By Andy May
This post was updated August 25, 2015 to clarify the text on sea surface temperatures, provide a correlation coefficient between the two warming periods and to discuss the Planck parameter.
Many writers, including Professor Richard Lindzen and Ed Caryl have noticed the remarkable similarity in global warming observed from around 1910 to 1944 and 1975 to 2009. The similarity in slopes exists in all global surface temperature datasets. Figure 1 shows the HadCRUT version 4 dataset and the NASA GISS land (GHCN v3) and ocean (ERSST v4) temperature dataset. We’ve identified the two periods of interest on the figure. All datasets also show some cooling between 1945 and 1975.
Figure 2 shows the two periods overlain with data from the HadCRUT version 4 dataset. This display is scaled to actual average temperature. Unlike Figure one this figure uses monthly smoothed data. In that way, we can see some of the variation within each year.
The left side of Figure 2 represents 1910 for the blue line and 1975 for the orange line. On average the earlier blue line is 0.36°C cooler than the later line. The later line also has a steeper slope, the earlier represents 0.144°C of warming per decade and the later line shows 0.192°C warming per decade. Figure 3 shows the yearly HadCRUT v4 anomalies from the mean with the two means forced to be the same.
Now we can easily see the similarity in the two warming periods. The linear correlation coefficient (also called the Pearson product moment correlation coefficient) between the two periods is 0.81, so they are highly correlated. Yet the IPCC in their AR5 Summary for Policy Makers states on page 17:
“It is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together.”
On page 14 of the Summary for Policy Makers they provide a description of the anthropogenic “radiative forcing” from man’s emissions and other actions. This is shown in Figure 4.