Optimal Sunshade Configurations for Space-based Geoengineering near the Sun-Earth L1 point

1. Climate Engineering Conference Berlin, Germany August 18-21, 2014 Optimal Sunshade Configurations for Space-based Geoengineering near the Sun-Earth L1 point Joan-Pau Sánchez & Colin McInnes

2. Space-based Geoengineering • All space-based methods for geoengineering aim at diverting incoming solar radiation before it reaches the Earth. • The estimated mass of the deployed structure is in the order of 107-108tonnes (Seifritz 1989; Early 1989; McInnes 2002; Angel 2006). Sunshades Reflectors Dust Optimal Sunshade Configurations

3. Optimal Sunshade Configurations for Space-based Geoengineering near the Sun-Earth L1 point • Revisiting the concept of deploying a large sunshade at L1. • Most of the previous work aims at a uniform reduction of the solar insolation by 1.7%. • Problem: a uniform insolation reduction of 1.7% would drive important changes to regional climates (Lunt et al. 2008). • Warming at high latitudes and cooling at the tropics. • Goal: optimal configurations of sunshades that offset regional differences such as latitudinal and seasonal difference of temperature. S=1367 W/m2 ΔS=23.24 W/m2 *at the SRP-displaced equilibrium point Optimal Sunshade Configurations

4. Tableof Contents GREB Optimization Results

5. GREB model (DommengetandFloter 2011) • Globally Resolved Energy Balance model (GREB) provides an insight of the effects of altering the incoming solar insolation into the Earth’s climate system. • GREB captures only the main physical processes by means of simplified models. It assumes fixed atmospheric circulation, cloud cover and soil moisture, which are given as boundary conditions. • Simple and fast Understanding of regional effects of climate change, while performing a numerically intensive search.

6. ClassicalIn-lineScenario • 2xCO2(680 ppm) + static sunshade at L1 • An iterative secant approach is used to find the size of the disk that yields a global mean temperature of 14 Co. 2xCO2+Sunshade Scenario 2xCO2Scenario Sun Earth L1 L2 *ΔT differencewithrespectthe control scenario (1xCO2world) 3D Energy Kick Function

7. Shade Patterns • A static sunshade at L1 casts an almost uniform shade onto the Earth. • By displacing the occulting disk, different shade patterns are achieved. Sun Sun Are there perhaps more suitable disk configurations that can reduce the impact of climate change further than what the Sun-Earth in-line configuration achieved? Earth Earth L1 L1 L2 L2

8. A Multiple-Objective Optimization • More suitable disk configurations that reduce the impact of climate at regional and seasonal scale? • 2 Mirrors of Shading areas A1 and A2 • 2 sinusoidal and displaced out-of-plane motions. Sun Design Variables Criteria Vector Earth L1 L2 Total Shading Area Geoengineering Performance Index

9. A Multiple-Objective Optimization • More suitable disk configurations that reduce the impact of climate at regional and seasonal scale? • 2 Mirrors of Shading areas A1 and A2 • 2 sinusoidal and displaced out-of-plane motions. Pareto Optimal Set Design Variables Criteria Vector Total Shading Area Geoengineering Performance Index

10. Geoengineering Performance Index • Optimal sunshade configurations were sought thatminimize J • J is the root-mean-square difference of temperature with respect to the control scenario, averaged over the entire Earth’s surface. • Return the largest fraction of Earth’s surface to a climate within ±0.1 Co difference of the surface temperatures of that of the 1xCO2 world.

11. Multiple-objective Optimization Problem Classical In-line Scenario • Pareto optimal design solutions that minimize both Jand the total shading area At required for 2 sunshades. J0 =0.325 CoClassicalinline Sc. Case I Case II ½J0

12. CASE I • A solution with At as in the classical in-line geoengineering solution. • J= 0,275 Co • Improvement of 0.05oC. • Case I returns nearly 40% of the Earth surface to pre-global warming temperatures, while the classical geoengineering scenario achieves less than 10%.

13. CASE I • The motion required cannot be generated with the natural periodic motion that exist near L1. • Specific control law are thus required.

14. CASE II • Minimum Atto achieve ½·J0 • 1.5 times the At of the classical inline geoengineering solution. • J= 0,162 Co • Environmental risk is reduced to a quarter.

15. Conclusions • This work provides new insights into the possibilities offered by space-based geoengineering using orbiting solar reflectors. • Optimal configurations of orbiting sunshades were investigated that not only offset a global temperature increase, but also mitigate regional differences such as latitudinal and seasonal difference of monthly mean surface temperature. • Two configurations of two orbiting occulting disks were presented that achieve clear gains with respect to a static disk near the Sun-Earth L1 point. 3D Energy Kick Function

16. Thanks for your attention Optimal Sunshade Configurations for Space-based Geoengineering near the Sun-Earth L1 point Joan-Pau Sánchez – jpau.sanchez@upc.edu

17. Appendix Optimal Sunshade Configurations

18. Space-based Geoengineering Fig. The effectiveness, affordability, safety and timeliness ratings of geoengineering methods analysed in a Royal Society report Shepherd at al. Geoengineering the climate , Report of Royal Society working group on geoengineering, 2009 Optimal Sunshade Configurations

19. Optimal Configuration for Sun-Earth L1 Occulting Disk Sun Libration Point Orbits Earth L1 L2 (McInnes et al. 1994)

20. Shade Patterns • A static sunshade at L1 casts an almost uniform shade onto the Earth. • By displacing the occulting disk, different shade patterns are achieved. Sun Sun Libration Point Orbits Earth Earth L1 L1 L2 L2