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Comment by jpfr | Hacker News Reader

As you write, plants can do this quite efficiently via photosynthesis. So there is at least one working example where CO2 capture is resource-efficient (even solar-powered). From photosynthesis we get sugar. From sugar we can get to raw (solid) carbon for long-term storage.

Forests are not the only option. Imagine large installations with tanks of algae under a white canopy. I imagine the economies of scale are huge once this is ramped up to industrial levels.

The question is who will pay for it. Nobody wants to indirectly subsidize coal plants on another contintent.

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Photosynthesis manages to convert about 1% of the sunlight into chemical energy. I wouldn't call that efficient. The big benefit of plants is of course that they're relatively low maintenance. You still need to cut them down and turn them into charcoal if you want to capture that carbon for good.

Now look at the gigatons of coal we already dug out of the soil and burnt. That's a lot of forests you need to plant, cut down, and turn into charcoal. It might be doable, but it certainly won't be cheap or easy.

AFAIK tree sequestration is marginal compared to the amount of co2 to be captured. Also, trees rot, and the decomposition process reverts a chunk of the process (releases co2)

"a worldwide planting programme could remove two-thirds of all the emissions from human activities that remain in the atmosphere today" https://www.theguardian.com/environment/2019/jul/04/planting...

> AFAIK tree sequestration is marginal compared to the amount of co2 to be captured

Nope. Tree sequestration is extremely effective on a global scale. That's why it's so important that we prevent deforestation.

"Intact forests and those re-growing after disturbance (like harvesting or windthrow) sequestered around 4 billion tonnes of carbon per year over the measurement period — equivalent to almost 60% of emissions from fossil fuel burning and cement production combined."

http://theconversation.com/explainer-how-much-carbon-can-the...

Well, the study considers already existing forests. Is it feasible to eg. increase global forestation by 100% to offset 60% of emissions? Not sure. Also, it considers a limited timespan. While it is not that relevant for undisturbed forests that usually have a stable ecosystem (trees born and die gradually), it may be an issue with disturbed ones; trees will tend to have about the same age and the same duration; this will cause release spikes related to decomposition. Also, "planting trees" usually is a placeholder response that largely ignores the impact of monocultures in the soil, subterranean water reservations and propagation of forest fires, plus the fact that co2 tends to concentrate higher in the atmosphere, way above the average tree line.

In summary: its not as bad as I thought when I wrote the previous reply, sure. But it is still far from feasible.

> plus the fact that co2 tends to concentrate higher in the atmosphere, way above the average tree line

I looked this up and seems it's the opposite. I'd politely suggest you check your assumptions are correct.

https://www.quora.com/Is-there-less-carbon-dioxide-at-higher...

https://667-per-cm.net/2015/08/04/atmospheric-concentration-...

The efficiency of photosynthesis isn't all that great.

There's a reason energy based on organic matter has gotten a bad reputation. It's just not a very efficient way of getting energy from the sun compared to e.g. solar.

The calculation for carbon storage will be a bit different, but not by that much.

The mechanisms for us (people everywhere) to begin paying for it is crucial, as there are a number of technologies on the cusp of being useful for tackling this problem, however they are not cheap. Cheap solutions to CO2 are rare and tend to only apply to point sources at best.

Large scale solutions that help mitigate the existing CO2 are going to require some significant capital and operational expenditure.

The cheapest large scale solution actually requires less money, and more legal / willpower. But I’d argue it’s also harder to measure concrete impact and “monetise” oceanic surface iron enrichment. It’s harder to convince people to pay for estimated results even ones based on measurement and science, than results containing physical outputs, like actual solid/liquid/gaseous CO2.

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