Climate Change—the effects of a warming world on the physical, biological, ecological, and social systems of our world—is emerging as the preeminent issue of the 21st century. Our ability lessen its impacts by effective policies, laws, and treaties that reduce the introduction of greenhouse gases (commonly referred to as mitigation), and our ability to adjust to the impacts which will occur regardless of mitigation or because mitigation is not as complete or effective as it needs to be (commonly referred to as adaptation), will largely dictate how severe and widespread climate change will be in this century and for centuries to come.