Read an article that asks the question Is the Earth Quarantined? It suggests that Earth has been placed 'off limits' through previous contact with alien worlds. For this reason aliens haven't landed and made formal contact and global warming has been fabricated to make the earth more accessible.
David Icke and his Lizard theory would suit the later as they would require a warmer environment. But is this planet a large germ bucket that other intelligent life wouldn't touch with a barge pole?