Numbers: I like numbers A tank of gas for a small to midsize car is 15 gallons ~60 liters. Gas has a s.g. of between 0.8 and 0.9 Call it 0.86 and you have just put 50 kg of gas in your car. Gas runs about 5/6 carbon so that's roughly 40 kg of carbon. A tree is mostly cellulose, which has the same empirical formula as sugar CH2O So a kg of carbon is 2.5 kg of tree. Dry weight. So, to balance out your tank of gas, you need to grow 100 kg of tree -- dry, or about 200 Kg wet weight. AND YOU CANNOT LET THAT TREE EVER ROT OR BURN. Now a fairly modest poplar will get that big in a few years, but it will fall and rot, or burn in, say 20 years. So now the price of your tank of gas is that you have to plant a tree and replant it every 20 years. In plantations poplar are planted on around 8 foot spacing. 700 per acre. If you use a tank of gas a week, you need to plant an acre every 14 years. And replant it appropriately. If everyone in the U.S (300 million) did this, it would take 20 million acres a year to balance out our personal gasoline use. That's 31,000 square miles a year. The U.S. is aobut 3 million square miles (ignoring Alaska) After 100 years the entire US is used to grow trees to suck up the carbon and keep it sucked up.