The ravages of climate change emerged as a central issue in the 21st century. In the U.S., the political divide over climate change and how to confront it expanded. While Democrats became increasingly determined to join international efforts led by the United Nations to combat climate change, Republicans largely balked. The country embraced climate leadership when led by Obama and Biden but when led by Bush and Trump, a rebuff ensued. Those opposing climate action included the fossil fuel industries and others that sought to profit from the status quo. Climate crisis activism of the 21st century is the story of the tension between those wishing to continue business as usual and those seeing their futures directly impacted by such choices and demanding climate justice. This is a guide to understanding climate policy and activism, highlighting the participants, the progression of environmental science, and the pendulum of national policy under changing parties in power.