Geoscience Reference
In-Depth Information
The range of plausible 21st century emissions
pathways leads to further global warming and
ocean acidii cation (Van Vuuren
et al.
2008b ;
Strassmann
et al.
2009). Projections based on the sce-
narios of the Special Report on Emissions Scenarios
(SRES) of the Intergovernmental Panel on Climate
Change (IPCC) give reductions in average global
surface pH of between 0.14 and 0.35 units over the
21st century, adding to the present decrease of 0.1
units since pre-industrial time (Orr
et al.
2005; see
also Chapters 1 and 3). Comprehensive earth sys-
tem model simulations show that continued carbon
emissions over the 21st century will cause irrevers-
ible climate change on centennial to millennial
timescales in most regions, and impacts related to
ocean acidii cation and sea level rise will continue
to aggravate for centuries even if emissions are
stopped by the year 2100 (Frölicher and Joos 2010).
In contrast, in the absence of future anthropogenic
emissions of CO
2
and other radiative agents, forced
changes in surface temperature and precipitation
will become smaller in the next centuries than inter-
nal variability for most land and ocean grid cells
and ocean acidii cation will remain limited. This
demonstrates that effective measures to reduce
anthropogenic emissions can make a difference.
However, continued carbon emissions will affect
climate and the ocean over the next millennium and
beyond (Archer
et al.
1999; Plattner
et al.
2008) and
related climate and biogeochemical impacts pose a
substantial threat to human society.
Thirty years ago, with their box-diffusion carbon
cycle model, Siegenthaler and Oeschger (1978) dem-
onstrated the long lifetime of an atmospheric CO
2
perturbation and pointed out that carbon emissions
must be reduced 'if the atmospheric radiation bal-
ance is not to be disturbed in a dangerous way'. The
United Nations Framework Convention on Climate
Change (UNFCCC) that came into force in 1994 has
the ultimate objective (article 2) 'to achieve . . . stabi-
lization of greenhouse gas concentrations in the
atmosphere at a level that would prevent dangerous
anthropogenic interference with the climate system.
Such a level should be achieved within a time frame
sufi cient to allow ecosystems to adapt naturally to
climate change . . .' ( UN 1992). Scenarios provide a
useful framework for establishing policy-relevant
information related to the UNFCCC. Here, the link
between atmospheric CO
2
level and ocean acidii ca-
tion, and the timescales of change, are addressed.
The outline of this chapter is as follows. In the
next section, we will discuss different classes of sce-
narios, their underlying assumptions, and how
these scenarios are used. Metrics for assessing ocean
acidii cation are also introduced. In Section 14.3, the
evolution over this century of atmospheric CO
2
,
global-mean surface air temperature, and global-
mean surface ocean acidii cation for the recent
range of baseline and mitigation scenarios from
integrated assessment models is presented. In
Section 14.4, the minimum commitment, as a result
of past and 21st century emissions, to long-term cli-
mate change and ocean acidii cation arising from
inertia in the earth system alone is addressed. In
Section 14.5, regional changes in surface ocean
chemistry are discussed. In Section 14.6, delayed
responses, irreversibility, and changes in the deep
ocean are addressed using results from the compre-
hensive NCAR CSM1.4-carbon model. Finally, in
Section 14.7, idealized proi les leading to CO
2
stabi-
lization are discussed to further highlight the link
between greenhouse gas stabilization, climate
change, and ocean acidii cation. The overarching
logic is to use the cost-efi cient Bern2.5CC model to
explore the scenario space and uncertainties for
global-mean values and the NCAR CSM1.4-carbon
model to investigate regional details for a limited
set of scenarios. The set of baseline and emissions
scenarios and associated changes in CO
2
and glo-
bal-mean surface temperature have been previously
discussed (Van Vuuren
et al.
2008b ; Strassmann
et al.
2009). We also refer to the literature for a more
detailed discussion of the NCAR CSM1.4-carbon
ocean acidii cation results (Steinacher
et al.
2009 ;
Frölicher and Joos 2010 ).
14.2
Scenarios and metrics
Scenario-based projections are a scientii c tool kit
for investigating alternative evolutions of anthro-
pogenic emissions and their inl uence on climate,
the earth system, and the socio-economic system.
Scenario-based projections are not to be misunder-
stood as predictions of the future, and as the time
horizon increases the basis for the underlying
assumptions becomes increasingly uncertain.
