As the founder of wikiHow, which is the wiki which has already become carbon neutral: http://www.wikihow.com/wikiHow:Carbon-Neutral I think I have a perspective on this issue which might be helpful to Wikimedians:

When wikiHow went carbon neutral the majority of community members and readers were positive about the move. For many it affirmed belief that wikiHow was more than just your average website. That said, there were some community members who opposed it for a variety of reasons including skepticism of carbon offsetting, skepticism of global warming and a belief that wikiHow should not be engaging in any forms of political statements. Others thought it was no more of a political statement than saying "I recycle" or "I clean up my garbage." It is this issue surrounding NPOV / and desire not to make political statements which is probably the thorniest issue here and something that the Wikimedia community would need to work out before proceeding.

Food for thought: If Wikimedia went carbon neutral it would have influence throughout the world that would go well beyond offsetting many tons of carbon. The Wikimedia Foundation has earned a good deal of moral authority. Other websites including wikiHow and many others look to the WMF for leadership on many fronts. By setting this example, other websites big and small would follow suit. Every person who has contributed to a WMF project has helped spread ideas that have made the world a better place: You have created and shared knowledge. You have educated others on the concept of free culture. You have proven it is possible to assume good faith and collaborate with total strangers. Now you have the chance to spread the concept that reducing and cleaning up your own pollution is a responsible way to run any corporation or non-profit organization. It is one more great idea that Wikimedia could spread widely.

On the issue of cost, this is a challenging but solvable problem. If a volunteer or group of volunteers did a separate fund raising campaign contacting organizations and people sympathetic to this cause, I think it could be successful. A pitch to the tune of "help the 10th largest website in the world set an example for the rest of the internet" would probably succeed at generating new donors that might not otherwise contribute to the WMF. It would also be possible to target existing donors. For example, over the past few years, wikiHow or me as an individual have contributed close to $30,000 to the WMF. If someone sent me an email asking me / wikiHow to chip in $1000 for this side fund raising plan, I'd say yes in a heartbeat! I'm sure I'm not the only one.

Best,