numbers, not adjectives — D. J. C. MacKay

9  Direct Air Capture

The idea in Direct Air Capture (DAC) is to run chemical processing plants which use technology to extract CO₂ from the atmosphere. Possible implications of DAC technologies include that maybe humanity shouldn’t worry too much about putting greenhouse gases in the atmosphere, since we can remove them again later if necessary. Of course, I’m enthusiastically in favour of any idea that could help solve our climate crisis. But I also have a habit of doing a back of an envelope calculation to understand what the proposal would entail.

About a third of the CO₂ in the atmosphere is currently (2024) human made, from burning of fossil fuels. For the sake of argument, to get a feel for the scale, let’s work out what it would take to remove 10% of the CO₂, this would correspond to about a third of what we’ve already added, or maybe to a smaller fraction if DAC is applied sometime in the future.

The first problem we encounter is that the atmosphere is HUGE, it weighs 5.5 10¹⁸ kg. Since we are 8000 million inhabitants on earth, that’s about 700 thousand tons per person. To remove 10% of the CO₂ we must process at least that fraction of the atmosphere, that stands to reason. But the chemical process isn’t going to be 100% efficient, let’s assume 50% capture rate. Ok, that means that for every inhabitant on earth, we need to finance, build, and implement, a chemical process plant capable of handling 140 thousand tons material. If you, like me, don’t think it is reasonable that poor people in countries that hardly contributed to creating the climate crisis in the first place should contribute, you can multiply that by a factor of 3 for those of us who should. Ok, so the scale is, a chemical plant to handle half a million tons of material per person.

And the chemical process involved isn’t simple; it’s not going to be wafting a gentle breze over a magic carbon sponge. There are fundamental energy and thermodynamic reasons that this is going to be difficult and will require huge amounts of energy.

So before we get excessively enthralled by DAC, we may want consult plans which clarify the size of the necessary investment, and who will pay, who will build and who will run these machines at an unprecedented scale. These plans would include an account of where the energy to run the machinery, likely requiring a doubling of global energy production, will come from. Unfortunately, no-one has presented such plans. Of course, you may reasonably think that DAC should only be a small contribution to solving the problem. Sure, how small a contribution? Let’s see the plan for that. Until we see these plans, we may at least be a little bit cautious about the potential of DAC.

Having read the above, a thought occurred to me: is it conceivable, that Direct Air Capture is a red herring?