Global warming doesnt necessarily mean the temperature will rise everywhere, no. Where it does rise is important, for example if the ice caps continue to melt, then this may cause extra freshwater and cooler water to mix with the current oceans... this is pretty complex and I don't claim to fully understand it but as an example, the extra freshwater can upset the salt balance and change the dynamics of the undersea currents. This will affect the massive belt of warm water that flows round and will therefore cool areas down. A few degrees centigrade of change in the climate can seriously affect an ecosystem, and most certainly will affect weather systems. Thats a vast simplification of how 'warming' can actually make things cooler and wetter. Here is an interesting article from New Scientist that covers this quite well: http://environment.newscientist.com/channel/earth/mg19926691.500-climate... (can be accessed fully from a university network, or if you are subscribed)
Hope that helps!