[wtracy]

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.

[guard-of-terra]

You can usually arrange in at least one ally (and now you're 2/6) and then tax other powers for premium features of the solar shade if they want them (that is, climate control).