Besides the obvious "it heats up the planet, dumbass" answer, global warming has many other effects on our weather. It can cause longer droughts, more rain, odd pressure systems, and severe storms. I've always wondered why this happened, and I found a good explanation in the August issue of Scientific American which I will paraphrase very loosely and briefly (I hope this is ok, bones!)

As the planet gets warmer, the higher temperatures accelerate the water cycle. Water vapor from the oceans evaporates into the atmosphere quicker and therefore becomes precipitation quicker. A warm atmosphere holds more moisture than a cool one, and drops water more frequently and in larger amounts. As the land heats up, parching occurs in dry areas. These large parched spaces increase the pressure gradients that cause winds to develop. This causes more violent storms, tornadoes and higher winds.

This is a very simplified explanation, but I hope it helps. This whole "our world is going to hell" deal can get pretty confusing sometimes.