Chapter 9 explores the questions: what are climate cycles and what causes them, and where do we presently sit within a climate cycle? This exploration begins by describing cyclic changes in Earth’s orbit around the Sun; the three most important cycles have periods ranging from 23,000 to 100,000 years, and cause cyclic variations in the amount of solar energy reaching the Earth (and driving Earth’s climate system). Those changes in insolation have been modified by feedbacks within Earth’s climate system to affect the distribution of carbon between the ocean and the atmosphere, to the extent that they have controlled the repeated growth and decay of large Northern Hemisphere ice sheets during the last several million years. One very long Earth-Sun cycle, with a period of 405,000 years, has been consistent for several hundred million years and can be interpreted from ancient sediments, providing very valuable time constraints on those older paleoclimate records. The increase in atmospheric CO2 due to human activity is now sufficient to change the natural cyclicity of Earth’s climate system.