Introduction - Climate Change: Identification and Projections - page 2

consensus in the scientific community. They are available on the IPCC
website and are regularly referred to throughout this work 3 .
According to the final press release, taken from the SPM (p. 17): “It is
extremely likely that human influence has been the dominant cause of the
warming observed since the mid-20 th Century”. Per the fourth report (AR4,
2007) this statement was only qualified as “ very likely ”. In the highly
standardized language of the IPCC, this means that its confidence in
attributing such warming to human influence has increased from 90%
to 95%.
This confidence is less evident in the texts. Of all the quantified
evaluations in the SPM, without a doubt, the most significant is what is
known as the planet's climate sensitivity. It quantifies the equilibrium
temperature change that would be caused by a possible doubling of the
concentration of CO 2 in the atmosphere 4 . According to the SPM (p. 14):
“equilibrium climate sensitivity is likely in the range of 1.5°C to 4.5°C ( high
confidence ), extremely unlikely less than 1°C ( high confidence) and very
unlikely greater than 6°C ( medium confidence ).”
According to the IPCC's future scenarios regarding concentration, the
rate of CO 2 may well increase four-fold in the course of the next century. In
the extreme cases, (1°C to 6°C), the respective consequences range from the
minor to the catastrophic: 2°C or 12°C. Moreover, the likely range has
broadened since 2007. The IPCC highlights the fact that “the lower limit of
the likely range evaluated (1.5°C) is therefore lower than the 2°C stated in
the AR4”. Aware that the alarmist nature of the message may be diluted,
scientists justified this modification to governmental delegates (p. 14): “this
assessment reflects improved understanding of climate sensitivity, the
extended temperature record in the atmosphere and the ocean, and new
estimates of radiative forcing”. Therefore, uncertainty has increased as
knowledge has broadened, despite the 95% confidence stated. It is on this
basis that international agreements are entered into, which involve the annual
3 http://www.climatechange2013.org/images/report/WG1AR5_SPM_FINAL.pdf
https://www.ipcc.ch/report/ar5/wg1/
4 This simple formulation contains a logarithmic law: if a doubling of CO 2 leads to
a temperature rise of S clim ( °C ), a further doubling will not lead to 4 S clim , but only 2 S clim .
A third doubling, eight times the initial level, would lead to a rise of 3 S clim , and so on. Note
that this is an empirical law where the extrapolation for high concentrations is questionable.