How to reduce the greenhouse effect while keeping the same amount of CO2?

So in my story, a group of people must colonize a planet that is habitable with a lot of sources of water and oxygen, and they need the right temperature, but I wasn't able to calculate the Greenhouse effect of 9% carbon, so my first question is how hot the planet will be? and if I place it a bit farther (Like 0.55-6 or so) how much it will cool down?

Even if the planet became colder than the average temperature of Earth but can support liquid it will be fine for me.

And if farther is not enough, how to reduce the carbon? or is there a way to make an anti-greenhouse effect?

Can the carbon cycle reduce the CO2 to reach an Earth-like amount?

My planet is orbiting a K type Star/Orange dwarf and doesn't have life on it.

Planet:(Unit is Earth = 1) Mass: 1.8 | Radius: 1.2 | Atmosphere: 88% N2 / 9% CO2 / 1.5% Argon / 1.5 other gasses. with Presure at sea level of 1 atm.

Semi-major axis(Changeable): 0.5 AU (Note that ~0.52 AU is the distance where the heat and energy received from the star is the same as Earth does from the sun), I don't know if it would be tidally-locked to the star, so just in case I made it have a very big moon with 0.2 Mass and 0.6251 Radius, so it can break the lock and make it locked to the moon instead, with a semi-major axis of 84500 km from the main planet center, which caused a day of almost a 48 hour(I'm fine with that). And I'm fine with binary-planets too.

With ~25% land and ~75% Sea of H2CO3(Carbon Cycle caused Low CO2 on the atmosphere + Sea being H2CO3) Of course, it does have a magnetic-field and plates tectonic.

Star: (Unit is Sol/Sun = 1) Mass: 0.72 | Radius: 0.78 | Luminosity: 0.2687 | Surface Temperature: 4710 Kelvin

And I don't know if my planet is plausible at all, so sorry you can ignore it and edit some properties if u want.

I forgot to mention that the atmosphere must be survivable, I want the atmosphere to be safe for the body so you will only need an oxygen mask to be able to live there. (Edited)

• Hey Khalid. Why do you need 9% carbon dioxide? Can you just assert the atmosphere and star hotness and planet distance is what it is to make your planet what your story wants? Or are you constrained to work with a planet someone else made up? Commented Feb 6, 2021 at 17:37
• The carbon dioxide was a random number xD, My story is about a group of people who were forced to live on a planet with only small resources that they got from the earth like cuttle, recycling machines, etc. I need CO2 to give them a resource of Oxygen and planets, they also can get from the ocean. Commented Feb 6, 2021 at 17:43
• If it's habitable as it is, it is probably the greenhouse effect keeping it warm. Reduce it to something like Earth's pre-industrial ~350 ppm, and the planet will freeze. (See "Snowball Earth".) The problem for your colonists is that 9% CO2 is likely to be toxic: en.wikipedia.org/wiki/Carbon_dioxide#Toxicity Commented Feb 6, 2021 at 18:15
• Would it be bad for the body itself? because I want them to only need an Oxygen mask. Is exposing your body to high concentrations of CO2 dangerous? Commented Feb 6, 2021 at 18:31
• @Khalid: Not as far as I know, but I'm certainly no expert. If I had to guess, I would say it's rather like water: you can swim in it, but not breathe it. Commented Feb 7, 2021 at 4:52

If you increase the reflectivity of the high atmosphere, less star energy will reach the surface and therefore the planet temperature will drop.

This is what happens with the nuclear winter (when the atomic mushroom shots a lot of dust high in the atmosphere) or with massive volcanic eruptions (which again shots high in the atmosphere a lot of dust and chemicals).

I guess you prefer the second option, since nuclear bombs seem to require some sort of technology to be detonated. If your planet has frequent large volcanic eruptions, comparable with Krakatoa or Tambora eruptions, your planet will be able to enjoy a high amount of CO2 without worrying for overly hot weather.

The eruption injected an unusually large amount of sulfur dioxide (SO2) gas high into the stratosphere, which was subsequently transported by high-level winds all over the planet. This led to a global increase in sulfuric acid (H2SO4) concentration in high-level cirrus clouds. The resulting increase in cloud reflectivity (or albedo) reflected more incoming light from the sun than usual, and cooled the entire planet until the sulfur fell to the ground as acid precipitation.

During the northern hemisphere summer of 1816, global temperatures cooled by 0.53 °C (0.95 °F). This very significant cooling directly or indirectly caused 90,000 deaths. The eruption of Mount Tambora was the most significant cause of this climate anomaly.[18] While there were other eruptions in 1815, Tambora is classified as a VEI-7 eruption with a column 45 kilometres (28 mi) tall, eclipsing all others by at least one order of magnitude

• Sorry but I forgot to mention that the atmosphere must be survivable, I want the atmosphere to be safe for the body so you will only need an oxygen mask there. Commented Feb 6, 2021 at 17:46
• @Khalid volcanic eruptions don't make the global atmosphere unbreathable. After those eruptions we are still here.
– L.Dutch
Commented Feb 6, 2021 at 17:48
• I know but wouldn't the H2SO4 be bad for our body? and I don't know if the planet will heat up and become hot again. Thanks anyway it gave me many ideas. Commented Feb 6, 2021 at 18:03
• @Khalid, high level cirrus clouds. That's around 8-9 km above sea level on Earth. Definitely not a place for mass gatherings.
– L.Dutch
Commented Feb 6, 2021 at 18:06
• There are no reports of people having trouble breathing because of Mount Tambora
– Mary
Commented Feb 6, 2021 at 18:48

In addition to L.Dutch's idea, you could build huge radiators above the atmosphere. Water is heated using heat pumps drawing thermal energy from the atmosphere, which is then pumped up a Space Fountain to the radiators, where it cools and then falls back down. This is a win-win, the water will be your coolant cooling your world and the working fluid holding your space fountain up, which is great because space fountains are helpful orbital infrastructure.