The study says that every 405,000 years, due to wobbles in our orbit caused by the gravitational pulls of the two planets, seasonal differences here on Earth become more intense. Summers are hotter and winters colder; dry times drier, wet times wetter.
At the height of the cycle, more rain falls in the tropics, allowing lakes there to fill up. This compares to the other end of the cycle, when seasonal rains in the tropics "are less and lakes have much less of a tendency to become as full," Kent said.
