The CO2 warming theory depends on a strong water vapor amplification mechanism, where the initial CO2 temperature forcing evaporates water into atmosphere whichg traps yet more heat, creating yet more water vapor, etcetera. As Roy notes, it is this warming and humidifying of the global atmosphere and the resulting acceleration of the hydrologic cycle that creates the upper troposphere hotspot. Ergo, no hotspot means no powerful water vapor amplification mechanism and no CO2-based account of late 20th century warming.