All Else Remaining Equal

Carbon dioxide in the atmosphere is expected to increase heat energy retained in the atmosphere by 1 to 1.5 degrees C for each doubling of concentration, "all else remaining equal".  All else remaining equal is a rather large assumption.

The Berkeley Earth Surface Temperature project was started because of questions that Dr. Richard Muller had about the transparency and accuracy of the surface temperature record used to determine the degree of warming due to increased carbon dioxide.  The project provide another independent estimate of surface temperatures which is fully open for inspection and has some very user friendly features.  Having Tmax and Tmin, the maximum and minimum recorded surface temperatures is really trick.  The chart above is Best versus the oceans.   It is a little hard to read.

To make it easier to see, here is just the Northern Hemisphere starting in 1900 using a 5 year centered moving average with a 1961 to 1990 baseline.  The 5 year averaging is just to reduce some on the noise without lossing too much of the signal and the 1961 to 1990 baseline is to make it easier to compare the instrumental data to the numerous plaeo climate reconstructions of past temperatures.  Note that the light blue curve is Tmin.  The slope of this curve is more consistently rising from the 1900 start than the Tmax and sea surface temperature(SST).  The end value of Tmin is approximately 1.8 degrees C warmer than it was at the start.  The greatest rise in the Tmin slope starts about 1970 and is approximately 1.3 degrees.  The start of that slope followed an approximately 0.3 degree drop beginning around 1960.

This chart is for the southern hemisphere.  The Tmin curve in the southern hemisphere also has a more consistent rise from the start in 1900.  There is not a much increase in the slope after 1970.  The SST and Tmax curves in this chart have a more uniform increase in temperature.  The total increase in the Tmin is the largest of the three and is approximately 1.5 degree maximum change.  Unlike the northern hemisphere, most of the increase in Tmin was before 1980.

In this chart there is a break in each series in 1965, approximately the maximum slope of both series with linear regressions for both segments of both series.  In the northern hemisphere, the change in slope is 3 times the initial slope.  In the southern hemisphere the slope doubled.  The northern hemisphere contains nearly 70% of the land mass which makes up about 30% of the total surface area of the globe.  The southern hemisphere contains about 30% of the land mass.  Coverage of the southern hemisphere is not a good as the northern hemisphere and the majority of the temperature data in the southern hemisphere Antarctic region, nearly 9% of the total global land mass, only started in earnest in the late 1950s.  There is some uncertainty, but the data is about the best we have.

Comparing the hemispheres, it doesn't look promising that "all things remained equal".  The CO2 increase should have its largest measurable impact on the average minimum surface temperature and should be greater over land than the oceans.  With the southern hemisphere having much more oceans influence than the northern hemisphere, the doubling of the slope in the SH versus the tripling of the slope in the NH is not unexpected.  The slope of both hemispheres prior to the 1965 change, would not as likely be due to CO2 increases.  As a rough estimate, half of the SH warming would be unrelated to CO2 forcing and possible one third of the NH warming would also be unrelated to CO2.  The curve in the SH series from 1965 to 2010 has the appearance of a dampened response, not in keeping with CO2 forcing, which implies more natural variability than generally mentioned by Anthropogenic Global Warming affectionadoes.

While swaying the "believers" in AGW is unlikely, rational skepticism of AGW being primarily due to CO2 forcing appears logical deduction not denial of proven science.  The "believers" perhaps should spend some time analyzing their own motivations.

Added Tmax and SST for the curious: