Geoscience Reference
In-Depth Information
However, by design they do not include explicit
policies to mitigate greenhouse gas emissions,
which would lower the extent of climate change
experienced over the 21st century.
Progress in developing multigas mitigation sce-
narios after the SRES report now allows for a com-
parison between consequences for the earth system
of climate mitigation versus baseline scenarios.
Figure 14.1 illustrates the relationship between sce-
narios and individual IAMs and how individual
mitigation scenarios are linked to a specii c baseline
scenario for the post-SRES set of scenarios. Many
mitigation scenarios were generated with several
IAMs as part of the Energy Modeling Forum Project
21 (EMF-21; Weyant
et al.
2006). The IAMs feature
representations of the energy system and other
parts of the economy, such as trade and agriculture,
with varying levels of spatial and process detail.
They also include formulations to translate emis-
sions into concentrations and the associated radia-
tive forcing (RF). The latter is a metric for the
perturbation of the radiative balance of the lower
atmosphere-surface system. Scenarios are gener-
ated by minimizing the total costs under the con-
straints set by societal drivers (e.g. population,
welfare, and technological innovation) and most
are related to SRES 'storylines'. Adding a constraint
on RF in a baseline scenario leads to a scenario with
policies specii cally aimed at mitigation. The miti-
gation scenarios analysed here are constrained by
stabilization of total RF in the period 2100 to 2150
with RF targets ranging from 2.6 to 5.3 W m
-2
. From
the wider set of baseline and mitigation scenarios
described in the literature, four have been specii -
cally selected and termed representative concentra-
tion pathways (RCPs). These include two mitigation
scenarios with a RF target of 2.6 and 4.5 W m
-2
and
two baseline scenarios with a RF of around 6 and
8.5 W m
-2
by the end of this century.
Two metrics appear particularly well suited for
characterizing the outcome of a scenario in terms of
ocean acidii cation. These are changes in pH and
changes in the saturation state of water with respect
to aragonite, a mineral form of calcium carbonate
(CaCO
3
) secreted by marine organisms. Ocean
uptake of the weak acid CO
2
from the atmosphere
causes a reduction in pH and in turn alters the
CaCO
3
precipitation equilibrium (see Chapter 1).
Recent studies indicate that ocean acidii cation due
to the uptake of CO
2
has adverse consequences for
many marine organisms as a result of decreased
CaCO
3
saturation, affecting calcii cation rates, and
via disturbance to acid-base physiology (see
Chapters 6-8). Vulnerable organisms that build
shells and other structures of CaCO
3
in the rela-
tively soluble form of aragonite or high-magnesian
calcite, but also organisms that form CaCO
3
in the
more stable form of calcite may be affected.
Undersaturation as projected for the high-latitude
ocean (Orr
et al.
2005; Steinacher
et al.
2009 ) has been
found to affect pteropods for example, an abundant
group of species forming aragonite shells (Orr
et al.
2005; Comeau
et al.
2009). Changes in CaCO
3
satura-
tion are also thought to affect coral reefs (Kleypas
et al.
1999 ; Langdon and Atkinson 2005 ; Hoegh-
Guldberg
et al.
2007; Cohen and Holcomb 2009).
The impacts are probably not restricted to ecosys-
tems at the ocean surface, but potentially also affect
life in the deep ocean such as the extended deep-
water coral systems and ecosystems at the ocean
l oor. The degree of sensitivity varies among species
( Langer
et al.
2006 ; Müller
et al.
2010) and there is a
debate about whether some taxa may show
enhanced calcii cation at the levels of CO
2
projected
to occur over the 21st century (Iglesias-Rodriguez
et al.
2008). This wide range of different responses is
expected to affect competition among species, eco-
system structure, and overall community produc-
tion of organic material and CaCO
3
. On the other
hand, the impact of plausible changes in CaCO
3
production and export (Gangstø
et al.
2008) on
atmospheric CO
2
is estimated to be small (Heinze
2004 ; Gehlen
et al.
2007). Other impacts of ocean
acidii cation with potential inl uences on marine
ecosystems include alteration in the speciation of
trace metals as well as an increase in the transpar-
ency of the ocean to sound (Hester
et al.
2008). The
changes in the chemical composition of seawater
such as higher concentrations of dissolved CO
2
are
also likely to affect the coupled carbon and nitrogen
cycle and the food web in profound ways (Hutchins
et al.
2009), and the volume of water with a ratio of
oxygen to CO
2
below the threshold for aerobic life is
likely to expand (Brewer and Peltzer 2009).
The saturation state with respect to aragonite, Ω
a
,
is dei ned by:
