[danaris]

Poverty, hunger, addiction, homelessness, etc, are all bad, but they are bad on an individual level, with secondary negative societal effects.

Global warming is an existential threat to modern civilization. If we allow it to continue unchecked, it will result in catastrophic changes in prevailing weather patterns, sea level, and animal and plant life. These might not be enough to wipe out all human life on the planet—we might very well even be able to recover from it, eventually—but it is likely to create enough instability worldwide to cause many major governments to topple.

Now, again, this is if we let it go completely unchecked, and none of this is stuff that would happen within a year or two. But we're already seeing some of the effects, and there's nothing to suggest we won't see the rest if we just shrug our shoulders and say, "Eh, there are other important things, too."