I just "bought" 21 trees as a gift to someone (to be planted in Denmark, not Iceland). This amount supposedly offsets what the average Danish person emits in a month according to the organisation I bought it from. Not sure where you're from, but the average American emits something like 2.5 times what the average Dane does, so that would work out to ~60 trees that need to planted monthly (if you're American). Obviously, your mileage may vary.

In any case, it really puts into perspective how messed up our continued use of fossil fuels is.