There is a little-known short story published by Jules Verne, that while technically inaccurate, raises a very interesting proposition. It is entitled Dr. Ox’s Experiment.
http://en.wikipedia.org/wiki/Dr._Ox’s_Experiment
The prosperous Dr. Ox enters a Flemish town known for its sedate and peaceful character. It rests within a closed and windless valley, so over the course of many years, the air there has a much higher percentage of heavier than air carbon dioxide. In fact, it is because of all this extra CO2 that the place is sedate and peaceful.
Dr. Ox offers them, free of charge, a electrolysis plant that will split water into hydrogen and oxygen. Then they can use the hydrogen to power their street lamps. However, his hidden motive is to flood their valley with oxygen to see its effects on people.
In any event, the story points out a possibility for agriculture. If you planted a crop in a depression shielded from wind, then added CO2 to the air, you would likely get much greater plant yields.
Importantly, to uptake CO2, plants have pores in their skin, which they open to get the gas. However, in the process, water evaporates out the open pores. But when there is a lot of CO2, the plants don’t need to open their pores as wide, so they lose less water. So they need to uptake less water.
This is a second good reason to do this, if you live in an arid region. To get higher plant yields *and* to conserve water.
Are you sure about that?