Sherwood Rowland’s comment at the banquet held to honor receipt, along with Paul Crutzen and Mario Molina, of the 1995 Nobel Prize in Chemistry, places the atmosphere at the center of some of the most influential scientific discoveries to have been made during human history. Within Rowland’s comment we can recognize Thales of Miletus who in the sixth century BC struggled to understand the different states of water and the process of evaporation, Lavoisier in the late eighteenth century discerning the exchange of oxygen between organisms and the atmosphere, and Arrhenius in the early part of the twentieth century calculating the relation between the carbon dioxide content of the atmosphere and the earth’s surface temperature. The importance of the atmosphere in the history of natural philosophy is clearly underscored by these seminal studies. Within all of these studies, however, is the undeniable influence of the earth’s surface and in particular the earth’s biosphere, on the chemical composition and dynamics of the atmosphere. The two are linked in a type of “co-dependency” in which processes and change can only be understood through studies that include both biotic and abiotic systems. The requisite nature of the nexus between the biotic and abiotic domains of the earth system is recognizable, albeit in extreme form, in the controversial concept of “Gaian homeostasis” laid out by James Lovelock and Lynn Margulis in 1974. While we (the authors) do not, in its entirety, endorse the tenets of a Gaian earth, we do recognize the value of this concept in defining the biosphere and atmosphere as coupled and interdependent systems. It is this interdependency, and the processes that maintain it, that will be the focus of this book.