You can probably only preach to the already converted, there will probably be a certain number in attendance at your talk who are sympathetic to your message, as for the rest you will be knocking your head against the wall.
How to get run out of the room? You will have to go to Freud, 1923, The Ego and the Id, "What is now holding sway in the superego is, as it were, a pure culture of the death instinct." "As is well known, in the superego there is all of human civilization with its structures, its taboos, its laws, its ideologies." Jose P. Miranda, Marx and the Bible, Orbis Books, 1974, p. 281.
(The core of Miranda's book is actually Heidegger-Bultmann-Tillich, and not Marx, but there is a wrapper of Marx, so to speak, on each end of the book.). Btw, Miranda is not writing a book about the environment, but he does write some things at the end about death, and even overcoming death.
We can get ourselves into a narrowly rational dynamic where we tell ourselves "it" cannot be stopped, we tell ourselves that we have lost control of the object that we ourselves created. We buy into our own Frankenstein story, because All-father, daddy, or in some way the parent (all the shoulds and musts) of our civilization told us "it", this thing, was the right way to go, even unto death.
The control of the superego acts to repress the big picture, rational talk has little chance to change the person, and forcing the issue can even result in a psychological crisis.
The large picture of the rational, gets lost in tomorrow's small "rational" irrational acts. So death talk, suicide talk, apocalyptic talk, nihilistic talk, starts leaking out of the system. Let's hope it that in the short term people can sustain hope, and work with the ship to turn the ship.
To understand further what is going on in the superego, particularly the effects of the authoritarian "Daddy" on the child (and the grown up child), see for instance: "The Broken Rebel; a study in culture, politics, and authoritarian character."Rupert Wilkinson, Harper & Row, 1972.
But you can't do any of this today, you can't mention Freud today in polite company, you'll be lucky to even find the books cited. You'll get thrown out of the room.
We have to understand that some people cannot, they cannot, psychologically, face up to this problem. In our society, we can gently try to move people away from apocalyptic visions and acting out, and move them gently away from nihilism (I.e. try to prevent Thanatos from running amok in our world) , but that is all we can really do, and in the meantime work with those who agree with us that this issue is important.
P.S. I am focused here on psychological issues which, if they have come into philosophy at all, came in from the Frankfurt School. But I am in no way suggesting that Marx qua Marx had the solution to our environmental problems, though some are trying mightily right now to make Marx into an environmentalist. Marx was in favor of our taking control of history, taking control of ends, our not being passive, I think it's safe to say that. In my opinion, if Marx came back today, he would first catch up on his history, then go to current events, primarily the economy.