[set fair (OP)]

I'm supposing we could deal with the polutants from the smoke. I mean if we kept felling and replanting. Not assuming the fastest growing but maintaining a mixed forrest.

[alancalverd]

Hugely dependent on population density and climate. No problem in Canada, Scandinavia (excluding Iceland - no trees!), Scotland, Siberia, maybe some central African states and possibly New Zealand. Almost impossible to make a significant contribution anywhere else.

[paul cotter]

That is how several civilisations collapsed when all the wood was gone.

[set fair (OP)]

Hugely dependent on population density and climate.

i'm thinking about the whole world.

Also I realise that there would be a whole host of problems, but thats another question although it makes my question just accademic. To make it a little simpler lets say 5% of Russian and Canadian forest were harvested annually - what % of mankinds energy needs would that provide?

[Origin]

The major use of wood currently is for fuel.

[alancalverd]

The population of all the countries I listed totals about 100,000,000and they are probably the only ones where wood burning could be indefinitely sustainable as a source of all artificial energy.

You can estimate 10 MJ/kg as the available energy output from wood burning, about 25% of the energy density of diesel fuel. To sustain a reasonable Western lifestyle you need about 400 MJ per day, say15 tonnes of wood per year. My guess is that there is about 10¹¹ tonnes of live wood on the planet, so you might harvest 5 x 10⁹ tonnes per year with some hope of sustainablity.

There are 8 x 10⁹ humans, so the demand is for 120 x 10⁹ tonnes of wood per annum.

No chance.  But with a bit of scraping and saving, you might just sustain one tenth of the present population at a recognisable (pre-industrial) standard of living by harvesting wood.

[Zer0]

@SetFair

Hope you do not mind a follow-up thought...

If ' we ' reduce the Population to around 1 billion, and could manage to accommodate everyone around the Equator, or say go back to a Wandering gypsy lifestyle scavenging resources as they grow & replenish, would that be a feasible plan?

[alancalverd]

Another approach to the calculation. The Carboniferous period lasted about 60,000,000 years. Suppose 1% of all the trees (i.e. 600,000 years' worth of growth, turned into coal. Coal reserves are estimated  to be sufficient for about 100 years as the primary world source of fuel at current rates of energy consumption. So if we reforested the entire planet and increased the CO2 content of the atmosphere to Carboniferous levels, we could only harvest about 1/6,000 of our energy needs sustainably.