Turning Excess Atmospheric CO2 into Dry Ice or liquid

First, it would be better to stop burning carbon entirely. That said, we would burn a lot more carbon if we start wars trying to force people to stop. Server sky and space energy is the best alternative to burning carbon, but it isn't here yet. We will have a lot of cleanup to do, so how can we do it?

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Assume we are REALLY STUPID, and make 500ppm (by weight) of CO2 before we come to our senses. How much CO2 is that?

The atmosphere weighs 15 pounds per square inch, or about 10,000 kilograms per square meter. 500ppm by weight CO2 would be 5 kilograms of CO2 per square meter, or 5000 tons per square kilometer, or 2.5 trillion tons for the entire earth. Dry ice weighs about 1.5 grams per cm³, or 1.5 tons per cubic meter, or 1.5 billion tons per cubic kilometer. 2.5 trillion tons of dry ice is 1700 cubic kilometers - a lot of dry ice, which must be kept refrigerated.

Liquid CO2

The ocean temperature at 1km depth is about 4C; the pressure is 100 bar. At much higher pressures, CO2 stays solid to higher temperatures, but at 100 bar the melting temperature is -78C. CO2 is a liquid at those pressures and 4C, with a density about the same as water. 2.5 trillion tons of Liquid CO2 occupies 2500 cubic kilometers.

So, imagine building an enormous 4 km high covered tank from 1km down to the sea floor at 5km, 30 kilometers in diameter. The tank might be made of steel, with gasoline floats holding it up, equal pressure inside and out. CO2 seems to compress better than water, so the CO2 at the bottom will be slightly denser and pushing out on the bottom, perhaps by a few atmosphere.

The ocean has an area of around 360 million square kilometers. Displacing 2500 cubic kilometers of ocean will raise sea levels by about 1 centimeter.

We capture CO2 out of the atmosphere (somehow), compress it isothermally to liquid (10MJ/ton? WAG) and push it down the hole, perhaps 3E19 joules. With 1TW devoted to the task, we could do all that sequestration in a year. Some of it might be handy to release if we want to stop an ice age.

Launching to Space

As we produce habitats for life in space, we will need carbon and energy for them. One place to get it is from our under-ocean store. Launching will use a lot of energy per ton; If we launch with 50% efficiency to 10,000 m/s, then we will need 100GJ per ton, 10,000 times as much energy as we needed to sequester it. At a 20TW rate, the process might take 400 years.

== Turning Back into Oxygen and Carbon =

The heat of formation of CO2 is around 400KJ/mole, about 10MJ/Kg, about 20GJ per ton at 50% efficiency. At a 20TW rate, the process might take 80 years.