Thank you everybody. Those advices realy came in handy.

About the desert, yes, I was talking about the classic, Saara/Atacama/whatever-like desert. Thankfully I developed a "damage control" rule for fixing such drastic nuisances. Something I called "catastrofic magic", which happens spontenously as much as any natural disaster, but is totally unpredictable. It conveniently served for explaining how the PCs could leave the artic-like country and reach a desert land so quickly in a ship travel. Pretty good tool, I could even build our current campaign centered on this weird phenomena. But nobody cares about it, right?

Something else I'd like to ask is that if anyone knows a program for simulating the tectonic plaques' movements (and, by extension, definining a large amount of important aspects of a planet's lands) and, if so, where can I get it.