The Earth system is a vast web of interconnected processes, with processes such as microscopic chemical reactions, ocean currents, forests exchanging carbon with the atmosphere, and humans altering the composition of the air all affect the system. However, given enough time, a small perturbation can alter its trajectory, known as the "butterfly effect", can cause significant changes in the system's direction. Major climate models have been developed around the world by universities, national laboratories, and government agencies. The author suggests that understanding the climate system is so complex that statistical models are no longer reliable guide to the future. At NCAR, scientists developed the Community Earth System Model, a comprehensive climate model that is used by researchers across the country and around the globe to study climate change, severe weather, and climate effects on wildfires.