Climate models from different research groups use (slightly) different sensitivities for CO2, other GHGs, aerosols, volcanic and solar forcings. 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 inplemented (see here), it is necessary to reduce the sensitivity for CO2 (forcing + feedbacks), 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 forcings x sensitivities and the mass of the oceans, as heat buffer. But it 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, 1/4 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 slighly better) with a strongly reduced sensitivity for CO2 and aerosols. This combination of sensitivities  is physically possible, as CO2 sensitivities are quite uncertain, especially cloud feedback, which has an observed inverse correlation with solar irradiation, but no certain correlation with GHGs...

On the net: 5 October, 2006.

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