Although only 1.2% of the land area of the Earth is currently considered urban, the spatial coverage and density of cities is expected to rapidly increase in the near future. The United Nations estimates that by the year 2025, 60% of the world's population will live in cities. Human activity in urban environments alters atmospheric composition; affects components of the water cycle; and modifies the carbon cycle and ecosystems. However, our understanding of the effects of urbanization on the total Earth-climate system is incomplete. The U.S. Climate Change Science Program (CCSP) strategic plan states that ...in a world that is more populated, urban, and interconnected than ever...A more integrated understanding of the complex interactions of human societies and the Earth System is needed.