ATTRIBUTION RESULTS FROM COMBINATIONS OF FORCINGS



Climate models from different research groups use (slightly) different sensitivities for CO2, other GHGs, aerosols, volcanic and solar forcing. Although solar is probably underestimated in most models, there is also a huge offset between the (quite uncertain) forcing from man-made aerosols and CO2. If aerosols have less influence than currently implemented (see here), it is necessary to reduce the sensitivity for CO2 (forcing + feedback's), or the increase in temperature would be too high, especially in the 1945-1975 period, when there was a slight cooling. If true, this has important consequences for future warming. A reduction with 50% indeed means that the increase in temperature will be 50% of what is predicted for any scenario. Or compared to the average 3 C increase for a doubling of CO2 as projected by models, it would be 1.5 C...

The "model" I used was a simple EBM (energy balance model) as given during a course at the University of Oxford. This model only calculates the increase in temperature, based on forcing x sensitivity for each component and the mass of the oceans, as heat buffer. The spreadsheet has as advantage that the different sensitivities can be changed. Here follows two examples, one with the "standard" sensitivity of 3
C increase for a CO2 doubling and a similar sensitivity for the forcing caused by aerosols, volcanic and solar influences. The other includes halve the sensitivity for CO2, 1/2 and 1/4 for aerosols and the previous sensitivity for solar. The results for the 1900-2000 period are:
 
Eemian trends Vostok
Oxford EBM model with standard sensitivities:
3 C for 2xCO2, 1 x aerosols, 1 x volcanic, 1 x solar.
Results: correlation = 0.870, R2 = 0.756

Eemain-last
        glaciation

Oxford EBM model with reduced sensitivities:
1.5 C for 2xCO2, 0.25 x aerosols, 0.5 x volcanic, 1 x solar.
Results: correlation = 0.884, R2 = 0.792

The results are similar (even slightly better) with a strongly reduced sensitivity for CO2 and aerosols. This combination of sensitivities  is physically possible, as aerosol sensitivity is quite uncertain, even the sign is uncertain (brown/black aerosol vs. white aerosol), besides other items like clouds which act as negative feedback, while they are a positive feedback in all climate models...

If we take these two results and extrapolate them to the full 21st century with increasing CO2 emissions (with fixed solar, human and volcanic aerosols) that shows:


model up to 2100

As one can see: halving the CO2 effect also halves the increase in temperature over the 21st century,
even if both "runs" show the same "skill" to simulate the past century.
That is mainly because two control knobs (CO2 and aerosols) are each other's antagonists.

There was an extensive discussion about the relative role of CO2 and human aerosols (specifically SOx aerosols) in the early days of RealClimate:
http://www.realclimate.org/index.php/archives/2005/07/climate-sensitivity-and-aerosol-forcings/ with my comment at #14
and a later discussion at RC:
http://www.realclimate.org/index.php/archives/2006/02/an-aerosol-tour-de-forcing/ with my comment at #6 and the following discussion.

An interesting side item is that such a simple EBM program on a spreadsheet gives similar and even better results than the multi-million dollar "official" climate models based on ten-thousands
 lines of code. That was not published ever in the main scientific press but still available from the authors whom performed the "benchmark" tests:
http://www.economics.rpi.edu/workingpapers/rpi0411.pdf

As the above graphs end with the data of the year 2000, an update with the latest figures would be interesting, as there are record emissions of CO2 but only moderate changes in temperature, aerosols and a drop in solar activity... That will be for another day...


On the net: 5 October, 2006.
Last update: 30 October, 2018.

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