"All this will happen before, and all this will happen again."

We can't actually tell if the Earth is going through a natural warming cycle or if we are causing it (or both). We'll never be able to tell because there is no way of predicting what the Earth would've done without us.

However, I completely agree with Winged Wolf on this: Way too many people look out their window, see cold weather, and proceed to scoff at global warming. The Earth is getting warmer. There is absolutely no doubt about that. Please note that the average temperature of the Earth is getting warmer. That's what global warming is. This does not mean every place on Earth will get warmer. (If you don't understand why, look up the definition of "average" in math terms.)

Weather is a whole lot more complicated that just your local forecast. Ocean currents, wind patterns, landmasses, mountains--all these things and more need to be taken into account when deciding what the weather will be like if global warming continues unabated. For example, the desert Southwest will get hotter and drier. However, some places will be cooler due to changing ocean temperatures and currents. Some places will be wetter. Some places will be completely under water!

Some references, for those more interested: Global Temperatures Plot