The ECCOH (pronounced "echo") chemistry module interactively simulates the photochemistry of the CH4
–CO–OH system within a chemistry climate model, carbon cycle model, or Earth system model. The computational efficiency of the module allows many multi-decadal sensitivity simulations of the CH4
–CO–OH system. This capability is important for capturing nonlinear feedbacks of the CH4
–CO–OH system and understanding the perturbations to methane, CO, and OH and the concomitant climate impacts.