[pjc50]

No: the atmospheric CO2 is at the bottom of the energy hill. In order to turn it in to fuel you need to pump in all the energy originally extracted from the fuel, then some more.

After you've re-expended the entire 20th century's worth of fossil fuels energy equivalent in sucking CO2 out of the atmosphere, not burned it, and stored it in a country-sized propane tank, then maybe we're back to 19th-century temperatures.

[bluGill]

The earth is not a zero sum game, we get energy from the sun. Photosynthesis reverses the process turning co2 into sugar and o2.

[pjc50]

And? So?

[bluGill]

That means co2 is not the bottom of the energy hill.

[Udo]

"Bottom of the energy hill" is a chemist's short hand way of saying that we can't practically extract energy from CO2 reacting with anything useful, meaning chemically this is at the bottom end of reactions that happen without energy input. To convert it into something reactive, you need to invest energy to split it up again into its constituent parts, which is what happens during photosynthesis.

[bluGill]

"Bottom of the energy hill" is a useful term in a lab. However the earth is not a chemistry lab. We cannot ignore photosynthesis outside of a lab as it is a factor that exists.

[Udo]

> We cannot ignore photosynthesis outside of a lab as it is a factor that exists.

I apologize if this wasn't clear. It's an expression about energy potentials, and as such it's not just useful in a lab, it's an essential piece of information about the substance.

For example in the original context of this comment thread, a person asked whether there was a danger of people exhausting our atmospheric CO2 because they got too greedy with this technology - a question that can in fact not be answered meaningfully without talking about energy deltas. The fact that CO2 cannot be practically processed in a way that releases energy is the only pertinent information when talking about this.

[api]

In the photosynthesis case the bottom of the energy hill is helium in the sun’s core.

[XorNot]

Well and also the CO2 isn't acting as a fuel, it acts as a battery or resource.

Plants make sugar from CO2, and also burn that sugar back to CO2 (plants respire overnight - they're not pure CO2 consumption machines).