Of course we are the reason.
In the year 1810 the world population reached 1 billion.
By 2012 it will pass 7 billion
Given, that over the same two hundred year period the World has seen the explosion of the Industrial revolution and the burning of fossil fuels it's not hard to do the math.
True there is quite a loud vocal group out there who are screaming denial, but I think that is only because to deny it is happening means they don't actually have to do anything or make any personal sacrifice...!