RESULTS FROM COMBINATIONS OF FORCINGS
from different research groups use (slightly)
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
currently implemented (see here),
necessary to reduce the sensitivity for CO2 (forcing +
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
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
for a CO2 doubling and a similar sensitivity for the forcing
caused by aerosols, volcanic and solar influences. The other
halve the sensitivity for CO2, 1/2 and 1/4 for aerosols and the
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
better) with a strongly reduced sensitivity for CO2 and
combination of sensitivities is physically possible, as
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:
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
That is mainly because two control knobs (CO2 and aerosols) are
each other's antagonists.
On the net: 5 October, 2006.
Last update: 23 November, 2022.