Stratospheric Aerosols and Climate Geoengineering

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Stratospheric Aerosols and Climate
Geoengineering
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Outline
Global climate change forecasts Climate control
factors Stratospheric aerosols as energy
modulators Engineering the climate system
Approaches Sulfur in fuel The 3 solution Soot
as a cooling agent Side effects? Conclusions
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Global warming does not justify severe measures
to control CO2 emissions but it does call for
extreme intervention through global environmental
engineering?
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Four greenhouse gases four reasons for concern
about global warming
R. Turco, 1997
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Retrospective Climate Change Predictions
IPCC, 2001
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Forecasts of Future Climate Change
Changes in global mean temperature (in degrees
Centigrade) predicted over the next century using
climate model ensembles and a range of estimated
greenhouse gas emission scenarios (A1B, etc.) as
developed by the Intergovernmental Panel on
Climate Change (IPCC). The forecasts are shown in
two time framesfor the future, and in an
historical context.
IPCC, 2001
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Projected Global Warming 2071-2100
IPCC, 2001
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Sea Level Change Predicted Over the Next Century
IPCC, 2001
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Cryospheric Impacts
Glacier National Park, Montana
Larsen ice-sheet, Antarctica
1911
January 31, 2002
2000
March 5, 2002
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Earth is Warming Up
The greenhouse effect of CO2 is strongly
implicated.
The consequences are very serious, and there is
wide consensus for action.
But we are addicted to fossil fuels.
So what actions are practical?
Is large scale geoengineering an appropriate
response?
What are the possibilities and pitfalls?
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Factors that Affect Climate

• Intensity of Sunlight, or the Solar Constant
  varies withDistance from the SunSolar
  Activity, or the solar cycleShadowing, or
  screening of sunlight

• Albedo, or Reflectivity, of the Earth varies
  withCloud coverageIce areaAtmospheric Dust
  and HazeVegetation and soil moisture

• Greenhouse Effect of Water Vapor and other
  trace gasesCarbon DioxideCH4, N2O, CFCs, O3

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The Sunscreen, or Shadowing, Effect
Modifying Earths Energy Balance
Reducing the effective solar constant by blocking
a fraction of sunlight from reaching Earths
atmosphere decreases the net source of energy,
leading to a lower mean global temperature.
Shading by 1 can cool the climate 1 degree
Centigrade.
R. Turco, 1997
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The Albedo, or Insolation, Effect
Modifying Earths Energy Balance
Increasing the reflectivity, or albedo, of Earth
decreases the net amount of energy absorbed by
the Earth, and reduces the mean planetary
temperature. The albedo is normally around
one-third (0.33). Changes of 0.01 can lead to
1 degree of temperature change.
R. Turco, 1997
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The Greenhouse, or Insulation, Effect
Modifying Earths Energy Balance
Modifying the export of energy by increasing the
quantity of greenhouse gases in the atmosphere,
thus insulating the surface and inhibiting the
escape of radiant heat, which warms the surface
reducing greenhouse gases has the opposite
effect. Doubling of CO2 concentrations causes 3
degrees warming.
R. Turco, 1997
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Volcanoes tend to cool the climate by injecting
sulfur vapors into the stratosphere that quickly
form fine sulfate aerosol particles, increasing
the albedo
Mt. St. Helens, May 18, 1980
Unattributed photo collage
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Historical volcanic eruptions have lead to
notable cooling events
Benjamin Franklin first noted the volcano-climate
connection in the aftermath of the Laki eruptions
of 1783.
Data from LaMarche and Hirschboek, Nature 1984,
based on tree ring analysis of bristlecone pines
in the Southwestern U.S. from R. Turco, 1997
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Volcanic Particles Reduce Sunlight(but still
allow heat to escape)
R. Turco, 1997
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A change in the stratospheric aerosol optical
thickness of 0.1 can lead to 1OC global cooling
at steady-state.
R. Turco, 1997
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Pinatubo volcanic cloud decay as seen by CLAES
(Cryogenic Limb Array Etalon Spectrometer)
observations.
CLAES websitehttp//www.spasci.com/CLAES
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CO2 Compensation Plans
Capture and sequestration (natural or mechanical)
Sulfur compensation, via emission and
transformation
Biological pumping
R. Turco, 1997
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Going Whole-Hog to Increase Earths Albedo
Balloons, Styrofoam and Artillery Shells
R. Turco, 1997
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Why Not Use the Volcano Effect?We burn coal,
which releases lots of sulfur
R. P. Turco et al., Nature, 1980 R. P. Turco,
"Global environmental engineering Prospects and
pitfalls," Chapter 7, Human Population and the
Environmental Crisis, B. Zuckerman and D.
Jefferson (eds.), Jones and Bartlett Publ., 1995
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Simulated increase in stratospheric aerosols
resulting from increases in the emissions and
background concentrations of sulfate aerosol
precusor gases, OCS, SO2 and CS2
R. P. Turco et al.,Nature, 1980
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Climate Engineering Through Direct Stratospheric
Sulfur Injection

• Direct injection into the stratosphere may be the
  most efficient and effective approach
• The amount and location are readily managed
• Termination of effects can easily be executed
• Effects can be assessed with current models
• The cost is relatively low
• We have nothing to lose

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Global average surface anomalies, geoengineered
5-Mt/yr tropical SO2 injection One 40-year model
realization
Results provided by L. Oman, John Hopkins U.,
A. Robock, Rutgers U., 2007
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Global average surface anomalies, geoengineered
10-Mt/yr tropical SO2 injection One 40-year
model realization
Results provided by L. Oman, John Hopkins U.,
A. Robock, Rutgers U., 2007
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Global average decadal surface air temperature
anomalies, geoengineered 5-Mt/yr SO2 injection in
the tropics
Results provided by L. Oman, John Hopkins U.,
A. Robock, Rutgers U., 2007
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Global average decadal surface air temperature
anomalies, geoengineered 10-Mt/yr SO2 injection
in the tropics
Results provided by L. Oman, John Hopkins U.,
A. Robock, Rutgers U., 2007
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Global average decadal precipitation anomalies,
geoengineered 5-Mt/yr SO2 injection in the tropics
Results provided by L. Oman, John Hopkins U.,
A. Robock, Rutgers U., 2007
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Global average decadal precipitation anomalies,
geoengineered 10-Mt/yr SO2 injection in the
tropics
Results provided by L. Oman, John Hopkins U.,
A. Robock, Rutgers U., 2007
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Problems with Sulfur-based Solutions(none of
which have been addressed in current GCMs)

• Quantitiespotentially 10s Mt-SO2/yr
• Deliveryaircraft, hoses, blimps?
• Injectionplume scale coagulation/removal
• Dispersionregional de-localization
• Microphysicsnonlinear growth and fallout
• Lifetimeonly 1 yr
• Toxicitysulfur dangerous pollutant
• Chemistrydepletion of the ozone layer
• Climateregional impacts unpredictable
• Side-effectsmilky skies
• Costslikely 10s100s B/yr
• Feasibilitypolitically unacceptable

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Observations of the dissipation of the Mt.
Pinatubo eruption cloud over the tropics based on
AVHRR (Advanced Very High Resolution Radiometer)
data 1990-93
Data presentation provided by A. Robock,
Rutgers University, 2007
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Nonlinear microphysics limits the lifetime and
optical effects of sulfur injections Below,
simulations of a 100200 Mt super-Tambora
eruption assuming stratospheric dispersion of
injected sulfur dioxide over 5-10 months.
J. Pinto, R. Turco, O. B. Toon, J. Geophys.
Res., 1989
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Balloon trajectories tracked over Antarctica,
with potential vorticity, 2005/06
Data provided by C. R. Mechoso, UCLA C.
Basdevant et al., LMD, ENS, 2007
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Balloon trajectories tracked over Antarctica,
with ozone mixing ratios, 2005/06
Data provided by C. R. Mechoso, UCLA C.
Basdevant et al., LMD, ENS, 2007
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Balloon trajectory tracked over Antarctica
balloon BP27 (air temperature indicates altitude)
Data provided by C. R. Mechoso, UCLA C.
Basdevant et al., LMD, ENS, 2007
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Smoke Aerosol as a Climate Moderator
Vegetation smoke consists of organic particles
that mainly scatter radiation. Lodi Canyon,
12/12/86.
Soot from burning liquid fossil fuels is mainly
absorbing. Sandia, JP-4, 3/13/87.
Courtesy of L. Radke, J. Hallett, 1987
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Footnote Assuming that a layer of smoke
particles lies above the tropopause, and is
uniformly distributed, the global average net
SOLAR radiative forcing is, F (1-A)
S/41-T/(1-AR). The albedo of the
unperturbed Earth is taken to be entirely due to
the troposphere, with a value, A 0.3. The solar
constant is, S 1370 W/m2. The solar
transmissivity of the ambient stratosphere is
assumed to be 1. The transmissivity of an
optically thin smoke layer is then, T
1-(2?a2? ?s) Chylek and Wong, 1995. The
corresponding reflectivity of the smoke layer,
assumed equal for solar and reflected
tropospheric light, is then, R 2??s. Here,
the absorption optical depth is ?a, and the
scattering optical depth is ?s. The backscatter
fraction, ?, which depends on particle size and
composition, is assigned an average value, ?
0.3. The tropopause forcings shown in the
accompanying figure assume that the extinction
optical depth, ?e?s?a (or equivalently, the
specific extinction coefficient, ?e7 m2/g, from
Turco et al., 1990), remains fixed as the single
scattering albedo , ???s/(?s ?a)?s/ ?e?s/?e,
varies. Effectively, the total mass of smoke is
held fixed in this case, while the elemental
carbon fraction is allowed to decrease as ?
increases, such that ?s ?a?econstant, or
equivalently, ?s?a ?econstant. The fraction of
the smoke particle mass that is not composed of
elemental carbon would presumably be in the form
of transparent or low-absorption organic
coatings, or similar materials. The smoke
particles assumed here have an initial single
scattering albedo of 0.3, with smaller values
ignored. Alternatively, if the absorption
optical depth, or specific absorptivity, ?a, is
taken to be constant (i.e., ?a 7 m2/g), then
low-absorption materials must be added to the
particles of smoke to increase their single
scattering albedo. The corresponding radiative
forcings for the initial optical depths of 0.07
(0.014) would be -37 (-7), -65 (-13), and -109
(-20) W/m2 corresponding to single scattering
albedos of 0.3, 0.8, and 0.9, respectively.
These latter forcings are larger than those given
in the figure because the absorption component is
held fixed while the scattering component is
increased, increasing the total extinction. The
smoke forcing corresponding to high single
scattering albedos are generally larger than the
forcings due to volcanic particles with the same
total optical depth because the smoke particle
sizes are taken to be smaller, which yields a
stronger backscatter componentin our case, ? is
roughly three times larger for smoke than for
volcanic aerosols, leading to about three times
the forcing.
Radiative forcing corresponding to 15 Tg of
stratospheric smoke injection, assuming a range
of absorptivity
O. B. Toon, R. P. Turco, A. Robock, et al.,
ACPD, 2007
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Variations in global temperature, precipitation,
and surface forcing 5 Tg of black soot in the
upper troposphere
The temperature, precipitation and surface
forcing anomalies persist for more than a decade
owing to inertia in the ocean-atmosphere system.
Compared to other major forcing events such as
Pinatubo, a smoke injection scenario creates much
larger and more tenacious climate perturbations,
exceeding by an order of magnitude (and of
opposite sign) the forcing associated with
greenhouse warming.
Robock et al., ACPD, 2007
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Variation in soot mass for subtropical injection
at various levels in the troposphere and lower
stratosphere
Year
The smoke injection is strongly stabilized
through radiative-dynamical interactions, and
especially the lofting to higher elevations
accompanied by the suppression of precipitation.
Robock et al., ACPD, 2007
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Global temperature perturbations for a 5-Tg soot
injection, compared to recent historical global
warming
Greenhouse Anomaly
Soot Anomaly
At the levels of cooling shown, significant
impacts would be expected on agriculture, and
perhaps on the frequency of severe weather events
(injection in the upper troposphere).
Robock et al., ACPD, 2007 Hansen J., et al., J.
Geophys. Res. 106,doi 10.1029/2001
http//data.giss.nasa.gov/gistemp/2005
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Surface air temperature changes for the summer
season one year after a 5-Tg soot injection
local decreases up to 7 oC
Robock et al., ACPD, 2007
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Summer precipitation anomalies one year after a
5-Tg soot injection changes, mainly decreases,
reach 90
Robock et al., ACPD, 2007
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Growing season changes predicted four years after
a 5-Tg soot injection local decreases are up to
60 days
Robock et al., ACPD, 2007
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Summer surface air temperature changes one year
after a 1-Tg soot injection decreases are up to
3 oC, but with warming at higher land elevations
Robock et al., ACPD, 2007
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Simulation of zonally-averaged total ozone column
densities for ambient and soot injection (5-Tg)
cases (ozone is averaged over the second
simulation year) the changes shown constitute a
semi-permanent global ozone hole
Mills et al., 2007
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Conclusions Where Do We Stand?

• Earth is a complicated and often chaotic
  placechange one thing and you affect many
  outcomes environmental engineering projects
  will never be straightforward.

• We could stem global warming by harnessing energy
  from the Sun in its many forms, replacing fossil
  fuelshowever, we must learn to use these sources
  more efficiently, and we must do that quickly.

• The application of technology can sometimes solve
  environmental problems, but often causes
  unfavorable side effects we cannot depend on
  technology alone to ensure a habitable futurewe
  must have the wisdom to impose constraints on
  growth and consumption.

• The fact that civilization has advanced to this
  point shows we are smart hopefully, we are not
  too smart for our own good!

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(Perhaps Not)THE END
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