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Perspectives of
Solar Radiation Management
Johann Feichter, Jan Kazil, Stefan Kinne and Johannes Quaas
Max Planck Institute for Meteorology, Hamburg, Germany
WE Heraeus Seminar, Bad Honnef, May 2008
Insurance against a bad climate trip (W. Broeker)
or
Are we going to open Pandora‘s Box?
Focus of my talk: Is it feasible?
Source: wikipedia
Magical practices
to control weather
in many cultures
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
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
the year after eruptionPresent-day anthropogenic
warming
~ 0.7 K
Pinatubo eruption June 1991
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
Numerical Model Simulations
Model simulations using ECHAM5/HAM
Considered Compounds:
Sulphate Black Organic Sea Salt Mineral DustCarbon Carbon
mixing statesize distributioncomposition
Prognostic variables:
Aerosol distribution: superposition of seven log-normal modes
The aerosol model
0,00
0,50
1,00
1,50
2,00
2,50
3,00
GHG AP GHG&AP
climate equilibrium simulationsglobal mean 30 year averages
Radiative forcing [W/m2]
Climate sensitivity T/F [K/W/m2]
Hydrological sensitivity P/T [%/K]
Sulfate burden [Tg S]
changes between year 2000 and 2030
aerosol reduced
GHG increased
Forcingchange in precipitation per 1 K temp. change
by courtesy of CA Perry
Decrease in solar irradiance reduces evaporation
Solar insolation
Surface wind
humidity
Aerosol reduce turbulent humidity transport
Eleven-year running mean of normalized anomalies of annual means of irradiance [W/m2]
Stanhill, EOS, 2007
Aerosol induced reduction in solar irradiance – solar dimming
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
- 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 SO
2
- continuous release - in layer above tropopause - in tropics between 10°S and 10°N
- Results are shown as GE - CTL
2. Stratospheric sulfur injection experiment
Results: Change in column sulphate concentrations
90°S EQ 90°N
90°S EQ 90°N
25%
0%90°S EQ
90°N
150%
0%90°S EQ
90°N
0.3
mg/m2
090°S EQ
90°N90°S EQ
90°N
0.5
mg/m2
090°S EQ
90°N
Absolute and relative change
( GE – CTL )
SO2
SO4
Results: Sulphate aerosol optical depths
90°S EQ 90°N
250%
0
90°S EQ 90°N
0.004
0
90°S EQ 90°N
Absolute and relative change
(GE - CTL)
90°S EQ 90°N
0.1
hPa
50
1000
Change in SO4
concentrations ( GE - CTL)
Results: Removal processes
Wet depositionabsolute and
relative change (GE - CTL)
90°S EQ 90°N
90°S EQ 90°N
90°S EQ 90°N
0.02 mg/
(m2 d)0
25
%
0
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
1 Tg S
10 Tg S
condensation
coagulation
3 days after injection
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 SRasch et al. 2008: - 0.6 Kour study: - 0.3 K
- Pinatubo: ~7-13 Tg S (Krueger et al., 1995)
→ cooling of -0.5°C in the year after eruptioncorreponding 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 more aerosol particles = more cloud droplets clouds become brighter
precipitation less likely
Latham, 2002
Bower et al., 2006
Forcing: F↓ Δα
F↓(α+Δα)
Measurable at the top of the atmosphere
albedo change due to increased aerosol
• ~ 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
A fit to the planetary albedo as retrieved by CERES is computed as a function of MODIS-retrieved aerosol optical thickness, cloud fraction, the area fraction covered by low-level liquid water clouds, and cloud optical thickness. Cloud optical thickness is a function of cloud liquid water path and cloud droplet number concentration.
Satellite data analyses – CERES & MODIS
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 oceansQuaas et al., JGR, 2007
Datasets:MODerate Resolution Imaging Spectroradiometer (MODIS) MOD08_D3 gridded data (1°x1°)
Clouds and the Earth's Radiant Energy System (CERES) SSF dataset including MODIS cloud retrievals
Daily data for Mar. 2000 – Feb. 2005 Coverage 60°S – 60°N
NPO
NAM
NAO
EUR
ASI
TIO
AFR
TAOSA
M
TPO
SPO
SAO
SIOOCE
Climate effect of seeding marine boundary-layer clouds?
0
-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
Forcing is largest where extended low-level 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
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
Davin et al., GRL, 2007
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
Geoengineering is feasible but
i. lack of accuracy in climate predictionii. 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
Conclusions (3)
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
to solve a policy dilemma apply effective policy
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