Long-term Climate Projections - Climate Change: Identification and Projections - page 107

10
Long-term Climate Projections
10.1. IPCC scenarios and projections
With the RCPs, the IPCC advantageously replaced its previous emissions
scenarios (SRES: Special Report on Emission Scenarios) with concentration
scenarios. The difference is a radical one. It was legitimate to hypothesize
on future human behavior, in particular CO 2 emissions, through the
combustion of fossil fuels. It is less so to build a scenario on future CO 2
concentrations, since these do not solely depend on anthropogenic emissions,
but on the whole of the “carbon cycle”: the way in which nature absorbs,
maintains or rejects the CO 2 according to the concentration which has been
reached, and the present global temperature. In principle, two coupled
models should be used: on one hand, the EBMs (or GCMs), where global
temperature is an output and CO 2 concentration is one of the inputs. On the
other hand, that of the carbon cycle whose output is CO 2 concentration and
whose inputs are the emissions rate and global temperature.
An extremely simplistic carbon cycle model consists of assuming that a
given fraction of anthropogenic emissions, known as the “airborne fraction”
(AR5, section 6.3.2.4), is accumulated in the atmosphere, and that the rest is
absorbed in various forms by the Earth's surface (solution in the oceans,
biomass, carbonates and other “carbon sinks”). It is simplistic because the
fractions absorbed and re-emitted obviously depend in part on the
atmospheric concentration itself, as well as on the global temperature. When
the IPCC announces that the so-called airborne fraction is 45%, this can only
be true temporarily, during the fleeting concordance between the generalized
phases of exponential growth which recently occurred (concurrently