Yes. It's happening right now. However, the question is if it's man made, or part of the earth's ever changing climate pattern. The earth swings between ice ages and being extremely hot on a regular basis. We are still in the end stages of the last ice age, as there is still extensive glaciation on multiple continents. For the past couple millennia the climate has been comparatively stable to humanity's limited perception, but this stability is an illusion caused by our tiny lifespans and record keeping. That said, it's been proven time and time again that humans have ridiculously outsized ability to affect our environment. My personal belief is that global warming is a combination of both our actions and natural cycles.