We have temperature station measurements in 1940 to estimate the 1940 temperature, and data since then to estimate the warming trend. (skepticalscience.com)
To estimate warming over the entire 21st century, scientists have to make assumptions about what happens beyond the INDCs. (carbonbrief.org)
So when Mr. Romm says that «[t] he best estimate warming for 1,000 ppm of CO2eq is 5.5 °C,» this means that (1) such a concentration should eventually lead to such a temperature change, and (2) this temperature change is versus pre-industrial temperature, not versus today's temperature. (cato-unbound.org)