Modellers have examined a wide array of ideal-world scenarios for regulation of greenhouse gases. In this ideal world … analysts have given inordinate attention to which countries participate in regulation—what we call “variable geometry …”—which has a strikingly small impact on total world cost of carbon regulations if international trade in emission credits allows …

contains several idealistic assumptions that could be violated in the real world where some technologies may not be fully … available, technology transfers and diffusion are imperfect, some world regions may not accept to reduce their GHG emissions …