How does this work?

You grow a ton of hemp. It captures X tons of carbon.

People use it for rope or whatever. Eventually throw it away. It decays or is burned. X tons of carb are released back into the atmosphere.

How did that capture any carbon?

It's about the long term. For hemp, in this case, Let's say 10 tons are captured per year, 1 ton is burned immediately as biomass. 9 tons is converted into goods which have an average duration of 10 years before being discarded, let's assume that they're burned immediately after 10 years, then you can get a net effect of removing 90 tons after the 10 year mark (hitting a steady state because 9 tons extra are being burned every year from that point on). If you can keep that 9 tons out of the atmosphere for 100 years, you remove 900 tons from the atmosphere after a century.

If those goods are discarded in a manner where the carbon isn't immediately re-released, so they're buried or introduced into a bio-system that can consume it and turn the carbon into some other non-gas form, then you can get further gains.

Lets say you burn it to generate power, as a lot of refuse is in Sweden. That's substituting for fossil fuels. Is it better to burn oil out of the ground, or carbon extracted from the atmosphere? You're creating a more sustainable cycle.

As a material, if you're using it in place of plastics again it forms a more sustainable cycle over a reasonably long time frame.

Yes we do need to sequester some atmospheric carbon long term, but establishing a more sustainable carbon materials cycle has an important role to play.

Sequestration remains an open problem. There are no large scale biomass capture projects in operation. But the science isn't hard: you bury it, basically. Or if you want to be fancy you burn it for biomass energy and capture the resulting CO2 for fancier sequestration techniques. Both are expensive, and no one is willing to foot the bill.

Right now the only realistic way to use photosynthesis to capture CO2 in a long term way is to plant a forest. There's some amount of this going on, though not enough to even cover deforestation elsewhere.

We need to start throwing our old newspapers down coal mines (or bury it) rather than recycling it - that will encourage more trees to be grown and solid paper is a great way to sequester carbon

If you want to be really fancy, you only partially burn it to make biochar, soak that in urea, and use that to make terra preta.

Two ways:

- the products last longer than a rotting plant. This is incremental.

- burned for energy. This reduces coal, NG etc uses and is roughly carbon neutral

after you're done using your hemp hacky sack you bury it miles under the earth's surface and it slowly begins to turn into coal

You can turn it into biochar. The biochar contains a large part of the plant's carbon and is stable for a few millenia at least. The process is even exothermic. It is also good for the soil if you till it under.

"because industrial hemp (as opposed to cannabis hemp)"

Are they really growing that much more efficient? Cannabis hemp was also intensively optimised for grow since generations.