Why is this depressing? I read and enjoy all the comments everyone made. There are quite a few questions in my head now: "When we can build a model that identify all the factors needed for a sustainable ecology", "What would be a more efficient way to tackle this problem: Drive straight to the environment issue, or make more assessments to the peripheral topics that influences this, such as human's nature (greed)"
From my point of view, as long as human continue to chase after wealth more than they need to survive (and to a certain degree of enjoyment), we are destined to extinction.
We are intelligent, but we are losing sight of the big picture. We are born without any purpose in life. If there is a purpose, I would say we, and every living beings on Earth, exist to keep the life cycles on Earth. Don't freaking destroy it just for our own species' temporary convenience.