I don't doubt that the Earth, the miracle of life, and probably even the human species have a long future, but our particular mode of living seems not long for the world. When you stop and think about the challenges that we're actually facing, and then you take a look at what we collectively invest our time and energy in, it's hard not to feel discouraged. This is what I'm referring to when I say there's an "undercurrent of doom" out there. I preach a dark future, etc.
Except really I don't. Apocalyptic thinking is contagious but ultimately not very insightful, and dispair has the downside of being both depressing and ineffective. Knowing is half the battle, so one place to start is thinking about the challenge.