Their whole premise is just based on lies, suppositions, and bad computer models that show whatever they want them to show.
It doesn't really matter one way or the other whether it is happening or not. Concern over the Earth's climate isn't important.
Solving global warming is not what the UN wants. If you read up on the Kyoto Protocol, it is really nothing more than an attempt at a massive transfer of wealth from industrialized countries to poor ones through the use of "flexibility mechanisms" (i.e. carbon credits). The global warming scare is how this transfer of wealth has been sold to the public.