Comment by kragen

Yes, if large amounts of energy are available. I haven't done the calculations in a long time, but I thought I came up with a ballpark of 10% of current global marketed energy consumption for a few decades for atmospheric carbon capture. We can place a very firm upper bound of about four times current world marketed energy consumption.

To avoid any concerns about scalability, as well as about energy supply intermittency, I made my estimate using only the oldest process in the chemical industry, lime burning, which predates plastics, oil drilling, steel, iron, bronze, writing, cities, ceramic, and possibly even agriculture. The only difference from the Neolithic method is that you have to retort the lime in a sealed chamber so you can collect the carbon dioxide it absorbed from the atmosphere after the last time you calcined it. This doesn't require an enormous amount of machinery, just very large machinery. Basically, a giant tin can similar to a water tower, maintained at a mildly negative gauge pressure as it's heated up.

My estimate, IIRC, was that, without soda for process intensification, you would need an amount of limestone a few times larger than the amount mined by the cement industry every year. That's fine, though; limestone is about 20% of all sedimentary rock, and sedimentary rock is 73% of Earth's land surface and 8% of the entire crust.

Very roughly, Earth is 6e24 kg, her crust is 6e22 kg, and its limestone is 1e21 kg. By contrast, the 427 ppm of carbon dioxide we need to capture half of is only 3.34 teratonnes, 3e15 kg. Limestone is 44% carbon dioxide by mass, so it absorbs 44% of its mass in carbon dioxide each time through the cycle. This absorption takes about 5 years if it's sitting in a paper bag dry, about a month when you whitewash a wall with it, or a second or so when you catalyze the absorption with a few percent of lye, like in a scuba diving rebreather.

The USGS publishes a lot of information about the world lime market at https://fred.stlouisfed.org/series/PCOALAUUSDA). So we know that producing a kg of quicklime can't require more than about 2 kg of coal, which provides 33MJ/kg or less (0.7¢/kWh or US$1.80/GJ), so 70MJ per kg of quicklime or of carbon dioxide. That's probably not a very tight upper bound, but I doubt it's high by more than a factor of 3.

Removing 1.4 teratonnes of carbon dioxide over 40 years is 1.1 million kg per second. At 70MJ/kg this multiplies out to 78 terawatts, roughly four times world marketed energy consumption.

Today, devoting a couple of terawatts to the problem would be unreasonably expensive, and tens of terawatts would require expanding world energy production considerably. At 24¢ per kg of carbon dioxide, removing 1.6 trillion tonnes of it would cost 400 trillion dollars, four years of world GDP (say, 10% of world GDP over 40 years). But that's just because the rollout of photovoltaic energy has just begun; the majority of the 18 terawatts or so of world marketed energy consumption is still supplied by fossil fuels, although they are clearly no longer cost-competitive with PV. PV manufacturing is still scaling up, though, and presumably PV energy production will exceed current world marketed energy consumption in a few years, and then continue to increase as new uses are found for the newly much cheaper energy.

78 terawatts is 0.04% of the 174 petawatts of terrestrial insolation.