Is it easy to shade the Earth for cooling?

[Kevin_Hall]

The idea is rather old: we put a bunch of Al foil 10 microns thick and reflect the solar flux over the daytime part of the planet. What sea level area will be shaded by 1 km² of such foil at an altitude of 1000 km?
Does it make sense to raise such an umbrella on the Geostationary orbit?

[Prometheus]

I remember seeing a back of the envelope calculation that estimated the amount of mass needed for such a shade to be effective at cooling would require an orbital infrastructure of epic sci-fi proportions (and didn't even attempt to calculate the warming caused by building a launching all those rockets).

I think it was a science communicator called Scott Manley if you want to have a google around to find it.

Can't find it, but i have an envelope handy.

Found this study that estimated the mass of a sun shield needs to be 10^7–10^8 tonnes. Let's take 10^7. That's 10^10 kg. The current cheapest vehicle per kilo is currently the Falcon 9 at $1400 per kilo. The cost of just launching this structure would be $14 trillion. Pretty expensive but economically possible i guess, if we ignore politics.

A Falcon 9 can launch 22,800 kg per launch, which would require ~ 440000 launches. Space X have one of the best launch cadences in the world but only managed 31 last year. If we started now we might get it done in 30 years if we can increase the launches to 15000 a year. 

Apparently a Falcon 9 produces 360,000 kg of C02 per launch. So we'd be adding 10^11 kilos, or 10^8 tonnes, of C02 into the atmosphere by the end of the project. 

All this is based on costs to launch to LEO. L1, the most feasible place it could sit, would be far more expensive (both $ and C02) - but those numbers aren't so easy to get.

I conclude that it's not feasible with current tech, and by the time it does become feasible it would be too late to significantly contribute i.e. the damage would be done. But feel free to check these numbers because i haven't.

[Prometheus]

Additionally geostationary orbit wouldn't work. This orbit means an object is always above the same point on Earth which means at night time its pointing away from the sun.

[swansont]

17 minutes ago, Prometheus said:

Additionally geostationary orbit wouldn't work. This orbit means an object is always above the same point on Earth which means at night time its pointing away from the sun.

It only works a very small fraction of the time (near noon near the equinoxes), because it’s only over the equator

2 hours ago, Kevin_Hall said:

What sea level area will be shaded by 1 km² of such foil at an altitude of 1000 km

The sun’s rays are very nearly parallel, so it’s about a square km

 

A shield that was perpetually blocking the sun would heat up and radiate, reducing the effectiveness of the shield.

[Prometheus]

2 minutes ago, swansont said:

It only works part of the time, and only over the equator

Why the equator? I thought the Earth's tilt took it out of the ecliptic plane?

[swansont]

4 minutes ago, Prometheus said:

Why the equator? I thought the Earth's tilt took it out of the ecliptic plane?

Geostationary orbit is equatorial

https://en.wikipedia.org/wiki/Geostationary_orbit

The tilt is why it won’t block the sun except when the sun is lined up over the equator

[Externet]

A 1km² reflective/shading foil in orbit at 1000km altitude will create the same shade as placing the same 1km²  foil on top of the earth soil at the warmest place or where cooling may be most needed (Sahara?, Arizona?) without the cost/energy of launching it.

 

[Prometheus]

2 minutes ago, swansont said:

Geostationary orbit is equatorial

Makes sense.

[swansont]

3 minutes ago, Externet said:

A 1km² reflective/shading foil in orbit at 1000km altitude will create the same shade as placing the same 1km²  foil on top of the earth soil at the warmest place or where cooling may be most needed (Sahara?, Arizona?) without the cost/energy of launching it.

 

I imagine the point of launching is that it doesn’t get in the way of doing things on the ground.

[Externet]

Placing the foil on the earth surface will not be restricted to equatorial shading, Will not last much under the wind, will create complains, but will equally shade/reflect.

 

[Ken Fabian]

2 hours ago, Kevin_Hall said:

we put a bunch of Al foil 10 microns thick and reflect the solar flux over the daytime part of the planet.

No unpowered objects will stay where they are put; the light pressure and solar wind will blow the foil away. Any mirrors/shades in space need to be able to do station keeping - ie move about with drives to keep them in place and aligned. There will be more mass than just foil.

My understanding is these approaches also are likely to result in regional climate changes above and beyond simply being overall cooler.

If emissions continue the ocean pH impacts will not be resolved.

 

I don't think there are any "just do x" solutions that are better or cheaper or quicker or easier or more compelling than a primary solution of "just build an an abundance of clean energy". I suppose it has value to explore the feasibility of all options but not at the expense of the options we already have or are close to viability.

My impression is that the space based ones are championed principally by the grand space dreamers for whom greatly expanding space capabilities and getting lots of humans into space - often with the express intention of being able to leave Earth and it's problems behind - are their priority. It won't go well to abuse and misuse the legitimate concerns about climate change to advance quite different goals and ambitions.

Edited January 6, 2022 by Ken Fabian
clarity

[J.C.MacSwell]

8 hours ago, swansont said:

It only works a very small fraction of the time (near noon near the equinoxes), because it’s only over the equator

The sun’s rays are very nearly parallel, so it’s about a square km

 

A shield that was perpetually blocking the sun would heat up and radiate, reducing the effectiveness of the shield.

How about controlling something/s at the Lagrange point between Earth and Sun. Close enough that it wouldn't need to be exceptionally large to have a considerable effect, and far enough that most of any re-radiation would not effect the Earth, assuming it's/they are shaped, oriented, and controlled correctly.

Edited January 7, 2022 by J.C.MacSwell

[swansont]

6 hours ago, J.C.MacSwell said:

How about controlling something/s at the Lagrange point between Earth and Sun. Close enough that it wouldn't need to be exceptionally large to have a considerable effect, and far enough that most of any re-radiation would not effect the Earth, assuming it's/they are shaped, oriented, and controlled correctly.

That’s the scenario from Prometheus’s post 

[mistermack]

18 hours ago, swansont said:

A shield that was perpetually blocking the sun would heat up and radiate, reducing the effectiveness of the shield.

That's likely to be a neglible effect. Most of the solar energy would be reflected by the shield back towards the Sun, not absorbed. And the energy it does radiate would be heading in all directions, only a small portion would be heading towards the Earth depending on how far from the Earth the shield is positioned. 

[Prometheus]

53 minutes ago, mistermack said:

That's likely to be a neglible effect. Most of the solar energy would be reflected by the shield back towards the Sun, not absorbed. And the energy it does radiate would be heading in all directions, only a small portion would be heading towards the Earth depending on how far from the Earth the shield is positioned. 

But it would threaten to knock the structure off the L1 point and so require a significant amount of fuel to keep it in place.

[swansont]

59 minutes ago, mistermack said:

That's likely to be a neglible effect. Most of the solar energy would be reflected by the shield back towards the Sun, not absorbed. And the energy it does radiate would be heading in all directions, only a small portion would be heading towards the Earth depending on how far from the Earth the shield is positioned. 

 

A shield as described would be send the net radiation perpendicular to the surface. Basically half would head to the earth. The shield would also reflect radiation from the earth back to the earth.

The distance doesn’t really enter into it.

[mistermack]

4 minutes ago, swansont said:

A shield as described would be send the net radiation perpendicular to the surface. Basically half would head to the earth.

Why is that?

10 minutes ago, swansont said:

The shield would also reflect radiation from the earth back to the earth.

That would also be negligible.

[swansont]

3 hours ago, mistermack said:

Why is that?

Symmetry. There’s no preferred direction for the radiation, so it should be isotropic, and it’s a 2-D system.

 

3 hours ago, mistermack said:

That would also be negligible.

Why? How small is negligible?

[mistermack]

3 hours ago, swansont said:

Symmetry. There’s no preferred direction for the radiation, so it should be isotropic, and it’s a 2-D system.

That seems to be completely self-contradictory. You previously said that the radiation would be emitted perpendicular to the shield, you can't get any more preferred than that. 

In any case, if that really was the case, you would only need to tilt the shield a tiny amount, and the Earth radiation and the radiation from the shield would miss the Earth completely. 

[swansont]

10 minutes ago, mistermack said:

That seems to be completely self-contradictory. You previously said that the radiation would be emitted perpendicular to the shield, you can't get any more preferred than that. 

Not at all. I said net radiation. Anything at some non-normal angle would be canceled out, when averaged over the surface. No preferred direction in the plane of the reflector.

 

10 minutes ago, mistermack said:

In any case, if that really was the case, you would only need to tilt the shield a tiny amount, and the Earth radiation and the radiation from the shield would miss the Earth completely. 

At 1000 km above the earth? Show me the calculation.

[mistermack]

37 minutes ago, swansont said:

Not at all. I said net radiation. Anything at some non-normal angle would be canceled out, when averaged over the surface.

No, you said "Basically half would head to the earth."

44 minutes ago, swansont said:

At 1000 km above the earth? Show me the calculation.

I thought it had already been established that that wouldn't work. The L1 point is what I'm talking about, and that's 1.5 million km away.

But it was you who said :

 

8 hours ago, swansont said:

The distance doesn’t really enter into it

 

[Ken Fabian]

On 1/7/2022 at 6:58 AM, Ken Fabian said:

No unpowered objects will stay where they are put; the light pressure and solar wind will blow the foil away.

For L1 positioning most of the light pressure may be compensated for by placement closer to the Sun - ie Sunward of L1. I hadn't considered that. I don't think that will work for anything in low orbit - and it would still not be stable near L1, ie still requires the means to align and move, but with less fuel.

 

On 1/7/2022 at 6:58 AM, Ken Fabian said:

My understanding is these approaches also are likely to result in regional climate changes above and beyond simply being overall cooler.

Doing a bit of reading reveals that it is hypothetically possible to have some active control over the relative dimming regionally as well as tropics vs poles - via paired occulting discs (iiuc) and that ability to move and align. I wasn't aware of that.

These issues are a long way from solved and it still leaves us with ocean pH change - maybe more so if space shades are used as the excuse to allow ongoing and unconstrained emissions.

Those aside we are still talking about (according to that source) about 10⁷ - 10⁸  metric tons for the array itself. The array will not be the entirety of what needs to be launched. So far there has been around 20,000 metric tons total launched into space - with much of that to low orbits and expected to return to Earth as orbits degrade. At US$1m per ton to launch 10's and 100's of millions of tons gets extremely expensive - and still extremely expensive even if we see costs reduced by another order of magnitude, ie US$1,000,000,000,000 - $10,000,000,000,000 .

I don't think it is possible to grow space launch capabilities that much or in time for it to be much help on our climate problem, especially for a major project that doesn't generate any income directly.

Aside from the technical matters... who pays for it?

 

 

Edited January 8, 2022 by Ken Fabian

[swansont]

11 hours ago, mistermack said:

No, you said "Basically half would head to the earth."

No? I can see where I said it. “A shield as described would be send the net radiation perpendicular to the surface. Basically half would head to the earth.”

 

11 hours ago, mistermack said:

I thought it had already been established that that wouldn't work. The L1 point is what I'm talking about, and that's 1.5 million km away.

That was part of my explanation as to why it wouldn’t work.

I said “the shield as described” so it should have been clear what I was talking about

 

[mistermack]

13 hours ago, Ken Fabian said:

Those aside we are still talking about (according to that source) about 10⁷ - 10⁸  metric tons for the array itself.

That sounds like a heck of a lot. Were they talking about a shield that could completely block out the Sun? To "correct" the climate, it would surely be only necessary to block a small proportion of the total sunlight. At a guess, less than one percent. 

In any case, it would be a risky thing to do, since in historical terms, we are overdue a major glaciation event. 

[Prometheus]

13 minutes ago, mistermack said:

That sounds like a heck of a lot. Were they talking about a shield that could completely block out the Sun? To "correct" the climate, it would surely be only necessary to block a small proportion of the total sunlight. At a guess, less than one percent. 

In any case, it would be a risky thing to do, since in historical terms, we are overdue a major glaciation event. 

That was from the 2015 study I gave. They didn't get those number themselves but provide these references to justify it:

1. Seifritz W. Mirrors to halt global warming? Nature. 1989;340(6235):603.
2. 10.Early JT. Space-based solar shield to offset greenhouse effect. Journal of British Interplanetary Society. 1989;42(567–569).
3. 11.McInnes CR. Minimum mass solar shield for terrestrial climate control. JBIS. 2002;55(9–10):307–11.