It's just because you can buy and sell weather now. Once people can make money off bad weather, they are going to try to cause bad weather. Currently, you can only buy and sell what are effectively average temperatures. They will almost certainly add rain and snow eventually. I wouldn't be surprised if they add hurricanes at some point. In case anyone cares, weather is traded at CME (www.cme.com).

More seriously, we still don't have good climate models. There is still a lot of uncertainty about what weather is normal. There is supposed to be a 30ish year hurricane cycle, which means that even without global warming things would be getting worse now. People just don't remember things for that long.