Introduction to the Governance of Solar Radiation Modification: Key SRM approaches and their governanceDouglas G. MacMynowski challenges

Douglas MacMartin Mechanical & Aerospace Engineering, Cornell University 1 Mt. Pinatubo Resulted in 30Mt of 1991 sulfate aerosols in stratosphere

Stratospheric Aerosol Injection (SAI): Could potentially do this with aircraft…

Volcanoes caused global cooling by putting small particles 2 in the stratosphere Marine Brightening Ship tracks due to aerosols

• Spraying salt-water into low might cool • Cloud-aerosol interactions are poorly understood! • Doesn’t work everywhere: spatially heterogeneous forcing • BUT: could use for regional cooling3 Could Cirrus Cloud cover be artificially reduced by Thinning seeding with efficient ice nuclei?

Unseeded cirrus clouds

Seeded cirrus clouds Solar Radiation Modification (SRM)

• Reflecting would cool the Marine cloud – Space-based brightening – Stratospheric aerosol injection (SAI) (MCB) – (MCB) – Surface • Could also enhance outgoing radiation – Cirrus cloud thinning (CCT)

Stratospheric Aerosol Cirrus cloud Injection (SAI) thinning (CCT)

5 What is the role for SRM? Business as usual

Cut emissions aggressively

1 CO2 removal (“CDR”) 3 2 Climate Impacts Climate SRM?

Time Climate Model Simulations (of SAI)

1° warming from CO2 offset by 1° warming from CO 2 1° cooling from strat. aer. Temperature (perdegree) Precipitation (perdegree)

MacMartin et al, JGR 2019 Some Key Governance Questions

• How confident do we need to be? – Some uncertainty will not be resolvable • Who gets to decide? – Everyone is affected… whose voices are at the table? • How will decisions be made? • What happens if some people/places are harmed? – Or are perceived to be harmed? • How do you ensure this isn’t used as an excuse not to mitigate? – “Moral Hazard” or mitigation deterrence • How do you manage deployment over very long timescales? – Possibly centuries, and without any interruption Impacts would depend on how it is deployed

SIMULATION 1 SIMULATION 2 Constant aerosol injection CHANGE IN Varying injection with time of year INDIAN PRECIPITATION

• This makes it hard to talk about “the” impacts of SAI • Decisions aren’t just yes/no Not a substitute for mitigation

Business as Cut emissions usual aggressively

CO2 removal

Climate Impacts Climate …

Time

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