On today's edition of Please Explain, we’ll look at the Earth’s atmosphere—what it's made of, how it makes life on the planet possible, determines weather patterns, and how human activity is changing it. We'll be joined by Dr. Joel S. Levine, Senior Research Scientist at NASA Langley Research Center, and Dr. Richard Somerville, Distinguished Professor Emeritus and Research Professor at Scripps Institution of Oceanography at the University of California, San Diego, and author of The Forgiving Air: Understanding Environmental Change.