Climate change is frequently referred to as one of the defining challenges of the twenty-first century. The authors of this chapter concur. In broad terms, the climate challenge is relatively straightforward. Global average temperatures are rising as a consequence of anthropogenic emissions of greenhouse gases. In the absence of deliberate and global action to substantially reduce and then eliminate (or even turn net negative) greenhouse gas (GHG) emissions, global temperature rise within this century is very likely to surpass two degrees Celsius (IPCC 2014), which is the (somewhat arbitrary) threshold set by the international community as a tolerable level. Continuation of current levels of emissions or continued growth in emissions throughout the twenty-first century could result in warming far above the two- degree threshold with very bad implications for the planet, for human societies, particularly poor people.