the most controversial scientific issues is determining the causes
of the gradual warming of Earths atmosphere over the past
century, especially the last 50 years. Lawrence Livermore scientists
have been among the leaders in modeling global climate change to
better understand the nature of the warming, to predict the probable
climate in the coming decades, and to determine the role of anthropogenic
(human) activity in climate change.
recently, the most important factor in global climate change appeared
to be the steady accumulation of greenhouse gases, mainly produced
by the burning of fossil fuels in cars, factories, and power plants.
These greenhouse gases, such as carbon dioxide and methane, are
known to trap sunlight and thereby warm the atmosphere.
observations of global temperature records over the past 50 years
have shown less global warming than predicted by computer models
that include only accumulations of greenhouse gases. The explanation
for this apparent discrepancy is that increasing concentrations
of anthropogenic aerosols in the atmosphere may be cooling the planet
and so partially counteracting the effects from the greenhouse gases.
past 10 to 15 years, scientists have also begun to consider how
aerosols, microscopic particles directly suspended in the atmosphere
or trapped in clouds, may be changing the planets climate.
Beginning in the early 1990s, calculations showed that aerosols
composed of sulfates (a form of sulfuric acid and a main component
of air pollution) could be cooling the atmosphere by backscattering
incoming solar radiation. The process works in much the same manner
as volcanic eruptions, which spew many tons of sulfates into the
higher atmosphere that eventually result in the cooling of Earths
climate. (See the box below.)
A Short Primer
Aerosols are concentrations of exceedingly minute particles
suspended in the atmosphere. Aerosol particles range
in size from 0.01 micrometer (millionth of a meter)
to several tens of micrometers in diameter. Particles
generated by pollution tend to be less than a millimeter
enter the atmosphere from many different natural and
anthropogenic (human activityrelated) sources.
For example, nature generates sulfate aerosols from
volcanoes, salt aerosols from sea spray, dust aerosols
from desert areas, and carbonaceous aerosols formed
from volatile organic compounds emitted by plants.
A growing fraction
of aerosols are byproducts of human activities, as seen
in the ubiquitous hazes that persist in the industrialized
regions of the world. Anthropogenic aerosols include
sulfuric acid, soot and smoke from the burning of fossil
fuels in factories, vehicles, power plants, cookstoves,
and fireplaces. The burning of forests and grasslands
to clear them for farming is another source of carbonaceous
aerosols. (Although dust is typically considered a natural
source of aerosols, human activities such as farming
or erosion caused by changing land use also kick large
amounts of dust into the atmosphere.)
a significant effect on climate. Whereas greenhouse
gases trap the Suns heat, thereby warming Earths
atmosphere and surface, aerosols mainly reflect solar
radiation, a phenomenon called the aerosol direct effect.
By reducing the amount of solar energy reaching the
Earths surface, aerosols serve as agents of climate
cool the climate indirectly, by changing the properties
of clouds, which cool Earth by reflecting solar radiation
back to space. (Of the daily average of about 340 watts
per square meter of solar radiation that reaches the
atmosphere, clouds reflect about 45 watts per square
meter.) Although commonly thought of as pristine sources
of water, clouds could not form without aerosol particles
(natural or anthropogenic) acting as cloud condensation
nuclei, which are sites on which water droplets can
Sunlight, Modifying Rainfall
of aerosols in the atmosphere lead to the formation
of clouds with water content spread over many more particles.
Clouds with smaller, more numerous droplets have a larger
surface area and therefore reflect up to 30 percent
more sunlight, a phenomenon called aerosols first
indirect effect. Whats more, the smaller water
droplets in the cloud fall more slowly, thereby prolonging
the lifespan of the cloud and strengthening its cooling
effect. This second indirect effect is believed to be
changing rainfall patterns in populated regions worldwide.
the scientific understanding of aerosols climatic
effects are recent satellite observations revealing
that aerosols of black carbon from biomass and fossil
fuel burning can absorb sunlight in the atmosphere,
thereby increasing the warming
effect opf greenhouse gases. Satellite observations
have also revealed that the absorption of heat by soot
can evaporate cloud droplets and thus reduce the presence
of clouds. This phenomenon, called the aerosol semidirect
effect, is particularly prevalent over heavily polluted
all of the direct and indirect effects of aerosols are
believed to increase Earths albedo (percentage
of sunlight reflected), thereby cooling the surface
and offsetting the warming effects of greenhouse gases
by 25 to 50 percent globally (and even much more in
climatic effects cannot be simply compared to those
of greenhouse gases because they are distributed in
time and space far differently. For example, greenhouse
gases are well mixed in the atmosphere and have a lifetime
of up to 100 years. In contrast, aerosols suspended
in the troposphere (lower atmosphere) last only about
a week before they are removed by winds and rain. (The
exception is the injection of sulfates into the stratosphere,
or upper atmosphere, where they can remain for a few
years. The global cooling observed following large volcanic
eruptions, such as that of Mount Pinatubo in the Philippines
in 1991, provides dramatic evidence for the climatic
influence of aerosols.)
Also, many anthropogenic
aerosols are localized and occur near or downwind from
their sources, such as power plants, factories, and
large urban populations. As a result, most aerosols
are found in the Northern Hemisphere, where most industrialized
nations are located.
Data, Models Aid Understanding
study aerosol distribution and composition requires
continuous observations from instruments located on
satellites and aircraft as well as ground-based field
stations. Data from these instruments, combined with
numerical models that mimic the formation of aerosols
and their interactions with clouds, have led to a much
greater understanding of how and to what degree aerosols
influence climate. Lawrence Livermore scientists have
been among the leaders in developing these models.
marked a turning point in January 2002, when more than
50 leading American atmospheric scientists (including
Livermores Catherine Chuang), together with representatives
from federal agencies, met to explore ways to achieve
breakthroughs in understanding and modeling aerosols
role in climate change. The meeting led to the formation
of a national Aerosol Climate Interactions Program supported
by several federal agencies. The programs goals
are to more accurately measure the sources, distribution,
and properties of aerosols and their influence on climate;
to more completely model the processes that govern aerosols
distributions and climatic effects; and to better quantify
the relative importance of aerosols and greenhouse gases
in global warming, including the effects on regional
could not form without aerosol particles (natural or anthropogenic)
acting as cloud condensation nuclei or sites on which
water droplets can condense. Anthropogenic emissions increase
aerosol concentrations and result in clouds with smaller
and more numerous droplets. These clouds have a larger
albedo (percentage of reflected sunlight) and a longer
lifetime, and thus they reflect more sunlight back into
Spotlight on Aerosols
In the past few years, intriguing data from ground
stations and satellites, together with insight gained from computer
models, have made aerosols a major focus of atmospheric research.
Ten years ago, the focus was on greenhouse gases. Now aerosols
are getting the attention, says Livermore atmospheric scientist
Chuang notes, however, that
large variations in aerosol concentrations have made it difficult
to confidently assess the magnitude of their effects on climate.
Aerosol chemistry and physics, especially in clouds, are complex
and not completely understood. Particles typically remain aloft
in the troposphere (lower atmosphere) for a week or less, in contrast
to greenhouse gases, which can persist for about a century.
Because they are short-lived,
aerosols do not mix homogeneously around the planets atmosphere,
and so concentrations differ greatly from one region to the next.
Whats more, aerosols come in a wide range of particle sizes,
with particles smaller than a micrometer exerting comparatively
greater climatic effects. As a result, says Chuang, one of the greatest
uncertainties in climatic change is the role played by anthropogenic
aerosols. To reduce these uncertainties, scientists are turning
to sophisticated computer simulations in an attempt to gain insight
into aerosols climatic effects.
During the past few years,
Chuang and colleagues including Joyce Penner (now at the University
of Michigan), Keith Grant, Jane Dignon, Peter Connell, Daniel Bergman,
and Douglas Rotman have used Livermores TeraCluster2000 multiparallel
supercomputer and the resources of the National Energy Research
Scientific Computing Center at Lawrence Berkeley National Laboratory
to model how anthropogenic aerosols affect global and regional climate.
The researchers simulations show in unprecedented detail how
aerosols are partially offsetting the effect of global warming and
changing the properties of clouds. In some industrial regions, the
generation of aerosols from fossil fuel combustion and biomass (forest
and grassland) burning may be as important to climate change as
greenhouse gases. Also, climate changes caused by aerosols vary
significantly by season and by region.
The research team belongs
to the Atmospheric Chemistry and Aerosols Group, part of the Atmospheric
Science Division of Livermores Energy and Environment Directorate.
The teams advanced simulations, whose findings have been corroborated
by field measurements at different geographical locations, build
on Livermores expertise in aerosols, climate, chemistry, and
supercomputer simulations. The research has received funding from
the Department of Energy, National Oceanic and Atmospheric Administration,
National Aeronautics and Space Administration, and Laboratory Directed
Research and Development. The work also contributes to fulfilling
the goals of the federal governments National Aerosol Climate
Interactions Program, an interagency effort created last year.
Chuang explains that aerosol
concentrations from natural sources, such as volcanoes, sea spray,
and desert dust storms, are believed to have remained generally
steady over the past century. However, like greenhouse gases, anthropogenic
aerosols have increased markedly since 1950. Based on satellite
data, models, and information on urban and agricultural activities,
scientists believe anthropogenic aerosols currently contribute about
half of the total submicrometer-size aerosols in the atmosphere.
Most of the anthropogenic aerosols are sulfates and carbonaceous
compounds produced by the burning of fossil fuels and biomass.
Global climate change by greenhouse
gases and aerosols since 1750. Factors above zero have a warming
effect; those below zero have a cooling effect. A vertical
line between two data points indicates scientific uncertainty
regarding the estimated contribution of a particular factor.
Reflection Means Cooling
When directly suspended in
the atmosphere, most aerosol particles exert a direct cooling effect
on the global climate by scattering sunlight back into space. Aerosols
also exert a significant indirect effect by serving as cloud condensation
nuclei (CCN) for raindrops to form. Increases in CCN result in clouds
with more but smaller droplets, thereby increasing the clouds
reflectivity of solar radiation, or albedo. Clouds with numerous
small droplets tend to last longer and so prolong the cooling effect.
Complicating matters is the
recently discovered influence of black carbon aerosols, such as
soot (incompletely burned carbon), that absorb heat instead of reflecting
it back into space. Black carbon aerosols are particularly prevalent
over parts of Europe, eastern China, and India, where much coal
Beginning in the early 1990s,
Chuang focused first on modeling the direct effects of anthropogenic
sulfate aerosols because they were thought to be the most important
compound involved in pollution over China, Europe, and the eastern
coast of the United States. She then added the contribution from
carbonaceous compounds because of their sizable emission from many
industrialized regions of the Northern Hemisphere and tropical regions
where agricultural burning is prevalent. The simulations also took
into account the solar absorptive properties of black carbon, the
first time this effect had been modeled.
The simulations showed that
biomass aerosols suspended in the clear sky cool the climate by
between 0.16 and 0.23 watts per square meter, while black carbon
from fossil fuels heats the climate by between 0.16 and 0.20 watts
per square meter. Also, sulfate aerosols cool the atmosphere by
between 0.53 to 0.81 watts per square meter. The sum of the cooling
effects ranges between 0.35 and 0.65 watts per square meter. (To
place these figures in perspective, about 340 watts per square meter
of solar radiation reaches Earths atmosphere daily.)
Chuangs research then
moved to the vastly more complex task of modeling the indirect effects
of anthropogenic sulfate and carbonaceous aerosols through their
interaction with clouds. These simulations indicated that the indirect
effects of aerosols are greater than the direct effects. The simulations
also showed that aerosols can mask the warming effects of greenhouse
gases, at least in regions with high pollution levels.
|Aerosols vary greatly from
region to region. These are predicted annual surface concentrations
of aerosols (natural and anthropogenic) composed of (a) sulfate,
(b) organic carbon (in terms of organic matter), (c) black carbon
from soot, (d) sea salt, and (e) dust particles. The red areas
indicate maximum concentrations of each type of aerosol.
|Simulations show the direct
(that is, without interactions with clouds) climate effects
of (a) carbonaceous aerosols from biomass burning, (b) carbonaceous
aerosols from fossil fuel burning, (c) anthropogenic sulfate
aerosols, and (d) all anthropogenic sources. Note that both
the fossil fuel and biomass burning release sulfur dioxide,
which later oxidates to sulfate.
simulations estimated that aerosols acting as CCN cool Earth by
about 1.85 watts per square meter, with 0.30 watts per square meter
associated with anthropogenic sulfate, 1.16 watts per square meter
associated with carbonaceous aerosols from biomass burning, and
0.52 watts per square meter associated with carbonaceous aerosols
from fossil fuel combustion. While concentrations of anthropogenic
carbonaceous aerosols are about equal in the Northern and Southern
hemispheres, aerosols of anthropogenic sulfates are more prominent
in the Northern Hemisphere.
Also, the simulations showed
that concentrations of aerosols vary with the seasons. The global
average of indirect effects by anthropogenic aerosols is greatest
in April through June, a period when biomass of savanna and forested
areas is burned in the topics. The indirect cooling effect is highest
in May, with 2.4 watts per square meter.
Chuang also addressed how
black carbon absorption affects solar energy in clouds. She found
that including this absorption does not decrease the overall cooling
effect by more than 0.07 watts per square meter on a global scale,
but that locally, it can decrease the cooling effect by as much
as 0.7 watts per square meter in regions that have significant black
carbon emissions. The model shows that if the effect of black carbon
absorption in clouds is not included, the indirect cooling effect
by carbonaceous aerosols may be overestimated by up to 25 percent
in regions where black carbon emissions are significant.
The Livermore assessments
were based on a three-dimensional general circulation model called
Community Climate Model-1 (CCM-1), which was developed by the National
Center for Atmospheric Research in Boulder, Colorado. General circulation
models predict global changes that result from changing concentration
of gases by dividing the global atmosphere into tens of thousands
of boxes and using the equations describing motion, energy, and
mass to predict the changes in climate. Chuang linked CCM-1 to GRANTOUR,
a three-dimensional global chemistry code for the troposphere, which
was first developed at Livermore in the late 1980s to simulate the
concentration and distribution of aerosols and their gaseous precursors.
Both GRANTOUR and CCM-1
were developed more than a decade ago, notes Chuang. They
lack advanced physics and modeling techniques that prevent us from
exploring in greater detail the interrelationship between aerosols,
clouds, and climate variation. We believe this interrelationship
is the leading source of uncertainty in predicting future climate
|Monthly averages of the indirect
climatic effect (interactions with clouds) caused by anthropogenic
(a) carbonaceous aerosols, (b) sulfate aerosols, and (c) total
carbonaceous and sulfate aerosols. The simulations show the
wide variation between January and July.
Era of Modeling
The research teams
goal is to move to what Chuang describes as a new era of modeling
that will link the most advanced atmospheric chemistry and climate
codes. To that end, last year Chuang and her colleagues added improvements
to Livermores integrated, massively parallel atmospheric chemical
transport (IMPACT) code so that it better represents aerosol chemistry
and runs faster on multiparallel supercomputers. IMPACT, which was
previously applied by Livermore researchers to global ozone calculations,
includes both the stratosphere and troposphere and uses databases
of monthly averaged emissions compiled by scientists and government
agencies worldwide to treat global chemistry processes.
With the recent revisions,
IMPACT can simulate the complicated reactions involving sulfate
aerosols that are formed from sulfur dioxide generated by power
plants and biomass burning. The code can also account for other
sources of sulfates, including the production of dimethylsulfide
by plankton, sulfur dioxide by volcanoes, and hydrogen sulfide by
soils, forests, and crops. The new version of IMPACT also predicts
the concentrations of black carbon and other carbonaceous compounds,
dust, and sea salt as well as their seasonal variations.
To better represent the ever-changing
size distribution of aerosol particles, Chuang is adapting an aerosol
microphysics module developed at Brookhaven National Laboratory.
The module simulates aerosol dynamics through complicated nucleation,
growth, and transport processes by tracking the lower order moments
of an aerosol size distribution in space and time.
Chuang plans to link IMPACT
and the new microphysics module with the Community Climate Model-3,
or CCM-3, the fourth-generation model developed by the National
Center for Atmospheric Research. This climate model allows more
realistic and higher resolution simulations of aerosol effects on
regional climate. For example, it can show how aerosols are transported
to different regions by strong winds and removed by rainfall.
Chuang notes that CCM-3 is
typically used by research centers at 300-kilometer resolution.
Such coarse resolution limits the codes usefulness because
it does not adequately represent topographic features that strongly
influence surface temperature and precipitation. Much finer resolutions
are required to examine regional climate change and the transport
of aerosols through the atmosphere. Chuang notes that a Livermore
team headed by Philip Duffy has simulated the effects of increased
greenhouse gases by using CCM-3 at 50-kilometer resolution to obtain
the finest resolution of global warming performed to date. (See
S&TR, July/August 2002, pp. 412.)
This simulation, using IMPACT,
shows the percentage of concentrations (averaged on an annual
basis) from all anthropogenic sources of aerosol.
all the modeling elements in place, Livermore atmospheric scientists
will be able to simulate early next year the global and regional climate
changes caused by both aerosols and major greenhouse gases. With
more complete chemistry and physics in our models, we hope to have
more accurate answers about how human activities are affecting our
climate, Chuang says.
Ultimately, she says, realistic
climate modelsaugmented by other dataprovide the only
viable approach for determining how aerosols are changing the planets
climate and for assessing the effects of future emissions. Models
are the only tools for making predictions about climate change so
that we can help policymakers arrive at the most informed decisions
for responding to changes in the environment.
She notes that at first glance,
it might seem that aerosols are a positive element because they tend
to counter the effects of global warming. However, purposely allowing
a greater buildup of aerosols to offset global warming would lead
to greater health and ecological damage. Aerosols that affect climate
are associated with air pollution and acid rain, lower visibility,
and decreased agricultural production.
The results from the advanced
Livermore simulations will surely help society as it decides to manage
air pollution, global warming, changing rainfall patterns, and the
unavoidable effects on human health and society.
aerosols, biomass, black carbon, cloud condensation nuclei (CCN),
Community Climate Model (CCM), dimethylsulfide, global warming,
GRANTOUR, IMPACT, National Center for Atmospheric Research, soot,
For further information contact Catherine Chuang (925) 423-2572 (email@example.com).
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