A physicist runs the math on direct air capture and warns it's a distraction

The article makes a valid point that the amount of energy required for direct air capture (DAC) is extremely prohibitive and far beyond current capacity, but assume for the sake of argument that this is a problem that could be solved.

This still leaves the problem that the industrial plant and chemical feedstock required to physically do the DAC at the scale of billions of tons per year is several orders of magnitude beyond what is possible with current supply chains and natural resources. You would have to strip mine the planet just to have the raw materials to build the necessary DAC infrastructure, and we need those resources for other critical things as well.

Some proposed DAC chemistries have upstream mineral dependencies that, if done at the scale required just to offset a year's emissions, would consume all known global reserves immediately. This is not solved by recycling because of compounding loss rates; recycling is not 100% efficient, and the amount of energy required to improve recycling efficiency is essentially exponential for rapidly diminishing returns, and we already have an energy problem with DAC.

DAC at any scale that is not an exercise in futility will be an exercise in environmental and economic destruction at unprecedented scale. The solution would almost certainly be worse than the problem. There really isn't a path forward that doesn't involve dramatically reducing emissions and letting nature remove the excess carbon.

Capturing only makes sense at the emission sources where the co2 concentration is high, like at power plants or tailpipes. But that would mean the costs would be borne by the emitters but they'd rather externalize those costs to all of us.

Enhanced rock weathering for CO2 removal potentially avoids the high energy requirement problem of DAC, since it relies directly on natural environmental processes to work. But it has it's own issues and question marks.

When I was in college, a physics professor said investment in solar is a waste of time and that we should invest in nuclear instead. A part of his calculation was the efficiency of solar cells at the time, but he didn’t take into account the radical increases in efficiency that investment was able to bring, making our investments in solar worth it today.

I think this may be similar, in that maybe the current solutions are not going to solve the problem, but by investing in these, in the long run, they may prove to be necessary as we get better at it.

Matt Ferrell I think talked a while back about an air battery that captured carbon dioxide while charging and release it while discharging. If the losses aren’t too bad, this could be a cogeneration solution where the batteries are used for load leveling. In essence you are using mostly surplus energy for the task anyway. The carbon footprint of “green” energy is often not that good if you consider the entire cradle to grave lifecycle. Carbon capture would make things less murky.

I agree we shouldn’t do direct air capture instead of reducing CO2 emissions in the first place, but 420ppm is already too high, we need to remove CO2 from the air somehow.

The numbers he posts are not all that bad really. It's 181kWh per metric ton CO2. At an electricity cost of 5 cents/kWh that's nine bucks per ton. That's quite a bit cheaper than the actual cost projected by real projects, so I'm not sure what he's debunking.

In terms of energy, some emerging tech is pretty close to ideal. Princeton has a project they claim can do 0.7 gigajoule/ton, or 195kWh [1]. And MIT is claiming one gigajoule/ton, or 278kWh [2].

If we transition to a wind/solar/battery grid, the cheapest option in the US at least is to have about four days of battery and 2X overproduction [3]. So that's a lot of extra energy basically available for free. Even today, with relatively small usage of renewables, utilities sometimes let electricity costs go negative [4]. If capital costs aren't too high, then DAC can run whenever there's excess energy available.

Of course, capital cost will be a big portion of the expense, though it'll decrease as we scale. Climeworks and Carbon Engineering have estimated $100/ton in total cost, which equates to a dollar per gallon of gasoline. Many places already have gas taxes higher than that. And the MIT and Princeton methods should be cheaper at scale than current methods would be.

Actually eliminating all our annual emissions by just doing this would of course be madness. But if we tried, and charged emitters for doing that, then most emitters would find it cheaper to stop emitting instead. Then we'd naturally end up only using DAC for emission sources that are very hard to eliminate.

After we hit net zero, we'd be smart to take CO2 down to a safe level. Call it 100ppm of reduction. Hopefully we can do some of that with reforestation and so on, but let's say DAC is our only option. One ppm is 7.8 gigatons CO2, so we're talking 780 gigatons to draw down. At 200kWh/ton that's 156 TWh, not far off from the world's entire energy usage for a year, matching the article's estimate.

But if we're at 2X overproduction on energy, then we'll have that much energy available. Only the capital cost of the equipment would really be important, and by the time we scale it that far, that should be fairly low too. The methods I linked use cheap, readily available materials.

So it looks pretty feasible to me. We can call this a "distraction" and put up with the terrible consequences of whatever high CO2 level we reach by the time we're at net zero, or we add DAC to a wind/solar/battery world and get down to a nice cool 350ppm. Since it takes time to scale, and climate feedbacks give us a time limit, we need to develop and start scaling DAC now.

[1] https://engineering.princeton.edu/news/2024/03/14/engineers-...

[2] https://news.mit.edu/2019/mit-engineers-develop-new-way-remo...

[3] https://caseyhandmer.wordpress.com/2023/07/12/grid-storage-b...

[4] https://archive.is/WfUIJ

Cheap direct air capture: grow biomass, sink to bottom of ocean.

Were they talking there about the Marxwell's demon?

Anyway, CO2 capture, by itself, is not the whole solution, but it should be part of it. There is an unbalance between the energy that comes in and the one that goes out. so things heats up (among other negative consequences, like i.e. ocean acidification). You won't solve the whole problem without addressing that eventually, so some way of carbon capture, going through the use of energy or something else, should be in the map.

And the complementary part of that is stop adding new (or, in other way to see it, very old, but that was stored away) carbon to the system. Even with the very worrying signs of the global climate going haywire with the corresponding loses of lives and money we keep adding each year even more fossil carbon than the previous one, no matter how much token energy generation is announced to be done by other means.

It may be expensive, or require technologies that we don't have yet, or compromises on what to lose, or whatever, but it is a problem that if left by its own means will eventually make the planet unlivable for us in a way or another. And that will turn all the saved money into something meaningless.

I know this is a somewhat flippant comment but...

Trees.