An Issue of National Security: Understanding how the Earth system evolves is one of the most difficult of sciences many grand challenges. The problem requires billions of data points from the surface, sea, air, and space-based observations. These data points are used to model the system on the most powerful computers through the application of physical sciences and applied mathematics and computational science. Unexpected rapid environmental change poses risk to national security, the economy, and public health. Livermore has been studying the climate and Earth system since the beginning of computer simulation. Its codes, developed to understand the complex dynamics of nuclear detonations, were adapted to study their impacts on the atmosphere, and eventually, the atmosphere itself. Today, Livermore contributes to Earth system and climate research on several fronts modeling the climate to understand the impacts of climate change, adapting models to study regional effects, for example, on the Western U.S. watershed, and testing the many existing climate models to understand how well they perform at predicting the atmosphere's actual behavior.