Shermer is right, of course. It's all about the butterfly effect, which refers to minor events that occurs in complex systems that potentially have a major impact.
Based on chaos theory, it is most commonly described in complex systems like weather prediction. The flapping wing of a butterfly represents a small change in the initial condition of the system, which causes a chain of events like pollen getting into the air, causing a gazelle to sneeze, thus causing a stampede of gazelles and other animals, thus causing additional dust to get into the atmosphere and block sunlight, thus changing the weather.
Since we can only look at gross changes and not every detail, it is very difficult to predict weather, and to predict the future. While we may not be able to track butterfly activity with any accuracy, we can track gross changes like dust in the atmosphere. Therefore, it does make sense for us to make short-term predictions, but not long term ones.