Explore the pivotal role of carbon dioxide in Earth's climate history through this Bjerknes Lecture delivered at the AGU Fall Meeting 2009. Delve into a comprehensive examination of CO2 as the primary control mechanism for global climate, spanning from ancient geological periods to modern times. Investigate fascinating topics such as Snowball Earth, the Great Dying, dinosaur extinctions, and ice core data. Analyze the interplay between CO2 concentrations, solar physics, and rock weathering in shaping Earth's climate. Examine techniques for measuring historical CO2 levels and their implications for understanding climate sensitivity. Address counterarguments and explore the significance of volcanoes, cosmic rays, and space dust in climate change. Gain insights into regional climate variations and confidence intervals in climate predictions. Conclude with a thought-provoking Q&A session to deepen your understanding of this critical aspect of Earth's climate system.