,
warming would occur much earlier than we
dust. Measurements from aircraft, ships,
which increase the reflection of solar radi-
might have imagined. My work with clima-
and surface stations involved in the experi-
ation by clouds, adding to the surface-cooling
tologist Roland Madden some twenty-½ve
ment found that biomass burning and fossil-
effect; and decrease the size of cloud drops
years ago revealed that the impact of global
fuel combustion contribute as much as 75
and suppress precipitation.
warming would become discernible by the
percent to the observed aerosol.
The link between aerosols and precipitation
year 2000. Meeting in 2001, the Intergov-
represents an added complication. As emis-
ernmental Panel of Climate Change, con-
As emissions from fossil
sions from fossil fuel and black carbon have
sisting of a group of over a thousand sci-
increased, monsoonal rainfall and surface
fuel and black carbon have
entists, con½rmed our prediction.
sunlight have decreased. Modeling research
Coincident with the greenhouse gas warm-
conducted by us with the climate-system
increased, monsoonal
ing is the appearance of atmospheric brown
model developed by the National Center for
clouds. If greenhouse gases are the ultimate
Atmospheric Research in Boulder, Colorado,
rainfall and surface sun-
end product of fossil fuel and
indicates that three factors may contribute
co2, then
light have decreased.
particulates in the air represent an interme-
to the drying effect. First, sunlight provides
diate phase. A brown haze is generally asso-
the energy to evaporate water from the ocean,
ciated with urban areas, but in 1999, the
Black carbon is probably the most insidious
causing rainfall. With reduced sunlight the
Indian Ocean Experiment (
component of the haze as far as health is
evaporation from the ocean decreases and,
indoex), in-
volving more than two hundred scientists,
concerned; it is also the most important fac-
as a result, the rainfall decreases. Second, as
focused on a brown cloud that spans an entire
tor in terms of climate change. During the
solar absorption by black carbon heats the
continent and ocean. As coleader of this ex-
indoex campaign we deployed a suite of
atmosphere, it cools the surface by shielding
it from the sun. This redistribution of sunlight
Bulletin of the American Academy Spring 2006 37

causes warmer air to overlie the colder sur-
by the increase in greenhouse gases from the
the regional level, affecting most severely
faces, suppressing rainfall. Finally, the brown
late nineteenth century to the present time
the poorest people on Earth. International
haze appears to slow summer monsoonal
has committed the planet to a global warming
cooperation among scientists and among
circulation, leading to a reduction in precipi-
in the range of 1°C to 3°C. We have realized
nations will be essential if we are to effec-
tation over South Asia. These ½ndings are
only a fraction (25–50 percent) of this warm-
tively address the formidable political, social,
important reasons for reducing air pollution.
ing. Some of this warming has been masked
and ethical consequences of global warming.
by the dimming due to brown clouds, and
Among the many obvious reasons for the
the remaining heat is stored in the depths of
urgent need for international collaboration
We continue to face a
the ocean to be released later. Through the
is the fact that the developed nations are the
process of convective overturning, oceans
major contributors to the greenhouse gases
number of uncertainties
transfer infrared energy to their deepest lay-
while the developing nations are the major
ers and hold the heat, delaying the impact of
sources for particles in the brown clouds.
in our efforts to predict
global warming. Whether this stored heat
will warm the atmosphere in a few decades
climate change.
© 2005 by Veerabhadran Ramanathan
or a few centuries is unknown. The delay of
the warming by decades to centuries by the
Lest you think that air pollution is a con½ned
flywheel effect of ocean mixing, when com-
problem, I want to emphasize that long-
bined with the century or more lifetime of
range wind transport means that pollution
on the East Coast of the United States is go-
2 (and molecules of other greenhouse
gases) in the atmosphere, presents policy-
ing across the Atlantic to Europe, European
makers with the central moral dilemma of
pollution is traveling to Asia, and Asian pol-
the global-warming problem. Every decade
lution is coming back to America. To better
we delay in taking action, we are commit-
understand how atmospheric brown clouds
ting the planet to additional warming that
impact the environment, climate, and the
future generations have to deal with.
quality of life, we have launched a project to
integrate scienti½c ½ndings with impact
assessment. With support from the United
Every decade we delay in
Nations Environment Program and the Na-
tional Oceanic and Atmospheric Adminis-
taking action, we are
tration, we are working with scientists from
Sweden, Germany, China, India, Japan, and
committing the planet to
other Asian countries to develop a system of
additional warming that
strategically located ground-based observa-
tories in the Indo-Asian and Paci½c regions
future generations have to
to monitor atmospheric pollution. Together
with satellite data and periodic aircraft mea-
deal with.
surement, these observatories will document
changes in aerosol content, pollutant gases,
and some greenhouse gases, enhancing the
The masking effect is equally troubling. We
predictive capacities of climate models. We
now know that the surface-cooling effect of
hope that in time this work will revolutionize
aerosols may have masked as much as 50
how we look at the atmosphere.
percent of the global warming caused by
greenhouse gases, presenting a serious di-
However, we continue to face a number of
lemma for the global community: If we at-
uncertainties in our efforts to predict climate
tempt to reduce air pollution because of its
change. First, by the beginning of the next
effect on health, we may see an ampli½ca-
century, the global population will reach
tion of global warming. At the same time, if
about nine billion, and people in the devel-
greenhouse gases are curbed because of our
oping world will be striving to achieve West-
concerns about global warming, the brown
ern standards of living. Their efforts will result
clouds may weaken the Earth’s water cycle,
in enormous amounts of atmospheric pol-
particularly the monsoonal rainfall in Asia,
lutants and other stresses on the environ-
leaving us with conflicting options involving
those regions negatively impacted by global
Second, there are the unsolved questions
warming and those negatively impacted by
surrounding the rate and masking of global
air pollution.
warming. The extent of global warming is
Although we talk about global averages in
not fully reflected in the Earth’s surface
discussions of global warming, we cannot
temperatures. The additional heat trapped
forget the marked changes that also occur at
38 Bulletin of the American Academy Spring 2006

Global Warming



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