This video segment from 'Earth: The Operators' Manual' explores how we know that today's increased levels of CO2 are caused by humans burning fossil fuels and not by some natural process, such as volcanic out-gassing. Climate scientist Richard Alley provides a detailed step-by-step explanation that examines the physics and chemistry of different "flavors," or isotopes, of carbon in Earth's atmosphere.

This high-resolution narrated video shows levels and movements of CO2 globally through the course of a year.