yes, but it also produces methane so beware!

I haven't seen anything about methane. How/why is it bad?

Methane (chemically CH4) is the primary component of natural gas. It is and odorless that like natural gas (methane with other natural parts) can kill you or at best make you sick. In high enough concentrations it can be explosive. They pump a lot of out of the ground in Texas.

I can't imagine that it would make enough CO2 to be worth the space.

You want more Carbon dioxide, throw a critter in the greenhouse !! I am sure some reptiles or amphibians or even canaries would be ok, assuming you can keep proper temps and feed them adequately!!

bradarmi-
Great insight! For optimal results, one or two cows would work well: it's been estimated that each animal produces up to 250 gallons per day of methane If you didn't want to keep the cows inside the greenhouse (obviously, another set of problems), you could get one of these devices...

Here is a link that might be useful: Cow backpack