The temperature changes are relative to the global average surface temperature of 1961 - 1990. (royalsociety.org)
It is worth noting that, in the absence of convection, pure greenhouse warming would lead to a globally averaged surface temperature of 72 °C given current conditions. (scienceofdoom.com)
It's easy to see that there was a sharp increase in global average surface temperatures from the 1970s through the end of the 1990s. (wxshift.com)