Global Warming
Global warming rates above nuclear war, terrorism, global bird flu pandemic and alien invasion as the most serious threat to the entire freaking planet. Seriously! A .001% increase in average global temperatures, and BAM! WE'RE ALL FUCKING DEAD!
Just take a look at this artist's rendition of global warming, and you'll see what we're talking about.
See? HOLY FUCK, we're in trouble.
Also, the evidence for global warming is so overwhelming that anyone who questions whether or not it's actually happening is a complete fucking moron. Are you a complete fucking moron?
Just check out this highly scientific graph if you don't believe us.

So, with global warming's legitimacy clearly established... what are we going to do about it? Well, I'm fucked if I know. I mean, I don't even understand the problem. But I do understand that it's going to KILL US ALL!
RUN! RUN FOR THE HILLS, YOU FOOLS! GLOBAL WARMING IS COMING! ROOOAAAAR!