Almost everything in liberalism, from behavior to beliefs, can be traced to its socialist roots. Global warming (and the environment in general) is no exception.
Obviously, socialism declares that government must have power over the economy. So good excuses are needed to justify such a handing over of power...especially in America, where the foundations of this country are LIMITED government control.
So they need a reason to regulate business...voila! Make accusations. Global warming and the environment are perfect for that. Claim that capitalism's industries are destroying the earth...and since we all live on the earth, that means capitalism is killing us all. It fits perfectly together.