Number 1 will be debated to death, until the polar ice caps melt completely atleast. [img]tongue.gif[/img]
There is acutally a proven way that humans have impacted local temputures in many areas and that is cities. More specificlly concrete, asphault, etc... There was a study just releases last year that showed temputures in cities are higher, longer then in the surounding country side. The reason is all that concrete in cities act as batteries, storing heat in the day and releasing it at night. IIRC the temp difference was as high as 10 degrees. This heat genereation also effect local weather patterns too.
note: Tried to goggle the article with no luck. too many hits