Can you be a long way along the 'green' continuum without being at least a bit authoritarian?
depends what you mean by that, but I guess there has to be at least some level of what could be referred to as authoritarianism within it.
for example, voluntary schemes for companies to keep their pollution within certain levels isn't exactly renowned for its success rates, whereas compulsory schemes back by legal sanctions and enforcement regimes have a pretty good success rate, though probably the least damaging and best supported are schemes that give a level of choice in how to meet the criteria required - set out the target, not the exact means by which it must be achieved.
without a level of compulsion, we'd still have had pea souper smogs killing thousands a year from coal fires, lead in petrol, ozone depletion causing serious increases in skin cancer levels etc etc.
on the other hand, recycling has been a pretty good success story for the voluntary approach based on provision of the service, and encouragement to use it, rather than legal sanctions for not recycling.
I doubt the aims can realistically be achieved via entirely voluntary means, they do generally need some form of legal backstop and enforcement to be successful, but at the same time they also need the vast majority of the population to support and go along with the measures and understand the need for them if they're to be successful.