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.

A sequence of five short animated videos that explain the properties of carbon in relationship to global warming, narrated by Robert Krulwich from NPR.

This static graph of changes in CO2 concentrations goes back 400,000 years, showing the dramatic spike in recent years.