What is a planetary sunshade?

A Planetary Sunshade is a space megastructure at Sun-Earth Lagrange 1 (SEL-1) could prevent the climate from experiencing a devastating temperature rise. It could also produce clean energy for use in space or on Earth.

How will a planetary sunshade reverse global warming?

Global temperature is regulated by the balance between heating from the sun and cooling through the atmosphere into space. Greenhouse gas emissions have destabilized this balance by trapping more heat in the atmosphere - this is called radiative forcing. A planetary sunshade reduces radiative forcing by reflecting sunlight back into space - about 1% shading is enough and a sunshade could be made smaller or larger as needed.

Don’t we need to stop burning fossil fuels?

Yes! There is no substitute for emissions reduction, and it should happen as fast as possible. A planetary sunshade deals with the consequences of carbon that is already in the atmosphere - and the less we emit the easier it will be to manage the consequences.

But climate change is already here, and will get worse before it gets better. We can stop making it worse by emissions reduction, but we can only make it better through carbon dioxide removal and solar radiation management.

Isn’t carbon dioxide removal enough?

Probably not. What’s hard about carbon removal is that it’s energy intensive - to a first approximation we’ll need to pay back the energy debt of industrial civilization to do it. That will require a global energy surplus to drive capture and sequestration of about a trillion tonnes of CO2.

It’s important to develop carbon dioxide removal technology now, but scaling it up to climate-relevant levels might not be possible until the renewable energy transition is largely complete. Once we get started at scale, capturing and sequestering all that carbon will take decades to centuries. Our climate crisis doesn’t allow that much time.

Isn’t solar radiation management controversial?

Yes. It’s a giant band-aid for the underlying problem, and some scholars and activists feel that it serves as a distraction from the need to reduce carbon emissions now. The urgency of emissions reduction is hard to overstate.

That said, climate change is here now, and getting worse. If we want to undo the damage, we will need to talk about solar radiation management alongside emissions reduction and carbon dioxide removal.

How is a planetary sunshade different from atmospheric geoengineering?

The cheapest and fastest technology for solar radiation management is atmospheric geoengineering by stratospheric aerosol injection. It would make the upper layer of our atmosphere more reflective and counteract radiative forcing from greenhouse gasses. These aerosols could be effective, but would need to be continually replenished to maintain the reflective layer. They also necessarily involve deliberate alteration of atmospheric chemistry, and study is required to know if the benefits would outweigh the risks.

A planetary sunshade would be outside our atmosphere, in deep space. It would not directly affect atmospheric chemistry, and could have fewer side-effects. It could remain in position as long as needed, and be moved out of the way when no longer required. Advanced concepts like a spectrally selective sunshade could better restore both temperature and rainfall, and a photovoltaic sunshade could possibly transmit enough energy to Earth to power carbon dioxide removal.

How would you build a planetary sunshade?

A planetary sunshade is conceptually a giant solar sail, a technology that already exists. The challenge is scale - it needs to be on the order of 1 million square kilometers to cool the planet by 1 degree C, and would mass at least 100,000 times as much as the international space station.

Because of this, planetary sunshades have been considered science fiction until now. But new heavy launch vehicles like SpaceX’s Starship are designed to make projects like this possible. Building a planetary sunshade is possibly easier than building a city on Mars because launch opportunities are available year-round, while the planets align only every two years. If SpaceX can build the 1,000 Starships needed to build a city on Mars, then there will be enough launch capacity to build a planetary sunshade.

Fundamentally, there are two possible construction strategies. The first is to construct solar sails on Earth, fold them up and launch them for deployment in space. The second is to source raw materials beyond Earth’s gravity well, from the Moon or near-Earth asteroids, and build solar sails in zero gravity. We are pursing both options, and think that if a planetary sunshade is built the initial phases of construction will be an Earth-launched architecture while the later phases will use space resources and in-space construction.

A project this large influences the technology used to build it. We can start at the scale we’re able to, and learn how to build a sunshade as we go. The Human Genome Project used a similar strategy - setting a goal and using the project’s growth to make exponential progress toward it.

How far along is the technology?

Solar sails are a proven technology that have flown on several missions. Starship is rapidly developing and should demonstrate an orbital launch soon. If the pace SpaceX’s earlier development is a guide, the system should be meeting its design goals around 2030. A mature Starship system will change spaceflight in the same way that going from biplanes to jet airliners changed air travel.

Space resource utilization is farther along than you might think. Since the dawn of spaceflight, researchers have been developing ways of using the mineral and energy resources of space to break the limits to growth on Earth. Until recently, spaceflight has been too expensive to demonstrate these experiments on the Moon, asteroids or Mars, but the first experiments are now flying and learning how to use space resources is a fundamental objective of the Artemis missions and of Mars exploration.

Would the international community allow a planetary sunshade?

The Outer Space Treaty states that the exploration and use of space shall be carried out for the benefit and in the interests of all countries, irrespective of their degree of scientific or economic development. We think that is a strong starting point to develop policy around.

It’s ultimately a political decision. Political systems tend to ignore future problems but respond to crisis and catastrophe after the fact. As climate change gets worse, it is already driving political change. It’s possible that the opportunity to restore Earth’s climate can foster international cooperation.

Who is going to pay for research and development of a planetary sunshade?

The Planetary Sunshade Foundation works with visionary donors and partners to develop this concept. Any deployment decision needs to be left to governments, but right now it is up to civil society to articulate planetary solutions for the anthropocene.