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OPTIMAL SUNSHADE CONFIGURATIONS FOR SPACE-BASED GEOENGINEERING NEAR THE SUN-EARTH L1 POINT Joan-Pau Sánchez & Colin McInnes Climate Engineering Conference Berlin, Germany August 18-21, 2014

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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 10 7 -10 8 tonnes (Seifritz 1989; Early 1989; McInnes 2002; Angel 2006). Space-based Geoengineering Optimal Sunshade Configurations ReflectorsDust Sunshades

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Optimal Sunshade Configurations for Space-based Geoengineering near the Sun-Earth L1 point Optimal Sunshade Configurations Revisiting the concept of deploying a large sunshade at L 1. – 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/m 2 ΔS=23.24 W/m 2 *at the SRP-displaced equilibrium point

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Table of Contents GREB Optimization Results

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Understanding of regional effects of climate change, while performing a numerically intensive search. GREB model (Dommenget and Floter 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

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2xCO 2 Scenario 2xCO 2 +Sunshade Scenario 2xCO 2 (680 ppm) + static sunshade at L 1 – An iterative secant approach is used to find the size of the disk that yields a global mean temperature of 14 C o. Classical In-line Scenario 3D Energy Kick Function Sun Earth L1L1 L2L2 *ΔT difference with respect the control scenario (1xCO 2 world)

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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 Earth L1L1 L2L2 Sun Earth L1L1 L2L2 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?

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A Multiple-Objective Optimization More suitable disk configurations that reduce the impact of climate at regional and seasonal scale? Sun Earth L1L1 L2L2 – 2 Mirrors of Shading areas A 1 and A 2 – 2 sinusoidal and displaced out-of-plane motions. Design Variables Criteria Vector Total Shading Area Geoengineering Performance Index

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A Multiple-Objective Optimization More suitable disk configurations that reduce the impact of climate at regional and seasonal scale? – 2 Mirrors of Shading areas A 1 and A 2 – 2 sinusoidal and displaced out-of-plane motions. Design Variables Criteria Vector Total Shading Area Geoengineering Performance Index Pareto Optimal Set

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Geoengineering Performance Index Optimal sunshade configurations were sought that minimize 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 C o difference of the surface temperatures of that of the 1xCO 2 world.

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Multiple-objective Optimization Problem Pareto optimal design solutions that minimize both J and the total shading area A t required for 2 sunshades. J 0 =0.325 C o Classical inline Sc. ½J 0 Case ICase II Classical In-line Scenario

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

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

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CASE II Minimum A t to achieve ½·J 0 – 1.5 times the A t of the classical inline geoengineering solution. – J = 0,162 C o – Environmental risk is reduced to a quarter.

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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 L 1 point. 3D Energy Kick Function

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THANKS FOR YOUR ATTENTION Optimal Sunshade Configurations for Space-based Geoengineering near the Sun-Earth L1 point Joan-Pau Sánchez – jpau.sanchez@upc.edu

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APPENDIX Optimal Sunshade Configurations

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Space-based Geoengineering Optimal Sunshade Configurations 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

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Optimal Configuration for Sun-Earth L1 Occulting Disk Sun Earth L1L1 L2L2 (McInnes et al. 1994) Libration Point Orbits

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Shade Patterns Sun Earth L1L1 L2L2 Libration Point Orbits A static sunshade at L1 casts an almost uniform shade onto the Earth. Sun Earth L1L1 L2L2 By displacing the occulting disk, different shade patterns are achieved.

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