Well. If you want a slow planetary catstrophie, it would entail that it starts quietly, so that no one at first notice whats going on.
Say plants are growing slower and slower each year, because of the depletion of natural minerals they require. No one would notice if the rafflesia took a year or a year and three months to bloom. It would only be noticed when crops start to take much longer to produce. As a result, oxygen production is decreased, and slowly the atmosphere of Earth would grow thinner and thinner. Because of the thinning atmosphere, less ozone is being replenished, and eventually, the ozone layer will die out, exposing the earth and humans to deadly UV radiation.
Or maybe the melting of the polar ice caps cause a slow shift in the climate, a la Day After Tomorrow ( i think ), only at a much slower rate. The overall lowering of the Earth's temperature causes the Earth to slowly and irreversibly sink towards an Ice Age. The people have to leave Earth because of the Ice Age which will kill anything on the planet.
Or the outer core of the Earth is starting to freeze at an alarmingly fast rate, causing the Earth's magnetic field to grow all screwy, causing a geomagnetic reversal to take place. At this point, you can come up with reasons for kicking humans off the Earth, because the exact effects of a geomagnetic reversal are unknown.
If you want more, I can always think up more.