That'd be a once off though. The dam may exist for such a long timescale that those decomposing trees under the waterline are irrelevant. I always feel we need

That'd be a once off though. The dam may exist for such a long timescale that those decomposing trees under the waterline are irrelevant. I always feel we need to simplify the carbon emissions to the point of "Carbon that was buried and is now in the atmosphere". This generally removes life in the carbon equation and reasonably so. The carbon in life was in the air relatively recently. Trees absorbed it from the air and other organisms will eat it and re-release it. In general that alone won't revert the earth to the state it was in billions of years ago. The carbon underground however hasn't been in the atmosphere for billions of years. That's the carbon that's going to return earth to the state it was in billions of years ago. That's what we should focus on.