I hate to cite Robert F Kennedy Jr as a source of a good speech because I have long thought of him as a dangerous extremist crank, especially for his crusade against vaccinations with the dangerously false and discredited claim that there is a connection between vaccinations and autism. In years past, massive deaths were the result of contagious diseases, which can come again if people turn against vaccinations preventing contagious diseases.
That being said, RFK Jr made a pretty interesting speech to the 2014 Goldman Award ceremony (environmental prizes). It's 12 minutes long, and more combative and extreme than my tastes, but interesting none the less.
I would summarize his main point as being that the destruction of the environment is kind of like the canary in the coalmine when it comes to democracy. That is, when corporations have so much control that they can pollute with impunity, the loss of democracy is already happening and democracy itself will perish.
He warns against the forming of an oligarchy in America, and it seems to me to be a good warning to pay attention to. Citizen's United has allowed unlimited money to be pumped into the political system by the super wealthy, and the result is big business, big finance, big agriculture running amok. The politicians dance to big money's tune, regulators are captured and neutered by big money, and democracy is endangered.
The best line of his speech is where he says that the domination of business by government power is Communism, and the domination of governance by corporate power is fascism. Freedom, indeed true free market capitalism, and democracy itself are in the middle road between Communism and fascism, between domination by government and domination by Corporations.
As to environmentalism itself, I believe the worst thing that ever happened to the environmental movement was when environmentalists decided to focus on predictions of catastrophic global warming rather than simply focus on reduction of pollution. I think it was a tactical decision based upon the assumption that the general public would only be moved to action if they were terrified of catastrophe. But, pollution itself has been opposed by both liberal and conservative people for decades (except for those who are in the thrall of the polluting corporations, of course). But when rivers burn, water tables become toxic, air pollution stinks and becomes visible, seas stop supporting life, etc. those are motivation enough to incite environmental protections.
There is large opposition to the hypothesis of man made catastrophic global warming that will have devastating effects in the future, but finding real, individual examples of pollution and calling for specific action seems to me to be a much more effective way of cleaning up the planet.