The science is clear that the Earth’s climate, including that of the United States, is changing, changing much more rapidly than generally occurs naturally, and it is happening primarily because of human activities. This chapter discusses the science underlying climate change and the current understanding of how our planet is being affected. In addition to the global analysis, there is special attention given to the findings for the United States. Humanity is already feeling the effects from increasing intensity of certain types of extreme weather and from sea level rise that are fueled by the changing climate. Climate change affects many sectors of our society, including threats on human health and well-being. Climate change will, absent other factors, amplify some of the existing threats we now face. The effects on humanity are already significant, costing us many billions of dollars each year along with the effects on human lives and health. Policy to respond to climate change is imperative—we have three choices, mitigation, adaptation, or suffering. Right now we are doing some of all three. The Paris Agreement begins the process internationally of really doing something to slow down change. But the current agreement is just the beginning and we will need to do much more.