The day the earth stood still was a movie from 1951 where aliens set to kill off humanity because it is a dangerous and warlike race, who is stockpiling weapons of mass destruction (nuclear weapons) and has a "shoot first, dissect later" approach to alien life. It is plausible, and it ends with humanity proving itself as capable of peace and rising above its baser nature.
So how does the 2008 remake fare? well, its about the same, only instead of warlike destroyers we are now polluters... the aliens decide to execute all of humanity because we pollute, since "life bearing planets are rare and precious" and apparently AGW will destroy all life on the planet. But don't worry, humanity proves itself to the aliens as capable of using clean energy and is spared complete annihilation.
Beyond the ridiculousness of the premise, one has to wonder at the mental status (psychopathy) and intent of the authors as well as anyone who positively rated the movie. Obviously the good guys are the aliens, the humans must learn their lesson or die. But unlike with "natural disaster from AGW wipes off humanity" films (which are bad enough as is), this one actually has an alien race execute humanity for it. The notion that genocide is a perfectly acceptable means to halt AGW is a chilling view into the mindset of the AGW pundit.
Although, maybe the idea isn't as laughable as I would like it to be... what if we encounter an alien species with a liberal government in power?