Geoscience Reference
In-Depth Information
sensitivity in the Bern2.5CC model is set to 3.2°C for
a nominal doubling of CO
2
.
Carbon emissions must drop if CO
2
is to be stabi-
lized. Carbon emissions are allowed to increase for
a few years to a few decades, depending on the
pathway, but then have to drop in all cases and
eventually become as low as the long-term geologi-
cal carbon sink of a few tenths of a Gt C only. This is
a consequence of the long lifetime of the anthropo-
genic perturbation. Atmospheric CO
2
thus rel ects
the sum of past emissions rather than current emis-
sions. Cumulative emissions by 2500 for the
Bern2.5CC model are in the range of 750 to 4000 Gt
C for a stabilization between 350 and 1000 ppmv.
Conventional fossil resources, mainly in the form of
coal, are estimated to be about 5000 Gt C.
If emission reductions are delayed (DSP and OSP
pathways in Fig. 14.10), more stringent reductions
have to be implemented later in order to meet a cer-
tain stabilization target. This is illustrated by the
two overshoot scenarios in which atmospheric CO
2
is prescribed to increase above the i nal stabilization
levels and by the delayed scenarios where CO
2
is
allowed to further increase initially.
The higher the emissions the larger the fraction of
CO
2
emissions that stays airborne on timescales up
to a few thousand years. This fraction is 20% for the
350 ppmv target and 40% for the 1000 ppmv target
by the year 2500. The higher airborne fraction for
high relative to low stabilization levels is primarily
a consequence of the non-linearity of the seawater
carbonate chemistry. The higher the partial pressure
of CO
2
, the smaller is the relative change in dis-
solved inorganic carbon for a given change in
p
CO
2
.
Thus, the partitioning of carbon between the atmos-
phere and the ocean shifts towards a higher fraction
remaining airborne the greater the amount of car-
bon added to the ocean-atmosphere system.
mitigation scenarios are idealized in many ways.
New technologies and policies are assumed to be
globally applicable and are often introduced over
relatively short periods of time. Especially in the
lowest mitigation scenarios, it is assumed that glo-
bal climate policies can be implemented in the next
few years to allow emissions to peak by 2020. These
scenarios do not deal with the question of political
feasibility and assume mitigation policies are imple-
mented globally.
Physical impacts in terms of ocean acidii cation
and climate change are lower in mitigation than
baseline scenarios. Global average surface satura-
tion with respect to aragonite is reduced to 3.1-2.4
by year 2100 in the mitigation compared to 2.3-1.8
in the baseline scenarios. The lowest scenarios result
in a decrease in saturation state of 0.6 by 2100 com-
pared with pre-industrial values and show only a
small difference of 0.1 between current and end of
century saturation conditions. These scenarios pro-
vide a guide to the range of global-mean surface
acidii cation that may occur, assuming an ambitious
climate policy. These low scenarios with forcing tar-
gets below 3 W m
-2
depart from the corresponding
no-climate policy baseline by 2015-2020 and incor-
porate the widespread development and deploy-
ment of existing carbon-neutral technologies. They
require socio-political and technical conditions very
different from those now existing.
Global emissions in the scenarios with a 4.5 W
m
-2
forcing target begin to diverge from baseline
values by about 2020 to 2030, with emissions drop-
ping to approximately present levels by 2100. CO
2
,
temperature, and ocean acidii cation start to diverge
from the baseline projections later than emissions.
This emphasizes the importance of early decisions
to meet specii c climate change mitigation targets.
Trends can be persistent and impacts of carbon
emissions may continue for decades and centuries,
long after carbon emissions have been reduced, due
to the inertia in the climate-carbon system. This is
exemplii ed by emissions commitment scenarios
where carbon and other emissions are hypotheti-
cally set to zero and subsequent changes can be
investigated. The projected global changes will
affect different regions differently depending on
their vulnerability to these changes. Widespread
year-round undersaturation of surface waters in the
14.8 Conclusions
We have examined a large set of projections for 21st
century emissions for CO
2
and for a suite of non-
CO
2
greenhouse and other air pollutant gases from
the recent scenario literature (Van Vuuren
et al.
2008b). Emissions scenarios provide an indication
of the potential effects of mitigation policies. Most
of the IAMs used to generate the set of baseline and
