Temperature observations during the 20th century reveal a warming trend unlike any other in many millennia. As one of the preeminent scientists looking for an explanation of this phenomena, Princeton University professor V. Ramaswamy provides an overview of existing climate models and explains in an accessible manner how various natural and anthropogenic variables potentially bring about climate change. Through his summary of available research, Ramaswamy argues that human-induced variables, particularly greenhouse gas emissions, appear to be primarily responsible for the warming trend of the 20th century.