By Steve Goddard
My friend Tamino says that “the modern global warming era starts in 1975.”
He goes on : “It’s an estimate of the time at which the trend in global temperature took its modern value.”
As you can see in Phil Jones’ HadCRUT graph above, the 25 year period from about 1975 to 2000 did warm about half a degree C.
You can also see that the 30 year period from 1910 to 1940 similarly warmed about half a degree C. At that time, atmospheric CO2 averaged about 305 ppm, well below Dr. Hansen’s suggested “safe level” of 350 ppm. See the graph below for that period:
Here’s an annotated HadCRUT graph to help you see the relevant periods and the changes of temperature versus changes in global CO2 concentration during the same period:
The video below superimposes the 1975 warming (blue line) on the 1910 warming (black line.) Note the similarity in slope, duration and patterns. It would be difficult to explain the 1910 warming as being due to CO2, because CO2 was barely above pre-industrial levels and rose only 10 ppm during that period.
Given the similarity between the 1975 warming and the 1910 warming, it is irrational to blame the 1975 warming entirely on CO2. The practice of good science tells us to look for a hypothesis which can explain both similar warming periods.
If there is an influence of CO2 in the recent warming, it appears small. And the warming stopped ten years ago, as shown in the HadCRUT graph, despite rapid increases in CO2.
Or perhaps one might conclude that climate sensitivity has decreased as CO2 levels have risen. In 1910, with CO2 at 300 ppm, it only took ten additional ppm to raise temperatures by 0.5°C. By contrast, in 1975 it took about fifty ppm more to produce the same 0.5°C warming by the year 2000.
There were also periods of time with rising CO2, and little or no rise in temperature. From 1940 to 1980, there was no net warming while CO2 rose by 30 ppm. Since 1998, there has been no warming – as CO2 levels have risen 30 ppm.
I feel a chill of La Niña coming on.