Deneen says in his book that the greatest threat to the environment is that liberalism* transformed the meaning of freedom. From the original meaning of “cultivating oneself with the goal of not being enslaved by one’s appetites” freedom acquired the opposite meaning of “recognizing no constraints on the satisfaction of one’s boundless appetites.”
As students learn on the first day of class in Economics 101, human needs are endless and can never be satisfied. Humans will not stop wanting to consume more until they eat up the whole planet. Deneen says that the only thing that will help the environment is for people to accept that the greatest freedom is not to desire endlessly but to be free from the tyranny of unbounded desires.
* Obviously, he doesn’t mean liberalism in the narrow US sense of “whoever opposes the US Republicans” but in the wider sense of the great emancipatory project that began in the 18th century and that aimed to liberate humans from political tyranny (think modern democracy), tyranny of nature (think antibiotics, heating, the AC), tyranny of their their bodies (think birth control, modern dentistry, reproductive assistance), etc.