National Geographic: Remember when tornadoes used to be something that just happened along about April and we waited for that first 70-degree day in May? Now we have terrible twisters in January and February and 70-degree days to boot. Is it global warming? Who knows at this stage, but something weird is happening. Those huge swings in temperatures are dramatic, but just a little change in the average readings for the planet could cause havoc, as a National Geographic Channel documentary shows …