Presumed Parameters - Climate Change: Identification and Projections - page 63

5.8. Review of uncertainties
As stated earlier, the assumed values, attributed here to the IPCC, are
based on its own quantifications, and its own fundamental concepts of
radiative forcing, static sensitivity and climatic feedback. Nevertheless, the
IPCC implies that the uncertainties could be significantly greater than stated.
Indeed, following discussions during the meeting for the SPM 6 , the
Coordinating Lead Authors (CLA) complements the definition of climate
sensitivity as follows:
“ The lower temperature limit of the assessed likely range is thus less than
the 2°C in the AR4 (2007), but the upper limit is the same*. The assessment
reflects improved understanding, the extended temperature record in the
atmosphere and ocean, and new estimates of radiative forcing”. * In a
footnote: “it is not yet possible to say which is the best estimate for climate
sensitivity, due to a lack of agreement between values resulting from various
studies and demonstration proposals.”
In the fourth SPM (2007), the most likely value was mentioned (3 ° C).
Rather than revise it downwards, it disappeared from the SPM 2013.
New estimates for radiative forcing are mentioned. However, since AR3
(2001), the IPCC has used the same formula (Myrhe, 1998), without
changing even one decimal, as if it was a definitively established physical
constant. Although the IPCC reports on a significant amount of later work, a
quantified consensus cannot be established. In the AR5, it is stated:
8.3: “The concentrations of WMGHGs can be determined from
observations […]. Thus the radiative forcings from WMGHGs are
determined entirely from observations.”
6 On equilibrium climate sensitivity, several delegations, including Australia, the Netherlands
and others, noted that the message that the lower limit of the assessed “likely” range of
climate sensitivity is less than the 2°C in the AR4 can be confusing to policy makers and
suggested noting it as the same as in previous assessments. The CLAs explained that
comparison to each of the previous IPCC assessments would be difficult, and new language
was developed adding that the upper limit of the assessed range is the same as in AR4.
(Kosolapova, 2013). This teaches us that values given for fundamental climatic parameters for
the planet are negotiated during the plenary session of the IPCC, between scientists and
governmental delegations.