Perspectives of Solar Radiation Management

1. Johann Feichter, Jan Kazil, Stefan Kinne and Johannes Quaas Max Planck Institute for Meteorology, Hamburg, Germany 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? Focus of my talk: Is it feasible?

2. Cloud seeding for water resource management and weather hazard mitigation Magical practices to control weather in many cultures Source: wikipedia

3. OUTLINE • Concepts • Impact of aerosols on climate - model studies • Sulfur injection into the stratosphere • Enhancement of cloud albedo • Land-use change

4. Geoengineering to Counteract GHG Warming

5. Can we compensate for changes in longwave radiation by changing solar radiation?

6. 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

7. 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 eruption Pinatubo eruption June 1991 Present-day anthropogenic warming ~ 0.7 K

8. Simulation and Observations warming Strato- sphere cooling Troposphere

9. 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

10. Prognostic variables: size distribution composition mixing state The aerosol model Model simulations using ECHAM5/HAM Considered Compounds: Sulphate Black Organic Sea Salt Mineral Dust Carbon Carbon Aerosol distribution: superposition of seven log-normal modes

11. change in precipitation per 1 K temp. change Forcing climate equilibrium simulations global mean 30 year averages • changes between year 2000 and 2030 • aerosol reduced • GHG increased Radiative forcing [W/m2] Climate sensitivity DT/DF [K/W/m2] Hydrological sensitivity DP/DT [%/K] Sulfate burden [Tg S]

12. Surface temperature response 2030 - 2000 GHG 1.20oC AP 0.96 oC

13. Change of precipitation GHG 0.07 mm/d AP 0.08 mm/d Aerosol effect: reduction of precipitation

14. Decrease in solar irradiance reduces evaporation Aerosol reduce turbulent humidity transport Solar insolation Surface wind humidity by courtesy of CA Perry

15. 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]

16. Pinatubo: Trenberth and Dai, GRL, 2007 Observed anomalies of precipitation between Oct. 1991 and Sept. 1992compared to the period 1950 to 2004 mm/day

17. 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

18. 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

19. Results: Change in column sulphate concentrations Absolute and relative change ( GE – CTL ) SO2 SO4 0.3 mg/m2 0 0.5 mg/m2 0 90°S EQ 90°N 90°S EQ 90°N 90°S EQ 90°N 90°S EQ 90°N 90°S EQ 90°N 25% 0% 150% 0% 90°S EQ 90°N 90°S EQ 90°N

20. Change in SO4 concentrations ( GE - CTL) 0.1 hPa 50 1000 90°S EQ 90°N Results: Sulphate aerosol optical depths Absolute and relative change (GE - CTL) 0.004 0 90°S EQ 90°N 90°S EQ 90°N 250% 0 90°S EQ 90°N

21. 0.02 mg/(m2 d)0 90°S EQ 90°N 90°S EQ 90°N 25 % 0 90°S EQ 90°N Results: Removal processes Wet deposition absolute and relative change (GE - CTL)

22. 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

23. 3 days after injection condensation coagulation Development of size distribution 1 day 1 Tg S 10 Tg S

24. 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

25. 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 Preliminary Conclusions (2)

26. F↓(α+Δα)‏ Forcing: F↓ Δα 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 albedo change due to increased aerosol Measurable at the top of the atmosphere Latham, 2002 Bower et al., 2006

27. ~ 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/m2anthropogenic 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

28. 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 EUR NPO NAM NAO ASI AFR TPO TAO TIO SAM OCE SPO SAO SIO Satellite data analyses – CERES & MODIS 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. 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

29. 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 Forcing is largest where extended low-level clouds exist. 0 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.

30. 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

31. Greenhouse versus Aerosol Effects: Is compensation feasible? Conclusions (1) • 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

32. 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

33. Conclusions (3) Geoengineering is feasible but lack of accuracy in climate prediction difficult to determine whether a weather /climate modification attempt is successful – internal variability regional climate response – winners and losers  policy implications 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! serious environmental problems which may be caused by high carbon dioxide concentration

34. 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