Carbon Dioxide and Plants

When you are in 2nd grade, your science teacher teaches you that trees take in carbon dioxide and "turn it" into oxygen. When you are in high school, your chemistry teacher teaches you that matter is neither created nor destroyed and that ordinary chemical reactions can't change atoms, just their rearrangement. You learn that carbon dioxide is a carbon with two oxygens CO2 and that oxygen in the air is two covalently bonded oxygen elements or O2.

Unfortunately, perhaps because you grew up not questiong that trees change carbon dioxide to oxygen, you don't reconcile the statements from your science class and chemistry class. The obvious question if both are correct is what do the trees do with the C in CO2? The answer is they use that carbon to make wood, leaves, flowers, acorns, etc. So, the amount of carbon that a tree takes out of the atmosphere can be no more (by mass) than the amount of carbon and hence the total mass of the tree. A plant in your office that weighs 2 pounds after you shake the dirt off and dry it out has taken 2 pounds (or less, some can come from the soil) of carbon out of the atmosphere during its whole lifetime. So much for cleaning all that air in in your office, some quick google searches turned up that humans about 400 lbs of carbon by breathing during 1 year (I didn't research this number much, so don't trust it).

Trees are not machines that magically turn CO2 into O2. An old-growth forest (aka, not growing) tends to be carbon neutral (not negative) because some new trees are growing, and some have died and are decomposing (which releases carbon dioxide trapped in the tree's wood/leaves/etc). Of course deforesting or burning that forest means that you release more CO2 and stop absorbing it, so it is still good to leave it standing.

So... planting trees doesn't really reduce global warming, at least not after the tree is grown up.