humans do not CAUSE global warming, but they definatly contribute to it.
the earth has been constantly changing even before humans were present.
it was once really hot and contained high levels of oxygen, and it constantly changes.
the ice age can be an example of this- surely back then humans were too small of a species to contribute greatly to it.
the earth would eventually warm on its own even if humans were non-existant, its a constant cycle.
although humans are making a large contribution to speeding it up, i dont think its possible to stop global warming, but it is definatly possible to drastically slow it down.