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> This seems very inefficient due to the high energy cost and the long timescales in the proposal.

This is efficient for what the paper is about: A solution to the sun's gradual evolution towards a red giant. So long timescale is not a problem, since we have 1B year before the sun becomes too hot for where we are today, and 5B years before being engulfed by it.

Space sunshades works well to reduce incoming radiations by a few % (e.g. for current climate change problem), it won't work so well when the earth will be within the sun's corona.

No--we have nowhere near a billion years. It's roughly a billion until all life is wiped from the planet, but it will be uninhabitable for humans *long* before that point.

We have only 50 million years before Earth's ability to adapt to solar warming pegs and the temperature starts crawling up--and note that even that will not be good for an awful lot of plant life because of low CO2 levels. Earth needs to start it's retreat before then.

> We have only 50 million years before Earth's ability to adapt to solar warming pegs and the temperature starts crawling up

This sounds like an oddly small estimate. Life has been around 3.5 billions of years and has passed through multiple extinction events. It would be a puzzling coincidence that human civilization appears and then we have only some tens of million years to avert catastrophe.

Doesn't necessarily sound like a bad estimate just based on approximate frequency of other extinction events. What they're describing doesn't necessarily sound like the end of life on Earth, only the end of life as we know it.

I'm not following what the mechanism is supposed to be though.

Mechanism: To keep the Earth cool the greenhouse effect needs to be turned down. That happens by CO2 being absorbed by water and turning into limestone. This is countered by limestone weathering and limestone getting sucked into volcanoes.

Over the eons the sun has grown warmer and Earth's CO2 levels have dropped, keeping the overall temperature pretty much stable. (There have been fluctuations in both directions that were major extinction events, though.)

Warming increases the rate CO2 gets converted into limestone, thus lowering the temperature. 50 million years is when that pegs and the mercury starts rising.

As for it being a puzzling coincidence--it might not be a coincidence at all. Think of the Fermi Paradox--where are the aliens? These days the astronomers tell us planets are very common. A look at the emergence of life on Earth says it happened about as fast as possible. Why are we alone? Enter the Great Filter hypothesis--*something* stops species from colonizing the galaxy. Looking at what has happened on Earth there are only three likely candidates:

1) The jump to multicellular life. It took billions of years. One data point isn't enough to determine an average but we can see it's not something that happens easily.

2) The emergence of intelligence. Same situation, again.

3) The lifespan of intelligent civilizations.

The odds of successfully crossing these three hurdles must be billions to one. Either intelligent races destroy themselves before reaching the stars or the billions to one odds are in the first two hurdles.

The most favorable interpretation for humanity is that the average time to cross those hurdles is well beyond the couple of billion years it took Earth, most planets fail to produce intelligence before their ecosystem gets fried or smacked with something substantially larger than the dinosaur killer. If this is the answer it's no surprise we squeaked in just under the wire.

And if that's not the answer humanity isn't going to be around for too much longer.

Putting them in Earth orbit would allow for local variation, but would be much less efficient overall. Firstly because they would only be between the Earth and the sun for a very small fraction of their orbital period, and also because they would be so close to the earth they would only shade a relatively small multiple of the area of the satellites.

This is why most proposals put the sun shade(s) in sun synchronous orbit between the earth and the sun. They would be a lot closer to the sun, and so intercept a lot more sunlight per area of satellite, and they would be blocking it all the time, or for as long as you wanted.

I think that's what the parent is referring to (from the wikipedia page they are in L1). Of course that does require the active orbital stabilization also described (though not as around L1). Being closer to the sun is a big advantage, but also impulse heavy to get there.

Perhaps an initial lift up to an unshadowed low earth orbit could boot strap a solar ion/sail to get them to L1?

> supercomputer to predict the chaotic effects.

This might be the hardest part of your proposal to achieve. Chaotic systems are fundamentally difficult to simulate, not just because of their differential equations but our lack of perfect knowledge of their initial conditions. Blanket solar reduction seems more feasible?

The Montgomery Burns plan. The Simpsons will have again predicted the future better than any science fiction writer could dream.

...except that 1) it hasn't happened in the real world, 2) it was proposed in December 1989 (if not earlier), about the same time as the first episode of The Simpsons aired. https://ui.adsabs.harvard.edu/abs/1989JBIS...42..567E/abstra...

“will have”

> you could use the swarm to focus sunlight on a certain part of the Earth

Whenever I see a proposal like this (e.g. orbiting solar power stations beaming microwaves down to terrestrial receivers) I think "super-weapon." I can't see why country A would ever allow disliked country B to implement such a scheme.

I believe, much before those timescales humans would have become a Type 2 or Type 3 civilisation.

A sunshade is almost feasible using current technology, though it would have some negative impacts.

Solar Cruiser is a 1,672 m2 solar sail being sent to L1 for 65 million. Which is of course nowhere close to cost effective for lowering global temperatures, but the basic components are there.

In Accelerando [1], humanity becomes a Type II civilization and builds a Matrioshka swarm. But it shines sunlight on Earth by opening up a hole in the swarm so that the sun is visible to Earth. That would allow harnessing almost all of the Sun's energy but would let Earth remain as a stable nature preserve for nostalgia purposes.

[1] http://www.antipope.org/charlie/blog-static/fiction/accelera...