Josh – “Fixed” means what it usually means; as in not mobile. But just to make it clear this term is often used as meaning “not active as a greenhouse gas” or “not in the atmosphere or dissolved in the sea” and hence not a problem to anyone. As an aside, natural seeps of hydrocarbons don’t cause a problem, even though methane is a much stronger greenhouse gas than CO2, because the residence time of methane is rather short – and also because methane seeps are by now part of the background.

Mongoose, I haven’t got a clue what you are talking about. There is fossil carbon buried in the earth; that is a given. Where it comes from, ultimately, is actually rather irrelevant; what matters is that whatever the burying process is, it is either over or much slower than the rate at which we are releasing it from its solid or liquid form.

As an example of why this matters, burning wood (or straw, or anything else formed recently) really isn’t much of a problem as long as you grow more trees or plant more grass. When it does become a problem is when one burns down forests and doesn’t replace them; this not only releases large amounts of CO2 as a gas that has been in the form of cellulose for a long time, but also removes a carbon sink. Which is why cutting down Amazonian forests to raise beef cattle for McDonalds is a poor idea, even if you don’t care about biodiversity or the people currently living there.