However, beyond this, the reference of this phrase (and so by implication, the scale of the threats we face) often becomes murky, which is why I generally avoid it. However, for the sake of clarity, let me offer a rough sliding scale of threats that the "planet" might need saving from and how plausible I think such threats are based on the current trajectory of human actions.
1. Destruction of the planet itself. Well-nigh impossible.This list has been expanded and revised from a comment I posted here. This phrase is also often associated with the even more common phrase "the end of the world", which has a similarly ambivalent list of possible referents. The end of the world as we know it doesn't necessarily mean the end of the world full stop.
2. Destruction of all terrestrial life. Very difficult.
3. Destruction of all human life. Difficult.
4. Destruction of our civiilisation and of the conditions under which large-scale human civilisation is possible. Possible.
5. Significant decline in human population and/or biological diversity. Fairly likely over the long term on our present path.
6. Downfall of/significant departure from the present mode of our society. Likely and probably imminent in the next few decades.
7. The ongoing catastrophe of history that we call progress. Presently underway.