Conclusions - Climate Change: Identification and Projections - page 131

legitimate to call into question. We know that water constitutes
the fundamental heat-transfer fluid in the climate machine, through its phase
changes: oceans and clouds (liquid)

↔ gaseous (invisible). Greenhouse
gases (including gaseous water) cannot act in isolation, without interacting
with cloud formations (liquid water). However, the IPCC admits that the
theoretical calculations regarding the greenhouse effect are valid in a
supposedly clear sky (AR5, 8.3.1). In fact, there is no experimental
confirmation of greenhouse effects by direct atmospheric measurements
(weather balloon, satellites), and the AR5 is particularly discreet on the
absence of a tropospheric hot spot which would nonetheless constitute its
primary sign (AR4, Figure 9.1). In such conditions, the hypothesis that
GHGs and human activity do not have a significant effect cannot be
dismissed. We went on to a statistical test of this hypothesis, which shows
that it cannot be rejected, with a significant certainty rate of 90%. We would
not go as far as to say that this is the most likely situation, but, it is certain
that anthropogenic sensitivity will remain very difficult to quantify for some
time, both through theory and identification. Indeed, climate history being
what it is, and given the forecasts of CO 2 emissions, we are unable to see
how, only using observations, the accuracy of the sensitivity to CO 2 could
improve in the foreseeable future. Paradoxically, this result is highly
significant: it contradicts the assertion that climatic observations prove the
anthropogenic origin of global warming.

12.3. Solar activity

The second fundamental climatic parameter is sensitivity to solar activity.
In the context of identification, the situation is more positive: the past
millennium saw successive large climatic variations: medieval warm period,
little ice age and recent warming. There were also transitory changes, the
most recent of which occurred around 1810, 1910 and 1940. Moreover,
since 1850, the quality of the thermometric measurements helps us to discern
the impact of 11-year solar cycles on temperatures. On all timescales, we see
correlations between these climatic fluctuations and solar activity, as shown
by measurements of irradiance, the observation of sun spots and archives of
cosmogenic isotopes. Nevertheless, the natural variability of the climate
clouds the perception of this: correlations which appear to be very revealing
over some periods, are contradicted over others. Furthermore, the analysis of
static correlation is misleading. As revealing as they may seem - precisely