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Some people think that carbon capture systems are the way to solve climate change (and yes, I recognise that not everyone thinks that carbon is even a problem to be solved)

Historically speaking, the most effective carbon capture in nature seems to me to have been sedimentary rock in the form of carbonates. At one point in the Archaean period, the earth was apparently at like 22 bar of CO2, and the period right before the oxygen catastrophe would have had much of that carbon being locked up in rock, and much of the oxygen getting sucked up by sinks like the plains of iron that were suddenly rusting.

Honestly, the oxygen catastrophe should be on of the most important geological events in human history, and something we should teach to every schoolchild. Besides changing the atmosphere into something made up of mostly oxygen, the oxygen catastrophe also caused a global ice age. Between the two, the earth saw over 90% of its life die out.

Sort of a warning in that, I think -- if we're not careful playing God we could wipe out most life on Earth...

On geological timeframes, the problem with most things like trees or fungi is that they die and decompose and release the carbon they had, whereas sedimentary rock just keeps on building up millennium after millennium. We haven't had trees store carbon on geological timeframes since the carboniferous period, the geological period after the evolution of cellulose but before the evolution of cellulose eating fungi when the world was covered in massive forests that just kept on growing and falling over and getting covered up and sent underground.

The last time trees did a great job of capturing carbon on geological timescales was an era called the carboniferous period. This was after trees evolved but before anything evolved that could decompose wood. That's where virtually all our coal deposits come from. After this period, wood decomposes back into CO2 thanks to fungus, so it doesn't do a great job of capturing carbon on long timescales. Instead it's more like the trees are just borrowing the carbon briefly.

Much of it is in carbonate rock. There's entire mountains of the stuff.

The carbonate is created when life takes the carbon out of the air somehow (it comes out using photosynthesis, but whether the life that did it or something that eats that primary consumer depends on which life it is) and then forms a hard shell. The hard shell is made out of carbonate, which when the creature dies falls to the bottom of the lake or ocean, creating the sedimentary layers of rock made out of carbonates.

It's important that we get the second step, because if you just pull carbon out of the air and use it or decompose it, then you're just taking it out and putting it back on non-geological timeframes. Cellulose was good because it didn't decompose for an era, but that era is long gone.

The fact that the earth once had 22 atmospheres of CO2 and that's where life evolved, and all fossil fuels on earth were produced while life existed on the planet suggests that the calls for apocalypse might be greatly exaggerated. Burn every fossil fuel on earth and there's still mountains of carbonates that aren't going anywhere.

The rock is produced by living things. Something creates some energy through photosynthesis, and either it creates a carbonate shell like coccolithophores or Coralline algaes, or ends up getting eaten by something else that goes up the food chain to feed something like a clam or a snail. What happens is those creatures live, grow, then die, and when they die their shells fall to the bottom of the sea and compact together, and these tiny creatures dying adds up over geological timeframes.

I ended up on a geological history of earth kick a few years ago, and stuff like this ended up being incredibly interesting because some really big things end up almost irrelevant to the geological record, but some really small things like microorganisms end up being immensely important.

I live in an area where you can drive for an entire day and it's nothing but trees. I totally agree with you that we need to fight to keep the earth green.

I think there's something we need to be careful of though, a friend of mine pointed it out: If we just min/max "more trees", then you end up engaging in certain practices where pack a bunch of the same trees in really tight in ways they wouldn't grow in nature because you can say you got more trees that way, but growing that way has negative consequences for the environment as well.

Monoculture, planting a bunch of one type of tree, for example, results in one bad disease for one species being able to wipe your entire forest. Canada's facing this in a lot of places right now, for example with the pine beetle outbreak harming British Columbia forests.

There's also practices like planting lots of trees really close together. In nature that's not how they grow, but it lets you say "Look at all the trees we planted!" but in the long term that's not a healthy forest and it's prone to things like fires as well.