Greed will lead to us fully expoliting what we can out of the Earth until there is nothing left. While this happens, we will continue to despoil the most basic of things necessary to our survival - air, water, what's left of the rusted and ever failing chain of nature. When we finally go to war over the very basic of necessities to survive, we will reduce our numbers to a low enough point where continuation of our species will be highly unlikely. The shape that the planet will be in after that will ensure a harsh, painful death for those unlucky enough to survive, as the few remaining humans will fall to disease, predators and the wrath of Mother Nature as the Earth slowly heals itself to the point where it can again sustain and nurture life. It should be noted, however, that the human race is a stubborn lot and may, yet, survive this scenario. If so, humankind will begin to re-establish themselves as the dominant species once again once their basic struggle for survival is conquered.
And then they'll start this whole mess all over again because the tragic flaw of human intelligence is that IT DOES NOT LEARN FROM MOST OF ITS MISTAKES.