Transcript First results on a stratospheric sulphate umbrella in the

WE Heraeus Seminar, Bad Honnef, May 2008
Perspectives of
Solar Radiation Management
Insurance against a bad climate trip (W. Broeker)
or
Are we going to open Pandora‘s Box?
Johann Feichter, Jan Kazil, Stefan Kinne and
Johannes Quaas
Max Planck Institute for Meteorology, Hamburg, Germany
Focus of my talk: Is it feasible?
Magical practices
to control weather
in many cultures
Source: wikipedia
Cloud seeding for
water resource management
and weather hazard mitigation
OUTLINE
• Concepts
• Impact of aerosols on climate - model studies
• Sulfur injection into the stratosphere
• Enhancement of cloud albedo
• Land-use change
Geoengineering to Counteract GHG Warming
Can we compensate for changes in longwave
radiation by changing solar radiation?
Solar radiation management concept
Reduce the solar radiation absorbed by the Earth-atmosphere
system to counteract greenhouse gas warming
Methods
• place space-borne reflectors at the Lagrangian point
Deflector diameter ~ 2000 km
the deflector would reduce incoming solar radiation by about 1%,
• injection of stratospheric aerosol
• enhance cloud albedo – aerosol particles
• enhance surface albedo in deserts
• de- or reforestation
• covering the oceans with white foam
Paul Crutzen’s proposal
Albedo enhancement by stratospheric sulfur injections: a contribution
to resolve a policy dilemma. [Crutzen, 2006]
Injection of sulfur
in the stratosphere
downscaling effect
by Mt. Pinatubo:
14-26 Tg SO2(= 7-13 Tg S)
injected into stratosphere
(Krueger et al., 1995) 
0.5 K cooling
Pinatubo eruption June 1991
the year after eruption
Present-day anthropogenic
warming
~ 0.7 K
Simulation and Observations
Stratosphere
warming
cooling
Troposphere
Numerical Model Simulations
1. Climate equilibrium simulations
• Atmosphere-aerosol model coupled to mixed layer ocean
• Integration 30 years after spin-up
• Effect of anthropogenic emissions (surface sources!!)
• Changes between the year 2000 and 2030 assuming a
further increase of greenhouse gas concentrations and a
decrease in aerosol emissions
Model simulations using ECHAM5/HAM
The aerosol model
Considered Compounds:
Sulphate
Black Organic Sea Salt Mineral Dust
Carbon Carbon
Prognostic variables:
composition
size distribution
mixing state
Aerosol distribution: superposition of seven log-normal modes
climate equilibrium simulations
global mean 30 year averages
3,00
changes
between
year 2000
and 2030
2,50
2,00
1,50
 aerosol
reduced
1,00
0,50
0,00
GHG
AP
Radiative forcing [W/m2]
Climate sensitivity DT/DF [K/W/m2]
Hydrological sensitivity DP/DT [%/K]
Sulfate burden [Tg S]
GHG&AP
Forcing
 GHG
increased
change in
precipitation
per 1 K temp.
change
Surface temperature response 2030 - 2000
GHG
1.20 oC
AP
0.96 oC
Change of precipitation
GHG 0.07 mm/d
AP
Aerosol effect:
reduction of precipitation
0.08 mm/d
Decrease in solar irradiance reduces evaporation
Aerosol reduce
turbulent humidity
transport
Solar insolation
Surface wind
humidity
by courtesy of CA Perry
Aerosol induced reduction in solar irradiance –
solar dimming
Stanhill, EOS, 2007
Eleven-year running mean of normalized anomalies of annual
means of irradiance [W/m2]
Pinatubo: Trenberth and Dai, GRL, 2007
Observed anomalies of precipitation between Oct. 1991 and Sept.
1992 compared to the period 1950 to 2004
mm/day
Preliminary Conclusions (1)
Higher aerosol load
• cools the earth atmosphere system
• reduces the solar insolation at surface
• reduces the evaporation and precipitation rate
• changes the precipitation pattern
2. Stratospheric sulfur injection experiment
- ECHAM5/HAM model, T63L31 resolution
- Climate conditions for the year 2000 (nudging)
- AeroCom aerosol emission inventory
1) CTL: Control
2) GE: Geo-engineered
- 1 Tg sulphur per year (~ 1.3% of total sulfur em.)
- as SO2
- continuous release
- in layer above tropopause
- in tropics between 10°S and 10°N
- Results are shown as GE - CTL
Results: Change in column sulphate concentrations
Absolute and relative
change
( GE – CTL )
SO2
SO4
0.3
0.5
mg/m2
mg/m2
0
0
90°S
EQ
90°N
25%
150%
0%
0%
90°S
EQ
90°N
90°S
EQ
90°N
90°S
EQ
90°N
Results: Sulphate aerosol optical depths
Absolute and relative
change
(GE - CTL)
Change in SO4
concentrations
( GE - CTL)
0.004
0
0.1
90°S
90°S EQEQ
90°N
90°N
hPa
50
250%
0
90°S
EQ
90°N
1000
90°S
EQ
90°N
Results: Removal processes
Wet deposition
absolute and
relative change
(GE - CTL)
0.02
mg/
(m2 d)
0
90°S 90°S
EQ EQ 90°N90°N
25
%
0
90°S
EQ
90°N
Optical properties and climate effect
Optical properties depend on the chemical
composition and the size distribution of the particles
Size distribution is controlled by aerosol microphysics
Development of size distribution
1 day
3 days after injection
condensation
1 Tg S
coagulation
10 Tg S
What controls the potential to cool the atmosphere?
• the higher the amount of sulfur injected, the higher the sulfuric
acid concentration and the particle size
• the higher the particle size, the stronger the sedimentation;
sedimentation rate controls the residence time of particles in the
stratosphere  saturation effect
• extinction efficiency ~ aerosol surface
• most efficient extinction if particle radius is about 500 nm and the
width of the distribution is small
• cooling effect due to extinction of solar radiation partly
compensated by a warming effect due to absorption of thermal
radiation (GHG effect); this effect is proportional to the aerosol
mass
next step: simulations using complex Earth System
Models with fine vertical resolution
Preliminary Conclusions (2)
- Geoengineering experiment:
stratospheric sulphate umbrella
- 1 Tg Sulphur / year in the tropical stratosphere
- Cooling depends crucially on
- aerosol microphysics – size distribution of sulfate
particles
- residence time of particles in the stratosphere
- amount and method of release (continous or
pulse)
cooling due to a strat. sulfate burden of 1 Tg S
Rasch et al. 2008: - 0.6 K
our study:
- 0.3 K
- Pinatubo: ~7-13 Tg S (Krueger et al., 1995)
→ cooling of -0.5°C in the year after eruption
correponding to 0.04 – 0.07 K per 1 Tg S
Albedo-enhancement of marine stratocumulus clouds
Use automatic vessels to generate seasalt aerosols which
act as cloud condensation nuclei
albedo change due
 more aerosol particles = more cloud droplets
to increased
 clouds become brighter
aerosol
 precipitation less likely
F↓(α+Δα)
Forcing: F↓ Δα
Measurable at the top of
the atmosphere
Latham, 2002
Bower et al., 2006
• ~ 40% of the oceans is covered by low level clouds
(=25% of the Earth)
• cloud albedo ~ 35%, cloud free ocean ~ 9%
• radiative forcing of marine low level clouds ~ -22 W/m2
anthropogenic climate effect = +1.6 W/m2
• to compensate for anthrop. climate effect
• enhance marine cloud cover or cloud optical depth
by 7 %
question:
what is the sensitivity of cloud optical depth against
changes of aerosol concentration
Satellite data analyses – CERES & MODIS
Datasets:
A fit to theResolution
planetary albedo
MODerate
Imagingas retrieved
by CERES is computed
as a
function of
Spectroradiometer
(MODIS)
MOD08_D3
EU
R
MODIS-retrieved
optical
gridded
NP data
NA (1°x1°)
NA aerosol
ASI
O
M cloud
O
thickness,
fraction, the area
AF
fraction
covered
by
Clouds
and
the Earth's
Radiantliquid
Energy
R low-level
water
andSSF
cloud
optical
System
(CERES)
dataset
including
TPclouds,
TA
TIO
thickness.
O cloudSA
MODIS
retrievals
O
M
Cloud optical thickness is a OC
function of
cloud
pathSIO
and
cloud
E 2005
Daily
data
forwater
Mar.
– Feb.
SPliquid
SA 2000
droplet
O number
O concentration.
Coverage
60°S
– 60°N
A linear regression yields the
sensitivity of CDNC to a change
in aerosol concentration. This
sensitivity, a measure of the
aerosol indirect effect, is found to
be virtually always positive, with
larger sensitivities over the
oceans
Quaas et al., JGR, 2007
Climate effect of seeding marine boundary-layer clouds?
-24
Radiative forcing by
the aerosol indirect
effect due to an
increase in cloud
droplet number
concentration to a
sustained uniform
400 cm-3 = -2.9 W/m2
0
Forcing is largest
where extended lowlevel clouds exist.
To obtain a uniform CDNC of 400 cm-3 over the world's oceans,
CDNC would need to increase in the mean by a factor of 4.3.
Given the relationship between CDNC and AOD from satellite data,
this would imply that in the global mean, an increase in AOD by a
factor of 10.7 is needed.
3. Albedo enhancement by land-use change
Davin et al., GRL, 2007
Changes in land-use (crops and
pastures)
between 1860 and 1992
Radiative forcing due to changes in
albedo and evapotranspiration
in W/m2
global mean -0.29
-0.22 albedo change; -0.07 evapotransp.
Changes in annual mean surface
temperature in K.
global mean -0.05 K
Conclusions (1)
Greenhouse versus Aerosol Effects:
Is compensation feasible?
• Greenhouse gas warming operates also in winter, during
nigh-time and in high latitudes
• Aerosol cooling is strongest in summer, during day-time
and in cloud-free regions (e.g. subtropics)
• Climate resonse depends on a multitude of interactions of
complex processes
• Enhancement of ice nucleation due to sulfur injections may
exert a warming
Compensation of warming feasible
but significant effects on hydrological cycle
Conclusions (2)
Albedo-enhancement of marine stratocumulus clouds
• formation of giant particles?
• does not seem feasible to balance a doubling of CO2
Enhancing surface albedo by land-use change
more bare soils  reduces storage of CO2 in soils and vegetation
Conclusions (3)
Geoengineering is feasible but
i. lack of accuracy in climate prediction
ii. difficult to determine whether a weather /climate
modification attempt is successful – internal
variability
iii. regional climate response – winners and losers 
policy implications
iv. huge difference in timescale between the effect of
greenhouse gases and the effect of aerosols  the
artificial release of sulfate aerosols is a commitment
of at least several hundred years!
v. serious environmental problems which may be
caused by high carbon dioxide concentration
my two penny worth
Is geoengineering a solution for a policy dilemma?
a world housing soon 9 billion people needs
responsible management of the resources
and not
‘wait-and-see’ politics
saving resources reduces the costs for the society but
might also reduce the gainings of some market sectors
as for instance of the established energy companies
and car manufacturers
to solve a policy dilemma apply effective policy