RESULTS FROM COMBINATIONS OF FORCINGS
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
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:
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
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.