Comment by photochemsyn | Hacker News Reader

Comment by photochemsyn | Hacker News Reader

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The biosphere is essentially in steady-state on human lifetime scales. Estimates are that 100 gigatons of carbon are released to the atmosphere as CO2 by fungal and animal and bacterial action on plant material, and an equivalent 100 gigatons are taken up by photosynthesis.

There were periods in the geological timeline where photosynthesis >> respiration and the result was massive accumulation of coal and oil deposits, such as the during the Carboniferous era (~350 million years ago).

Of course there are many good reasons to do reforestation - prevention of erosion, a supply of timber, habitat for wildlife, retention of water, etc. Offsetting fossil fuel emissions is not one of those reasons.

[edit: on medium length (10K year) timescales, there is an effect of storage of photosynthesis products, one that's involved in the past ~2 million year ice age cycle (with periods of around 100K years). There are two processes: storage of photosynthetic products in the deep ocean, related to decreased circulation, and storage in permafrost, related to growth of permanent ice, that slowly decrease atmospheric CO2 during the descent into ice ages.]

While it's a good point that plant sequestration of CO2 is slow compared to digging up coal, oil or methane and burning it, it's not meaningless.

It's not meaningless, but it's an example of humanity chasing its own tail.

Photosynthesis is not the most efficient process in the equation by far. It is competing against two things; a system that can use lots of oxygen as an oxidizer to release energy, and titanic stockpiles of hydrocarbons waiting to be burned. An animal can either consume plants after they've done their job or consume other animals, and use animated lungs to consume a constant stream of oxygen that can be used to oxidize the energy substrate. Plants can only use photosynthesis, which is much less efficient of an energy transfer than respiration (creating molecules instead of breaking them apart). Because plants are stuck with photosynthesis as a food source, they are limited by its speed. Plants still need oxygen to respirate the food they create for themselves, but they cannot afford lungs and a cardiovascular system to utilize more oxygen. If they could, then they might as well be eating other organisms that already performed the hard work for them.

What I'm saying is that humans, being respiration animals, are always competing with plants, and do even more so when they release carbon from reserves in the earth. The ability of artificially propagated plants to compete against that system is minuscule. It can only compete with surface area, and for humans to be able to even make a dent in atmospheric CO2 by planting trees would mean planting way more trees than is likely worth their own expended energy. In such a scenario, the tables are flipped, and the animals (humans) are competing against the inefficiency of photosynthesis and must use enough energy to compensate. That is neither a proven strategy nor sustainable. Just as body fat can in theory be lost entirely through exercise, it is orders of magnitude more efficient to simply not introduce more energy into the system (i.e. consume less food, and eat food with fewer if any deleterious side effects).

Certainly it gives a perspective: I've read that the amount of fossil fuels we've burned is already greater than the currently living biomass on the whole planet. We've burned almost all of it during the last 100 years.

So yes we can't operate as we do and fix it by on the side planting a few trees.

The point is, plant matter is not geologically sequestrated in the present world. The oceans are oxygenated and circulation is pretty strong, and permafrost is decreasing, not increasing. Hence, any photosynthesized carbon material will be returned to the atmosphere as CO2 relatively quickly.

The only way to sequester carbon is to make something like limestone (Ca/Mg + CO2 + H2O -> (Ca/Mg)CO3) and that requires calcium or magnesium ions. There are some sources of those ions, but not that many. Another option is production of something like carbon fiber material, or even better, blocks of diamond.

Why can't you just create oil or coal or similar products?

Well, the only reason to do that is to combust those products to create energy, right? If you want to bury the oil or coal as a means of carbon sequestration, well I suppose you could do that with wood instead, but if people really want to stablilize atmospheric CO2, the first step is to stop using fossil fuels entirely.

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