With regards to CO2 tanks, multiple people have already pointed out that this is CO2 that was already in the atmosphere, captured, and bottled.
But if you're really curious about how much CO2 I go through for kegs, I have a 5 pound tank of CO2. That lasts me about 5 homebrewer kegs (at 5 gallons apiece). That gets me both the initial carbonation and all of the pouring. In comparison, burning one gallon of gasoline gets you about 19 pounds of CO2 released into the atmosphere.
I've never put the fermenter on the scale before and after fermentation - I imagine that would be the best way to track CO2 emissions, as the only thing that should leave the fermenter in this time is CO2. However, let's assume the volume of the beer doesn't change that much during fermentation. I start with 5 gallons at a specific gravity (density of the beer / density of water) of 1.06. That's 42.4 pounds. I end up with about 5 gallons at a specific gravity of 1.01. That's 40.4 pounds. So assuming the volume of the beer doesn't increase, that's 2 pounds lost. In reality, since alcohol is less dense than water, there should be a larger volume in the end, and so the final weight is probably above 40.4 pounds.
So in the worst case scenario, there's 3 pounds of CO2 involved in the fermentation and serving of 5 gallons of beer. I bet that having my stove on full blast for 2 hours to boil the water emits much more CO2 than that. Heck, me driving to the homebrew store 6 miles away definitely emits more than that.
So in conclusion - I think it makes much more sense to focus on the costs involved with distributing the beer and heating the water that makes the beer, and much less on the fermentation and kegging systems.
"Life is a garment we continuously alter, but which never seems to fit." -- David McCord