The IPCC has published estimates of climate sensitivity of between 1.5 °C and 4.5 °C, but are unable to provide guidance on the likely actual level, whether it is in the middle of this range, or at either extreme. This is not science!
How can it be refuted (i.e. tested) if the premier organization does not dare make a prediction? Therefore, it is not science, since there is not an explicit statement that can be tested, and if necessary, refuted.
The expression “climate sensitivity” represents the warming theoretically expected if CO2 doubled from pre-industrial levels.
The use of this expression might have been an attempt to make an indirect proposition more understandable to lay-people. If so, one can say that has been fairly unsuccessful, and I believe it has stifled understanding and debate, rather than encouraged it.
In scientific circles, the effect of GHGs on the atmosphere is expressed more directly as “forcing” calculated as watts per square metre (W/m2). Scientists have calculated that the additional forcing for a doubling of CO2 will be 3.982 W/m2. (At least this part of the argument is reasonably treated as “settled science.”) So, expressing it another way, the IPCC are saying that additional forcing of 3.982 W/m2 is likely to result in an increase in global average temperature somewhere between 1.5 °C and 4.5 °C.
Complicating the use of this expression of “climate sensitivity” is the fact that CO2 is not the only greenhouse gas (GHG). What they actually mean is that any increase of GHGs resulting in a total increase in forcing of 3.982 W/m2 is likely to result in an increase in global average temperature somewhere between 1.5 °C and 4.5 °C.
One might be entitled to say that the definition and implementation of the expression “climate sensitivity” is too complicated to be useful in public debate. I prefer to use a new way of expressing this, referring instead to a “forcing multiplier,” even though it may superficially seem to be more complicated,
Converting the IPCC’s range of climate sensitivity into this new measure, we can say that the IPCC’s imputed estimate of this forcing multiplier would be from 0.38 to 1.13.
Using the “forcing multiplier” we can directly compare forcing and warming.
We know that the additional forcing (since industrialization) from all GHGs has been 3.05 W/m2. Over that period the global average temperature has increased by 0.94 °C (from -0.44 °C to +0.5 °C). This means that the observed forcing multiplier can be simplistically calculated as 0.94 / 3.05 = 0.31. Alternatively, using a more sophisticated calculation, taking into account a number of different variables, and every annual value from 1850 to 2014, we can say that the actual observed forcing multiplier is 0.37.
We can see that the actual observed values for the forcing multiplier are slightly below the bottom end of the IPCC’s imputed range.
Alternatively, we can just say that the observed climate sensitivity is slightly below the bottom end of IPCC’s range for climate sensitivity.
This is a fact, but it is not something you are likely to find in a peer reviewed “climate change” journal.
The major difficulty faced by most climate scientists is that their models predict a forcing multiplier much higher than the actual observed forcing multiplier. Indeed, the empirical data does not confirm their scientific analysis, so their proposition remains in limbo, and effectively “not proven.”
On the other hand, scientific propositions that are supported by the observed data are not being widely canvassed in the scientific literature. (Such propositions do exist, but they quickly disappear from view. Why?)
Currently there are no (accepted) scientific propositions to establish the forcing multiplier where both science and observations meet. We are in limbo on this subject.
We can present this problem graphically. Here we show just how far the IPCC upper range estimate of climate sensitivity (or forcing multiplier) is from the actuals. From direct communications I know that some climate scientists are expecting ocean temperatures to gradually increase, with the equilibrium (higher) temperatures of the ocean taking centuries to be achieved. While temperatures in the ocean are probably lagging the land temperature increases, there is no observable indication in the following chart that such a “catch up” has had any effect so far.
On the other hand, those who try to reproduce temperature data without taking into account GHGs will continue to struggle to win the argument. As can be seen, the other variables included here, like the 11 year sun-spot cycle, volcanoes, and El Nino, are not sufficient to explain the long-term trend of an increase in temperatures.
If the forcing multiplier remains around the level of 0.38 per W/m2, the long-term strategy dealing with the likely continuing increase in global average temperature is clear. A steady reduction in CO2 emissions in developed nations (plus China) of about 2% per year is necessary.
If, on the other hand, if the forcing multiplier rises to 1.13 per W/m2, as the IPCC seem to either expect or are not willing to dismiss, more radical action on CO2 will be required.
Yet 165 years of evidence weighs against the wildest of the IPCC estimates. Accepting and acting upon an extreme estimate is likely to lead to extreme political difficulties right across the whole planet. The precautionary principle suggests we should wait for more definite evidence before acting on such unsupported claims.