Concrete reabsorbs carbon over time, so the net carbon footprint dynamic is not unlike that of wood. See https://www.architectmagazine.com/technology/concrete-become... Concrete's long-term net carbon footprint is simply the fossil fuel energy input minus whatever unoxidized material we dredge up. While theoretically we could manually plant trees and manually bury them in a way to minimize decomposition, we could likewise dig holes to expose and crush unoxidized rock. The former might be marginally more efficient energy-wise than the latter, but the energy consumption dynamics of both fundamentally suck.

I agree, many small solutions can help. More efficient use of existing resources is a large reason why American per capita energy consumption has remained steady for the past decade. (At least if we ignore our outsourced carbon emissions.) But many, small efficiencies is a privilege only rich countries with strong institutions are likely to achieve. Certainly historically that has been the case. Yet such countries have enormous carbon footprints. Similarly, reforestation can be helpful in a strict sense, but our primary problem is deforestation in developing countries, and even with global warming increasing the growth of temperate and boreal forests, it's rather difficult to make up the loss of tropical forests.

More importantly, none of these measures can help nearly as much as nuclear energy or renewables like wind and solar, which are both necessary and at least theoretically sufficient on their own. I think we just need to be clear about the efficacy of various measures. By all means, plant more trees. We can certainly walk & chew gum at the same time. But when people think these activities are even remotely similarly efficacious, and when the lesser one is easier to accomplish policy-wise than the other (e.g. in a country where we've effectively already accomplished an efficient market for wood products), I worry about misdirected attention.

More specifically, we need to discard this widespread notion that trees (naturally, or even in an industrial life cycle) are perpetual carbon sinks. That's really what I was getting at by pointing out the Carboniferous period--that the fundamental dynamic of carbon sequestration by trees does not and cannot work as commonly believed. The article for this thread is interesting (though only experimentally showing what was already well predicted), but it's misguided to believe it's of any consequence in the fight against global warming. The oceans are a different matter, though I don't know how greater is the long-term sequestration potential--I don't think we understand the carbon cycle as well as we do on land; for example, we know less about the microbiota, especially on the ocean floor.