Fig. 1. (a) Global temperature anomalies (NASA, 1880–2013) as functions of radiative forcing using the carbon dioxide (CO2) forcing as a linear surrogate. The line has a slope of 2.33°C per CO2 doubling. Some dates and corresponding annually, globally averaged CO2 concentrations are indicated for reference. GISS, Goddard Institute for Space Studies; ppm, parts per million. Adapted from Lovejoy [2014b, Figure 1a]. (b) The residuals from the straight line in Figure 1a; these are the estimates of the natural variability. The vertical dashed lines are the same as in Figure 1a. The arrows indicate notable events. Adapted from Lovejoy [2014b, Figure 1c].