[stickyricky]

> almost any other carbon source is much more available

Two things: the author addresses this by saying these other sources will become more scarce in the future (undeniably true as they're non-renewable however who knows how the economics of this will actually shape up).

The second thing I'll say is that despite the availability of alternatives there are externalities to burning it (i.e. climate change). Air extraction may be less efficient but that inefficiency may be worth it to A) prevent continued CO2 pollution and B) reverse existing CO2 pollution.

[dgroshev]

There is effectively unlimited amount of carbon in the form of limestone, at the very least.

[marcosdumay]

The air is the most limited source of carbon we have. There is probably more of it in coal, and certainly more in fossil fuels.

But anyway, a huge part of the Earth's crust is composed of carbon-rich rocks. If we ever take the carbon from the air, it will be to regulate its amount. Taking it from rocks is much easier and requires a comparably tiny amount of infrastructure.