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greenhouse gases (methane, etc.), by industrial aerosols, or by changes in
land use, the impact of which then adds or subtracts from the influence
solely restricted to CO
2
.
Adding these elements as distinct inputs into an identification model
would cause a number of problems. Firstly, the various greenhouse gases are
not driven by identical laws (logarithmic for CO
2
, square root for methane,
etc.). Moreover, some factors have an impact due to their concentration,
others by their emission rate, which effects whether the time they remain in
the atmosphere is respectively short or long. The concept of GWP (Global-
Warming Potential) attempts to bring together their effects. In any case, they
are all highly correlated between each other, due to the simple fact that, since
the start of the industrial era, they have all grown exponentially. By
identification, it would therefore be difficult to distinguish between their
respective effects.
If we limit anthropogenic inputs solely to the concentration of CO
2
, this
must then be interpreted as an indicator of human activity, and the sensitivity
identified will aggregate the sensitivity to all the other factors not listed,
including all the approximations stated above.
The identified sensitivity
S
clim
will then be an anthropogenic sensitivity,
whose assumed value it is important to establish. For this reason, we have
examined the information provided by the IPCC on the various
anthropogenic forcings listed. Amongst them, some are positive (other
GHGs), others negative (aerosols and variations in albedo due to a change in
land use). The overall result is likely to be positive, which would move the
range upwards. Given the uncertainties, the extreme lower limit is
maintained (
). Since there is only
medium confidence
in the upper
limit, this is also maintained (
1C
°<
S
c
lim
), rather than resorting to risky
S
<°
6C
c
lim
calculations.
In the end, no distinction will be made between the ranges of uncertainty
for climate sensitivity in the strictest sense (relating to CO
2
) and for
anthropogenic sensitivity.
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