How to promote reduction of carbon footprint for conference participation?
Personal, disputable view follows.

Even if you avoid a conference, you save only a few kilograms of carbon. There's millions of other fliers that don't care about it and are travelling at the same time.

The best way to protect the environment is through technological advances: planes that work with eco-friendly fuel, are faster or more effective, better virtual reality technology that allows one to avoid travels, maybe even quantum teleportation and all that kind of stuff. 

How do we advance technology? Through scientific research. How do we get it earlier? By advancing scientific knowledge as fast as we can, in the most efficient way. So go to that conference, don't worry about that minor O(1) saving, and do your part to obtain the O(n) or O(n^2) one later on in the history of humanity. Maybe that theorem in homological algebra that you are presenting today is going to be instrumental to quantum teleportation in 100 years.