John Nyboer, University Research Associate at the Canadian Industrial Energy End-use Data and Analysis Centre at Simon Fraser University answers:
Trees are fine and in fact there is considerable encouragement to do what you suggest in many circles. But there are a number of issues.
The scale of the problem is much greater than any solution that trees can provide. For one thing, the amount of CO2 conversion provided by trees is dwarfed by that provided by ocean algae, the number one source for the capture of CO2 on Earth for billions of years. Ocean plants (mostly algae) account for about 80% of the biological CO2 fixing on the planet while terrestrial plants handle the other 20%. Trees are a fraction of that 20%. There have been numerous proposals to "fertilize" the oceans with things like iron and nitrogen (which are limiting factors).
Marginal farm land may also be marginal forest land. Also much farm land is on the prairie where historically few trees were growing (this is currently the case, unless supplemented by water, another increasingly scarce commodity). One would have to plant a lot of trees if one were to compensate for the release of greenhouse gases from burning fossil fuels. In fact, it's probably not even possible. Let's just say that there are significant limits to your suggestion which means it could solve only part of the problem. That said, it is an approach used by many industries and utilities as an offset.
Finally, carbon is only held in the tree for the life of the tree, unless the wood from the dead plant is stored somewhere where decomposition won't take place. That is, trees as a CO2 storage method is a time bound limited solution.
making a small donation to science.ca.