There's a piece in the latest TREE on how to have a carbon-neutral conference. The reference list also shows that emissions caused by conferences are an issue that several people have thought about. Sample title: Are international medical conferences an outdated luxury the planet can’t afford? Yes.
None of this is exactly new. And while local food, paper plates and so on are all better than nothing, I still think the answer is fewer meetings, not offsetting. "Most importantly," write Oliver Bossdorf and co, "[meetings] provide ample opportunities for networking." Well, so does the web, and you can do that at home.