Long-term Climate Projections - Climate Change: Identification and Projections - page 111

driving the freely identified models (with the weak constraint mentioned
above). For forced simulations, we use the strong IPCC's constraint
2

−

S irr , leaving all the other parameters free and identified. The
zero temperature in the figure is the conventional preindustrial equilibrium
temperature (reference 1850-1900). We highlighted the future year 2100, as
well as the critical level of a 2-degree warming, which is the basis of
negotiations during international climate summits.

< 1 62 ° C / Wm

Figure 10.3. Projections under RCP 8.5

By severely limiting the contribution of solar activity, the IPCC harms
the reproduction of millennia-old temperatures (see Chapter 7), and therefore
indirectly reinforces the anthropogenic radiative forcing coefficient
(see Chapter 9). Consequently, the bundle of curves in graph (b), resembles
the one in Figure 10.1 (under RCP 8.5). In all cases but one, the projections
to 2100 cross the line indicating a warming of two degrees above the
conventional preindustrial temperature.

The range of free simulations (a) is much less alarming. In some cases,
future temperatures decrease, when anthropogenic sensitivity is estimated as
being zero, or that the drop in solar activity over-rides the effect of an
insignificant anthropogenic sensitivity. In only a small number of cases, all
of which come from the Jones and Mann reconstruction, is the critical
threshold of 2 ° C exceeded - though only by a small margin - before the end
of the century. Several centuries into the future, some projections may still
become alarming, but their reliability depends on the accuracy of other