What is the "tipping point" for consumers on major trends?

I have been trying to wrap my head around the whole idea of “green” and sustainability. I am trying to understand what the tipping point might be for the population at large to accept this trend and start living it (without really having to actively think about it so much).

I thought about seat belts, and when people started to accept them as something they should do for their own good.

What do you think causes people to finally “give in” to a cause and start living it out? What will it take for “green” to become this widely accepted?