Re: Wrong way round @ Phil
"So by chopping them down and burning them for useful energy we're not actually making things worse for the atmosphere, but gain in terms of fossil fuel deplacement?"
It depends how you look at it. Plants need time to regenerate, and a young forest system does in fact fix pretty substantial amounts of CO2, as opposed to a mature one like a rainforest. It's why "sustainable forestation" is a pretty safe bet when it comes to Sensible Greenery. ( amongst other things. There's soil fixation, desertification, and a number of other factors in play as well.) Our use of energy is far greater than the regenerative capacity of plant growth, so that would put a crimp on things right there..
The other problem is the Coriolis effect, effectively segmenting our atmosphere in separate zones, which severely limits transfer of a lot of things, including CO2. It's one of the reasons why the whole "Carbon Bond" idea is pretty much ridiculous. CO2 generated in the northern hemisphere does not magically end up in the Amazon, or other equatorial rainforests, however much Politicians and Hippies wish it so. So if you're taking a lot of carbon from below the equator, and burn it up north, you're effectively creating a localised effect in greenhouse gases.
( El Reg had a lovely article, with video, about NASA showing that effect over a couple of years a couple of months ago. It also shows you how the Russian Taiga waking up for spring literally gulps down the "excess CO2" from Western Civilisation, except for the bits that bleed through to the polar belt, making the CO2 level ( and greenhouse effect) rise ever so slightly there... )
"Yes, but it did give us all the fossil fuels we need today. zero-sum profile again?"
Only if you're talking geological timescales, which is a bit long-term , especially if you consider the absolute guarantee of the disappearing of us naked apes anyway when you take that long a view ;)