Simple answer: Tragedy of the commons. If your nation consists of 1/6th of the world's population, you bear 1/6th the burden of the global warming triggered by

Simple answer: Tragedy of the commons. If your nation consists of 1/6th of the world's population, you bear 1/6th the burden of the global warming triggered by your fossil fuel use, but get 100% of the energy output from your fossil fuel use. Conversely, if you unilaterally decide to build a giant shade for the earth, you foot 100% of the bill yet only receive 1/6th of the overall benefits.