I've been doing preliminary research on what kind of high-density trees grow reasonably fast in different climates. If the wood has a high enough density it will sink in water without treatment. Wood that has been submerged can take 100s of years to decay. Hence it might be possible to lock away a lot of carbon and give us some breathing room, by repeatedly clear-cutting and reseeding forests and sinking the logs. Even without high-density wood, it might be worth the effort to treat the timber so that it will stay sunken. I did some estimates previously that suggest the amount of land area that would have to be dedicated to this to reduce atmospheric CO2 is not unreasonable.