4. The Coupled Model
Intercomparison Project Phase
5 (CMIP5) presents an
unprecedented level of
information on which to base
projections including new
Earth System Models with a
more complete representation
of forcings, new
Representative Concentration
Pathways (RCP) scenarios and
more output available for
analysis.
New experiments and
studies have continued
to work towards a more
complete and rigorous
characterization of the
uncertainties in long-
term projections, but
the magnitude of the
uncertainties has not
changed significantly
since AR4.
5. Pour établir le cinquième Rapport d’évaluation du GIEC, la communauté scientifique a défini un
ensemble de quatre nouveaux scénarios, appelés profils représentatifs d’évolution de
concentration (RCP).
Ces RCP sont identifiés par leur forçage radiatif total approximatif pour l’année 2100 par rapport à
1750:
-2,6 W m-2 pour le RCP2,6 scénario d’atténuation conduisant à un niveau
de forçage très bas
- 4,5 W m-2 pour le RCP4,5 scénario de stabilisation
- 6,0 W m-2 pour le RCP6,0 scénario de stabilisation
- 8,5 W m-2 pour le RCP8,5 scénario aux émissions GES très élevées
Les RCP peuvent ainsi représenter toute une gamme de politiques climatiques pour
le XXIe siècle, par comparaison avec le rapport spécial sur les scénarios d’émissions (SRES), dont les
scénarios d’émissions étaient utilisés dans les troisième et quatrième Rapports d’évaluation, et qui
ne prévoyait pas de politique climatique.
6.
7.
8. Global mean temperatures will
continue to rise over the 21st
century if greenhouse gas
(GHG) emissions continue
unabated.
Temperature change
will not be regionally
uniform. There is very
high confidence that
globally averaged
changes over land will
exceed changes over
the ocean at the end
of the 21st century by a
factor that is likely in the
range 1.4 to 1.7.
10. Temperature (left) and precipitation (right) change patterns derived from transient simulations from
the CMIP5 ensembles, scaled to 1°C of global mean surface temperature change. The patterns
have been calculated by computing 20-year averages at the end of the 21st (top) and 22nd
(bottom) centuries and over the period 1986–2005 for the available simulations under all RCPs,
taking their difference (percentage difference in the case of precipitation) and normalizing it, grid-
point by grid-point, by the corresponding value of global average temperature change for each
model and scenario.
11. Global mean
temperatures will
continue to rise over
the 21st century
under all of the
RCPs. From around
the mid-21st century,
the rate of global
warming begins to
be more strongly
dependent on the
scenario .
12. It is virtually certain that, in most places, there will be more hot and fewer cold temperature
extremes as global mean temperatures increase.
These changes are expected for events defined as extremes on both daily and seasonal
time scales. Increases in the frequency, duration and magnitude of hot extremes along with
heat stress are expected; however, occasional cold winter extremes will continue to occur.
14. Changes in average precipitation in a warmer world will exhibit substantial
spatial variation. Some regions will experience increases, other regions will
experience decreases and yet others will not experience significant
changes at all. There is
high confidence that the contrast of annual mean precipitation between
dry and wet regions and that the contrast between wet and dry seasons
will increase over most of the globe as temperatures increase.
Annual surface evaporation is projected to increase as global
temperatures rise over most of the ocean and is projected to change over
land following a similar pattern as precipitation.
Prominent areas of projected decreases in evaporation include southern
Africa and north western Africa along the Mediterranean.
Surface drying in these regions as global Temperatures increase is likely
with high confidence by the end of this century under the RCP8.5 scenario
21. It is very likely that the Arctic sea
ice cover will continue shrinking
and thinning year-round in the
course of the 21st century as
global mean surface temperature
rises.
At the same time, in the Antarctic,
a decrease in sea ice extent and
volume is expected, but with low
confidence.
It is very likely that NH snow cover
will reduce as global
temperatures
rise over the coming century. A
retreat of permafrost extent with
rising global temperatures is
virtually certain.
Fig. 12.29
22. The global ocean will warm in all RCP scenarios.
The strongest ocean warming is projected for the surface in subtropical and
tropical regions.
Best estimates of ocean warming in the top one hundred meters are about
0.6°C (RCP2.6) to 2.0°C (RCP8.5),
and about 0.3°C (RCP2.6) to 0.6°C (RCP8.5) at a depth of about 1 km by the
end of the 21st century.
23.
24. Long-term Climate Change, Commitment
and Irreversibility
Global temperature equilibrium would be reached only after centuries to millennia if
radiative Forcing were stabilized.
A large fraction of climate change is largely irreversible on human time scales, unless net
anthropogenic CO were strongly negative over a sustained period.
For scenarios driven by CO2 alone, global average temperature is projected to remain
approximately constant for many centuries following a complete cessation of emissions.
The positive commitment from CO2 may be enhanced by the effect of an abrupt cessation
of aerosol emissions, which will cause warming. By contrast, cessation of emission of
shortlived GHGs will contribute a cooling.
Some aspects of climate will continue to change even if temperatures are stabilized.
Processes related to vegetation change, changes in the ice sheets, deep ocean warming
and associated sea level rise and potential feedbacks linking for example ocean and the
ice sheets have their own intrinsic long time scales and may result in significant changes
hundreds to thousands of years after global temperature is stabilized.
