It is only a matter of time.
Perhaps not in your lifetime, but all logic and reason point to our eventual poisoning of the planet so much so that our numbers will decrease dramatically. Whether it's terrible climate change or a disruption in the balance of oxygen in the atmosphere, we are continually adding to the risk of major disruption.
The hourglass has been turned over, we just don't know how long the hour is.
How much mercury do you have to ingest before it's toxic? Well, that's a bit different for every creature, but at an eventual level, it kills all living things. So it is with the Earth. We have been pumping gases and chemicals into the atmosphere continuously and in increasing quantities since the beginning of the industrial revolution. I am the grown up when I say, "Hey, don't you think we ought to stop shitting in our own bed? I mean, eventually we won't be able to sleep there, right?"
To those who balk at the specter of climate change, and to those who pooh pooh the dangers of slashing the Amazon rainforests and decimating sea life, and especially to those who insist that the world continue burning fossil fuels until the very last drop is extracted from the earth, I say
PULL YOUR HEAD OUT!