It's civilization's practice run for the rare but certain future catastrophic events. A warm-up run if you like. If we can't handle everything that global warming throws at us, how can we ever hope to cope with the next asteroid strike etc.
It's like my previous post. If we're too afraid to get ourselves embroiled in local minor conflicts, how are we ever going to cope with the big ones.
I think this is the strongest argument for listening to young Bjorn and lower our priority of counteracting greenhouse gases over other environmental concerns. Unless of course that means we won't build any more hydro or nuclear power stations :)