I've actually worked this out in one of my notebooks. It turned out to be a pretty hefty figure. I've only worked it out for the sugars consumed in the beer, for example, a five gallon batch of 1040 OG beer, fermented to 1011, will produce about 1kgof carbon dioxide. I agree, it sounds like a lot, but that's science for you.
http://www.guardian.co.uk/environment/green-living-blog/2010/jun/04/carbon-footprint-beer
Weep as you add in the airmiles of your beautiful American hops...
Cascade is grown in the UK, if you can get it, though it's mostly under contract. Still a lot of miles for your beer. Buying, growing, picking and using local is the only answer. To look about growing your own hops, here's a handy link!
www.gyohops.blogspot.ie
If I can do it in northernmost Ireland...