Only the leaves (and maybe other green and exposed parts) of a tree do photosynthesis. The rest of the tree provides structure and protection and nutrients and

Only the leaves (and maybe other green and exposed parts) of a tree do photosynthesis. The rest of the tree provides structure and protection and nutrients and most of the carbon is sequestered in the structure and roots, not in the photosynthetic part. Human civilization, as it becomes more reliant on solar power, essentially becomes photosynthetic. Already, sustainable harvesting of trees to make long-lived structures can sequester more carbon than a tree does as it decomposes on the forest floor at the end of its life (emitted CO2 and a bunch of methane, which is worse). But we can also do “photosynthesis” more directly by using photovoltaics to produce hydrocarbons from direct captured CO2 and water. In fact, going back to the space meme, this is already done on the International Space Station as a side effect of the life support system. Water is split into oxygen (for the crew) and hydrogen. The hydrogen was originally just vented to space along with CO2 (expelled by the crew) captured from the cabin air, but they realized they could combine the hydrogen with the CO2 and recover the oxygen in the form of water, with the hydrocarbon methane as the byproduct. The methane is vented to space normally, but then they realized they could use the methane and reform it into polyethylene and hydrogen (which can be recycled to capture more oxygen form CO2). The polyethylene could be used in the 3D printers on the space station to produce structures. (NASA hasn’t accomplished this methane to polyethylene goal, yet, although methane-to-polyethylene/polypropylene is done on Earth via methane to methanol, then methanol to olefins). This is, essentially, a synthetic kind of photosynthesis. NASA funded SBIR grants to try to demonstrate this process. And there’s no reason you can’t make any kind of hydrocarbon this way from water and CO2 (hydrogen and CO—which can be made from CO2, as Perseverance demonstrated—are synthesis gases which can be used via the Fischer-Tropsch process to produce any hydrocarbon). In this way, as the mass of a photovoltaic-centric human civilization (buildings and infrastructure and machines) in the form of carbon fiber and wood and resins grows, it is sequestering carbon in its own structure, just like a tree does. So humans (with their houses and buildings, etc) can, in fact, become carbon negative as we transition to clean energy including for utilizing synthesized (or natural) hydrocarbons (or sustainably harvested wood) for structural materials.