Trees come in an abundance of shapes and sizes, so their capacity to absorb CO2 varies wildly, from about 12 lbs per tree per year in forests typical of the US Northeast, to 50 lbs CO2 / tree-year for tropical trees. For the estimates below, I use 25 lbs CO2 / tree-year for a mature tree.
The question is: how many miles can you drive on the carbon absorbed by one tree? The answer, not surprisingly, depends on the fuel economy of your car. To find the number of miles offset by a tree in a year, you take the MPG of your car, multiply that by the amount of carbon absorbed annually by a single tree (25 lbs CO2 / tree-year), and divide by the amount of carbon released by burning a gallon of gasoline (19.4 lbs CO2 / gal). In short,
Miles / tree-year = 1.29 * MPG
It's a good reason to move ourselves around in a way that emits as little carbon per mile as possible.
Americans drive, on average, 13,500 miles a year. Supposing, generously, that you do that in a high-efficiency car (50mpg). You would need 210 trees to offset your annual driving, or about a third of an acre of forest (assuming 700 trees per acre).
You couldn't fit 210 trees in the average back yard.
By all means, plant a tree: we need every one of them (besides, they are beautiful). But it has more, and more immediate, impact to adjust your buying habits to prevent the cutting down of tropical forests. For instance, palm oil (called "vegetable oil" in ingredients lists) comes from palm plantations that have replaced old-growth tropical rainforest. A floor made of exotic woods may stroke your vanity, but the trees the wood came from are probably better off left unculled from the Brazilian rainforest.I'm sure you can think of many more products that lead to the destruction of forests that act as the planet's life support system. Time to start avoiding those products!