Geoscience Reference
In-Depth Information
subtropics, with a decrease from pre-industrial lev-
els exceeding 4 to values below 2.5 at the end of the
21st century.
Mitigation leads to lower levels of atmospheric
CO
2
and hence less climate change and ocean acid-
ii cation. Global surface-ocean mean Ω
a
decreases
between 0.1 to 0.8 and pH decreases by 0.04 to 0.19
units. Mitigation options that lower atmospheric
CO
2
will thus effectively alleviate the impacts of
ocean acidii cation on marine systems. Due to the
inertia of the earth system, the benei ts from early
mitigation of atmospheric CO
2
increase will
become increasingly important after 2100. The
main shortcoming of present coupled climate-car-
bon cycle models is their coarse resolution and
over-simplistic representation of biological proc-
esses. This makes it difi cult to evaluate different
emission pathways and mitigation options at the
regional scale and in particular in terms of biologi-
cal impacts.
CO
2
. In the high scenario, the volume occupied by
undersaturated water increases from 60% in pre-
industrial times to 75% in 2100 and reaches a maxi-
mum of 90% in 2300, 200 yr after the emissions are
set to zero. The volume of undersaturated water
increases up to 83% in the low scenario. Changes
projected for the historical scenario are much more
modest, yet the volume of supersaturated waters
decreases too. These drastic and lasting changes in
deep-water carbonate chemistry suggest the loss of
suitable habitats for calcifying organisms.
Past and 21st century CO
2
emissions set the extent
of ocean acidii cation over the coming centuries.
Due to the inertia in the earth system, impacts of
CO
2
emissions on seawater carbonate chemistry are
delayed and will continue to perturb the biogeo-
chemical cycles and marine ecosystems for centu-
ries to come.
As mentioned in the previous section, the main
shortcoming of present coupled climate-carbon
cycle models for evaluating future changes in ocean
carbonate chemistry is their coarse resolution. These
models converge in their projections of the chemical
consequences of ocean acidii cation at global as well
as basin scale. However, studies mostly report
changes in the mean state of properties (e.g. mean
surface-ocean pH or saturation state averaged over
a year or a decade). In order to integrate modelling
studies with the growing understanding of the dif-
ferential response of contrasting ocean regions,
future research will need to focus on the assessment
of impacts of ocean acidii cation at the regional
scale. The interplay between ocean physics, chemis-
try, and biology will need to be investigated at sea-
sonal and interannual timescales.
15.2.1.5 The legacy of historical and 21st century
fossil fuel emission will be felt for centuries
It is known with a very high level of coni dence that
the legacy of historical and 21st century fossil fuel
emission will be felt for centuries. Coupled climate-
carbon cycle models are typically used to explore
the long-term commitment to ocean acidii cation
caused by the uptake of CO
2
since the onset of
industrialization and over the 21st century follow-
ing different emission scenarios. To this end, emis-
sions are unrealistically set to zero in the year 2000
(historical scenario) or in 2100 for scenarios of the
IPCC SRES B1 (low) and A2 (high) to explore the
legacy of past emissions (see Chapter 14). For
atmospheric CO
2
levels peaking around 850 ppmv
(high scenario), Ω
a
decreases by 50% in 2100. It
increases subsequently as emissions are set to zero,
but remains substantially lower than the pre-indus-
trial value until the year 2500. Atmospheric CO
2
falls below 350 ppmv within a few decades in the
historical scenario, and the perturbation of ocean
chemistry remains relatively small. While the sur-
face-ocean carbonate system responds quickly to
changes in atmospheric CO
2
through air-sea gas
exchange, the interior of the ocean shows a delayed
response due to the centennial timescales involved
in the surface-to-deep transport of anthropogenic
15.2.2 Biological and biogeochemical
responses
15.2.2.1 Ocean acidii cation will adversely affect
calcii cation
The precipitation, dissolution, and preservation of
CaCO
3
are the processes which have been investi-
gated most in the context of ocean acidii cation,
both in the fossil record and in perturbation experi-
ments. In Chapter 4 Knoll and Fischer provide an
overview of the fate of calcii ers during earth's his-
tory. They show that several events in the geological
