Let's have a planetary greenhouse effect only in winter?

Question

Suppose there's zillions of tons of CO2 in the atmosphere of a planet and summer temperatures become unbearable. The amount of CO2 is so big it cannot be removed anymore, or it would become an endless effort.

What could a hypothetical technologically advanced civilization on a planet do, to cause release of all that heat in the atmosphere selectively, that is to occur only during the wet and winter seasons ?

(This question is not science based and the civilization is near future SF, imagine a few hundred years, on Earth or on a similar colonized planet that requires terraforming. No magic. Frame challenges are welcome)

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Notes on comments below:

Indeed, I want to know how an advanced civilization with a world under a major greenhouse effect would cool down their world during the summer.

Southern and northern hemisphere on this planet have opposite seasons, as a result of axial tilt.

Also, comments have pointed out this idea is very location-specific. Southern and northern hemisphere have alternating seasons. I now wonder if some region with lowered CO2 concentration could be established, where the low concentration persists during summer. Then, this region with lower concentration of greenhouse gasses migrates (in some way) to the other hemisphere, to stay there for another half year. Arrange some kind of cycle in the atmosphere..

Answer 1

the civilization is near future SF, imagine a few hundred years ... No magic

You can't

There are no even remotely possible ways to control the content of the atmosphere at different locations independently with near-future tech. It would require a huge amount of energy and crazy tech/infrastructure, or else be a violation of the principle of entropy (i.e. magic).

The only scientifically possible way I can think of doing it is to just set up a huge system of pipes that somehow filter huge amounts of CO₂ out of the air and move it to the other hemisphere, but you would need to be moving it constantly because the minute it comes out the other end it's going to start dispersing towards the summer-side hemisphere again. This is not feasible for a Kardashev Type-I civilization because even just the modern-day Earth atmosphere has about 3.2×10¹⁵ kg of CO₂ in the atmosphere.

Short of erecting a huge 100km wall around the Equator or something (again, not Type-I feasible), there is no way to regionally filter the atmosphere.

As for controlling heat directly, that's even more far-fetched; even removing 0.1°C from half of the atmosphere would require a transfer of about 3×10²⁰ Joules of thermal energy (multiplied by whatever inefficiency your mechanism has), which we have no way of storing and so would just have to be released elsewhere at which point it would immediately re-disperse across the whole planet.

Answer 2

giant space sun-shade

have a giant parasol hanging above earth. Make it able to maneuver and/or to tune its optical absorbance.

I leave the details up to you. It could be a giant disk moving around to position itself over the right hemisphere at the right time. Or a swarm of reflective heat shields that can adjust their formation and angle to let more or less light through. Or lenses that refract light away. Or even a giant sphere around earth whose opacity at any point can be controlled (e.g. by filling it with varying thickness of some liquid or gas with suitable absorbance).

"Realistically", I'd go for the swarm thing. Make it solar powered, and keep it mostly around the L1 point.

You can pick whatever designs suits you but the idea remains the same. Instead of trying to pull off some insanely high tech, incredibly flexible, unbelievably precise, and horribly energy intensive solution to locally pump CO2 in and out of the atmosphere every 6 months, just deal with the source of heat directly, and let in only as much light as you need in the first place. Technologically and economically it's gonna be way easier this way

Disclaimer 1: It could be considered a frame challenge, but well, greenshouse effect essentially blocks radiation from the planet, and this solution proposes to block radiation to the planet, so you could argue it's a reverse greenhouse

Disclaimer 2: I don't personally think that it can be qualified as near-future tech, but it shouldn't be too hard to sell as such

Answer 3

Okay, this is not a near-future idea, its a wacky, far-future, experimental technology idea, but hear me out.

A network of strategically placed, carefully outfitted space elevators.

A space elevator is designed to pierce through the many different layers of the atmosphere. If we take for granted that it has been built so that it won't fall down regardless as to what is thrown at it, we have to also imagine it will have some affect on the climate. Putting up several in a calculated formation could, conceivably, have a strong effect on wind movement, cloud formation, and so on. If planned out very intentionally, their placement could be the first step in deterring super-storms from forming, or towards stabilizing a seasonal system that is spiraling out of control towards.

However, that's just what they do by existing. Now lets talk about what's on them. The elevators could have specialized equipment for more directly controlling weather in large ways, using electrical interference shot out above, below or next to cloud formations in order to artificially grow or shrink them. Thus, across the network of multiple towers, the amount of water moving through the skies at strategic locations could be at least measured and at best regulated and perhaps even controlled.

The elevators would also be outfitted with algae farms up and down their exteriors. This would be probably the most important part of the plan, using a large volume of simple plant life to directly soak up the growing CO2 levels and pump more oxygen into the atmosphere. The algae habitats would be fully controlled on an individual level, allowing people to shut off or turn on any number of segments worth of plant life in response to the atmosphere's current balance.

So, this world would have seasons and superseasons. Superseasons refer to the artificial cycle of CO2 and Oxygen, and occur across multiple years - letting CO2 build up, letting heat build up, water levels rise, etc, (Red years) and then activating the algae farms on a large scale, letting that CO2 turn into O2, only to stop the process as O2 levels get out of balance (Green years). Within that larger cycle, normal seasons would be regulated to allow for hot, warm, wet and cold in varying degrees based on the position in the superseaon.

Obviously, anything this wacky raises as many questions as it answers. How could such a system possibly be built? What unexpected effects would it have on weather, climate, atmosphere, society, evolution? What if one of the towers actually broke and fell? I'm not gonna pretend this idea isn't ridiculous. But it's an idea, and that's what makes SF so fun.