evidence: Even more subtle motions exist. The circle traced out in the sky by the North Pole has little wobbles on it, because of a sort of rocking motion called nutation. Over a cycle of about 41,000 years, the tilt of the spin axis actually varies from about 22 to 25 degrees. In the first part of the 20th century the Serbian mathematician Milutin Milankovitch proposed that such variations in tilt, along with slight changes in the shape of Earth’s orbit, have triggered climate change in the past. Climate scientists now widely accept this model as explaining one of the many triggers of the onset and ending of ice ages. [...] Taking into account all that has been learned about Earth’s past and present, one can speculate in an informed way about what the future may hold for the planet. This involves exploring two basic scenarios: one in which humans have no effect or role and another in which people influence the course of events in various possible ways.