That trees are the major tools to remove CO₂ from the air is clear, but how many new trees need to be planted to compensate all human CO₂ pollution (6 gigatons / year?)? We should work with standard tree and count a tree in the tropics double since they absorb year round. If yearly net uptake of growing trees is 20kg CO₂ in temperate zones, global average might be 30kg/year. This means that "only" 6 × 10¹² kg ÷ 30 kg = 200 × 10⁹ = 200 bn trees are needed. I say "only" because this means only 10% extra forest, while I was counting on a doubling of global tree cover is needed (and possible).

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    I would also like to add that if we were doing a global geoengineering project we should consider that trees sequester different amounts of carbon throughout their life. A fully grown tree does not sequester nearly as much carbon as a growing tree. To maximize the carbon sequestration you would want to plant quick-growing trees, chop them down and bury them in landfills, then keep replanting. Of course, it would be better if we didn't dig up fossilized trees and put them in the atmosphere to begin with... – crypdick Jan 4 '16 at 19:24
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    Interesting, but where do your figures on absolute and relative uptake come from? – Chris H Jan 5 '16 at 15:31
  • Thanks replying, I seek this information for 5 years already, but yesterday I googled the subject and spend a few hours, but only found the 20kg with FAO and 15MT per Ha, what could be correct. Extremely confusing and out of date figures are available on google and wiki. – Ferdinand Swart Jan 5 '16 at 22:41
  • a few years ago I made an educational puzzle with carbon cycle and found the best figures with Stand-up Economics a UK stage performance on CC. May be science can help me further :-) – Ferdinand Swart Jan 5 '16 at 22:45
  • In fact I am talking about class tropical lumber harvested in 10 years and sold (in Africa about $20,-) for construction. The reason to grow for lumber is to avoid the clearing of old-grown forests – Ferdinand Swart Jan 5 '16 at 22:51

Trees follow the logistics curve for growth. More complicated, the number of trees descreases as they get larger. E.g. after a forest fire, lodgepole pine sprout at about 10 per square foot. In 10 years there is about 1 per square yard. At 30 years it's one per 6 foot square.

The fastest mass growth rate is when a given area is mostly at the 1" diameter stem. Some variation by species. Google biofuels for more info on this.

A mature forest is carbon neutral. Over the course of a year the process of decomposition equals the process of sequesterization. Most tropical rain forests are carbon neutral.

Some numbers: An immature aspen forest (3" to 6" trees)increases biomass at roughly a metric tonne per year per acre. At GT of carbon is about 1.5 GT of cellulose & lignan. So it would take about 1.5 billion acres of aspen forest per GT of carbon, or 9 billion acres for your 6 GT.

About 14 million square miles, or about 4-5 times the area of the continental U.S.

Now my numbers are from the Canadian Praire, latitude 54. Growing season about 115 days. While longer growing seasons will help, remember that a lot of those days here have 18 hours of sun, so it doesn't help all that much. But you might gain, say 30% that way.

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