6B,
Pumping oil out of the ground and burning it means the carbon stored in it is released. More CO2 = more plants is where the removal of said carbon back into a 'store' isn't happening; rainforest removal, deserts expanding, that sort of thing. Nature will make a balance, and revert to the kind of atmosphere the Earth had last time that amount of carbon was around.
Using renewable sources for current energy demands will affect local surroundings but won't have a global impact eg the Severn estuary tidal power scheme (one of only 7 identified locations globally which is suitable for such a method of power generation as it happens) will remove power from the water flow. This will affect the local area with altered tidal flow patterns but as the source of tidal power is the moon's orbit slowly decaying, something it will do anyway, using the power to produce electricity won't have global consequences.
Wind power, wave power, hydro-electric all have similar arguments, the only suspect production method is geothermal which has been accused of causing earthquakes in otherwise stable areas - such as Devon!