Human activities now match (and often exceed) the natural forces of the Earth System (Steffen/Sanderson/ Tyson/Jäger/Matson/Moore/Oldfield/Richardson/ Schellnhuber/Turner/Wasson 2004). Recent ice core data show that current levels of carbon dioxide (CO2) and methane are well outside the range of natural variability over the last 800,000 years (Luthi/Le Floch/ Bereiter/Blunier/Barnola/Siegenthaler/Raynaud/Jouzel/ Fischer/Kawamura/Stocker 2008). Roughly half of the world’s ice-free land surface has been altered by human actions. Humans now fix more nitrogen than does nature. Particles emitted by human activities alter the energy balance of the planet, as well as having adverse effects on human health. Human choices about how we use resources are at the heart of many of these changes. These may seem to be unrelated issues; however, over the last decades, we have gained a deeper understanding of the degree to which all of these separate issues are linked. The Earth System is a very complex coupled system with myriad feedbacks, and it has and inevitably can still exhibit rapid, globalscale responses to changes in environmental conditions (Costanza/Graumlich/Steffen/Crumley/Dearing/Hibbard/ Leemans/Redman/Schimel 2007).