But the outcome will be decisively influenced by two nations that are now failing to do enough: the United States and China. While India is also growing fast in importance, it should be absolutely clear that it is the two largest CO2 emitters - most of all, my own country -- that will need to make the boldest moves, or stand accountable before history for their failure to act.
Both countries should stop using the other's behavior as an excuse for stalemate and instead develop an agenda for mutual survival in a shared global environment.
These are the last few years of decision, but they can be the first years of a bright and hopeful future if we do what we must. No one should believe a solution will be found without effort, without cost, without change. Let us acknowledge that if we wish to redeem squandered time and speak again with moral authority, then these are the hard truths:
The way ahead is difficult. The outer boundary of what we currently believe is feasible is still far short of what we actually must do. Moreover, between here and there, across the unknown, falls the shadow.
That is just another way of saying that we have to expand the boundaries of what is possible. In the words of the Spanish poet, Antonio Machado, "Pathwalker, there is no path. You must make the path as you walk."
We are standing at the most fateful fork in that path. So I want to end as I began, with a vision of two futures - each a palpable possibility - and with a prayer that we will see with vivid clarity the necessity of choosing between those two futures, and the urgency of making the right choice now.
The great Norwegian playwright, Henrik Ibsen, wrote, "One of these days, the younger generation will come knocking at my door."