Impact of climate change mitigation on ocean acidification projections - Ocean Acidification - page 276

amount of CO 2 emissions that would cause the
same radiative forcing over a time period of 100
years; IPCC 2007). Emissions growth slows down in
the second half of the century in all baseline scenar-
ios, because of a combination of stabilizing global
population levels and continued technological
change. The mitigation scenarios necessarily follow
a different path, with a peak in global emissions
between 2020 and 2040 at a maximum value of 50%
above current emissions.
The projected CO 2 concentrations for the baseline
cases calculated with the Bern2.5CC model (Plattner
et al. 2008) range from 650 to 960 ppmv in 2100 using
best-estimate model parameters (Fig. 14.2C). The
CO 2 concentrations in the mitigation scenarios
range from 400 to 620 ppmv in 2100. Uncertainties
in the carbon cycle and climate sensitivity increase
the overall range to 370 to 1310 ppmv (bars in Fig.
14.2C ; Plattner et al. 2008). Uncertainties are particu-
larly large for the high end. The two scenario sets,
baseline and mitigation, are also distinct with
respect to their trends. All baseline scenarios show
an increasing trend in atmospheric CO 2 , implying
rising concentrations beyond 2100. In contrast, the
mitigation scenarios show little growth or even a
declining trend in CO 2 by 2100.
Projected global-mean surface air temperature
changes by the year 2100 (relative to 2000) are 2.4
to 4.2°C (Fig. 14.2D) for the baseline scenarios
and best-estimate Bern2.5CC model parameters.
Uncertainties in the carbon cycle and climate sensi-
tivity more than double the ranges associated with
emissions. For the mitigation scenarios, the pro-
jected temperature changes by 2100 are 1.1 to 2.1°C
using central model parameters. The mitigation sce-
narios bring down the overall range of CO 2 and
temperature change substantially relative to the
baseline range. As for CO 2 , the greatest difference
compared with the baseline is seen during the sec-
ond part of the century, when the rate of tempera-
ture change slows considerably in all mitigation
scenarios in contrast to the baseline scenarios. In
several mitigation scenarios, surface air tempera-
ture has more or less stabilized by year 2100 (Van
Vuuren et al. 2008b ; Strassmann et al. 2009 ).
The evolution of the global-mean saturation state
of aragonite (Fig. 14.2E) and pH T ( Fig. 14.2F ) in the
surface ocean mirrors the evolution of atmospheric

2 + 2

3

−

[Ca ][CO ]

W =

*

a

K

sp

where brackets denote concentrations in seawater,
here for calcium ions and carbonate ions, and K * sp is
the apparent solubility product dei ned by the equi-
librium relationship for the dissolution reaction of
aragonite. Similarly, saturation can be dei ned with
respect to calcite which is less soluble than arago-
nite. Uptake of CO 2 causes an increase in total dis-
solved inorganic carbon ( C T ) and a decrease in the
carbonate ion concentration and in saturation (see
Chapter 1). Shells or other structures start to dis-
solve in the absence of protective mechanisms when
saturation falls below 1 for the appropriate mineral
phase. A value of Ω greater than 1 corresponds to
supersaturation. Supersaturated conditions are pos-
sible, as the activation energy for forming aragonite
or calcite is high.
The pH describes the concentration or, more pre-
cisely, the activity of the hydrogen ion in water, a H ,
by a logarithmic function:

pH =− log a .

T 10

+

H

The activity of hydrogen ion is important for all
acid-base reactions. In this chapter, the total pH
scale is used as indicated by the subscript T.

14.3 Baseline and mitigation emissions
scenarios for the 21st century: how much
acidii cation can be avoided?

Figure 14.1 shows the cumulative carbon emissions
over this century for the recent set of baseline and
mitigation scenarios (Van Vuuren et al. 2008b ) and
Fig. 14.2 their temporal evolution. Cumulative CO 2
emissions are in the range of 1170 to 1930 Gt C for
the seven baseline scenarios and between 370 and
1140 Gt C for the mitigation scenarios, with the
highest emissions associated with a high forcing
and a weak mitigation target. In the baseline (no cli-
mate policy) scenarios, the range of increase in
greenhouse gas emissions by 2100 is from 70 to
almost 250% compared with the year 2000 (here,
emissions are measured in CO 2 -equivalent—CO 2 -
equivalent emissions of a forcing agent denote the