Your first and third arguments hold the most merit. It's always interesting to consider the existence of aliens, and the sheer size of the known universe provides nearly endless possibilities.

Some food for thought:
Humans are weak physically, and not very resilient to weather. So, of course it is true that historical civilizations on Earth have eventually collapsed (when weather conditions changed, resources dried up, or conquerors arrived). But what is to say that an alien race would be remotely similar to us biologically? That is a heck of a stretch based on the [nothingness] we currently know about alien life.

Also, you should check into Co2 a bit more. It follows a logarithmic curve, where each additional molecule contributes to less of a warming effect than the previous. Additionally, increased Co2 in the atmosphere significantly increases the rate at which plant life grows on this planet. The catastrophic views about our climate are blown way out of proportion - as they have been for the last twenty years. There have been dozens of climate models predicting catastrophe in that time - and guess how many have come close to getting it right as time went by? Zero. I know this wasn't a climate post, so if you disagree, no worries. Just something that curious minds in the past have found intriguing; thought I'd share.