Tapping the power of fossil fuels over the past century and a half has propelled a massive expansion of human enterprise and prosperity, yet it has also released toxic amounts of carbon dioxide into the atmosphere, thereby endangering the future viability of human civilization. If global average temperatures rise 4 to 6 degrees above the level of preindustrial times, climate scientists nearly unanimously conclude that severe disruptions of global ecosystems will result. These disruptions include: self-reinforcing spirals of global warming caused by feedback factors like melting permafrost, ice loss, water vapor, and wildfires; acidification of ocean waters and flooding of coastal regions as sea level rises by as much as one foot per decade; growing frequency of severe weather events such as droughts, floods, superstorms, and heat waves; the spread of tropical diseases into temperate regions; and the collapse of agriculture in many parts of the planet, leading to waves of desperate climate refugees.