Would it be feasible to deploy massive solar sails to cast a shadow and prevent solar heat from hitting the earth, so that over enough time it might reverse some of the effects of global warming?
I think the short answer is 1. it's much cheaper to simply reduce CO2 emission, and 2. "An Earth with more CO2 and less incident sunlight" is not the same as "An Earth with less CO2 and the same sunlight as before" (e.g., think of ocean acidification).
It's an idea that's been floated in scifi. The scale of it is huge, though, and you'd have to consider the CO2 emissions of the necessary launch and manufacturing. Probably only feasible if you could make it on the moon.
It might not be as expensive as it sounds. First of all a shadow can be cast using very very light-weight material, maybe even get the mass from objects already in space. Another benefit is that it can be controlled much more carefully than other proposals that have been proposed by serious publications that involve dispersing stuff into the atmosphere itself.
> First of all a shadow can be cast using very very light-weight material
Not in space. Unless the material is an absolutely perfect reflector across the entire EM spectrum, which simply isn't possible, any structure would melt pretty much instantly. You'd have to make radiators that are as big or bigger than the shade structure in order to radiate that heat away and you'd have to make the entire structure much bigger than the shade part because otherwise you'd just be radiating the absorbed heat back at the Earth.
> perfect reflector across the entire EM spectrum, which simply isn't possible
You don't need it to be a perfect reflector across the entire spectrum, 200 to 2000 nm is enough.
And according to "Solar Absorptance and Thermal Emittance of Some Common Spacecraft Thermal-Control Coatings", e.g. Kapton film reflects 92% of the entire solar spectrum.
Sorry I meant be completely transparent or reflective. If even < 1% is absorbed in any way, the heat will steadily build up and eventually start to melt the material, which changes its properties usually causing it to absorb more and more heat (causing a runaway heating effect). You can either radiate that heat back towards the sun (not going to work) or towards the earth which defeats the purpose. This is one of the most basic design constraints in any spacecraft simply because there is no way to convect heat away like we do within our atmosphere.
Since black body radiators are very inefficient, you'd have to build a massive umbrella with an oversized "heatsink" that dumps way more heat to the sides than it does towards the earth or absorbs energy from the sun. I don't even want to imagine how massive of an engineering challenge that would be.
Look into the research and tests on solar sails. It's not a difficult problem. Most of the light is reflected, and the rest is not enough to overheat it. Casting a shadow does not require 100% of photons be blocked.
Solar sails are designed to move away from a large source of photons which decreases their energy input exponentially as they move. They are also attached to a spacecraft with its own heat dumping mechanism which means most of the energy can conduct away from the sails into the rest of the more massive thermal system.
Please, remember that we literally live on solar energy. That's what we eat. And that's also the most promising alternative from the global warming causing fossil fuels.
Engineering aside, you could perhaps position this shadow strategically over some place that is uninhabited and which wouldn't cause too much weirdness in our climate. Or perhaps many small shadows. My guess would be that you would want them over the oceans. Or maybe Kim Jong-un's house.
Alternatively, if geostationary is too high up, perhaps just wrap them at a particular latitude near the equator, spaced out so they just make an effect like a perpetually cloudy day. You then just have to keep your solar panels and needy crops off that latitude.
If you want a big enough effect to detail global warming, you'll need a big enough effect to change the climate. There's no place it "wouldn't cause too much weirdness in our climate".
If you place them over the equator, you've just destroyed the places with the highest plant growing and solar energy harvesting potential. Granted it is not the most used place of the world today, just the one with most potential.
If you place the shadows over the oceans, you'll completely change the environment there, and help destroy its bigger species, that we eat too.
Well, the whole point is that the Earth, due to greenhouse effects, is taking in too much sunlight compared to what it is radiating back out into space. So yeah, you're going to be reducing the total potential harvestable sunlight getting to Earth. However, that's not really a constraint for us at the moment. We just care about tapping sunlight in places near where there is a demand for energy. So, you could probably find a nice band around the Earth where the inability to place highly efficient solar panels wouldn't be missed sorely.
By 'too much weirdness' I'm referring to instigating entirely new and dangerous weather patterns. Ideally, you could keep the weather roughly the same, with the lowered temperatures disbursed enough to create a measurable but not easily noticeable effect.
And we're also not talking about plunging the area into permanent darkness, we're talking about enough reflection to reduce average global temperatures by a few degrees (or at least prevent them from rising further). I expect that could be done, if spread out enough, without enough of an impact on any one area to impact local environments.
Of course, this is all just theoretical. I doubt it's actually a reasonable course of action. I just don't think this is why it isn't.