Yes Andrea, those misconceptions are all really common ones. My hunch is that they are getting all the different types of global change mixed up when they hear about it in the news. I have found that students often come in with those misconceptions, but they are pretty straightforward to unravel once you address each topic in turn. Concept sketching really helps, as do dynamic activities where they start with index cards with various topics on each card (just one topic per card, such as methane; CFCs; melting permafrost; fossil carbon; etc) and they arrange the cards into processes. Some cards will go together and others belong in a separate area. Students can do their arranging on a lab table, and then groups of students can visit each others' cards and add/subtract/rearrange. Ultimately, all of the processes could be linked together in a global system.
Kai's talk today had students work with boxes of balls where each box represented a carbon source or sink, and ping pong balls represented carbon. Students moved the balls back and forth to different boxes to match the global carbon cycle. I thought this was a neat approach.
Both of these approaches are active, in that the students work with something physical. This is helpful because atmospheric gases are an abstract concept. I'd be interested in hearing what other approaches people have used.