Geoscience Reference
In-Depth Information
CHAPTER 14
on ocean acidii cation projections
Fortunat Joos, Thomas L. Frölicher, Marco Steinacher,
and Gian-Kasper Plattner
14.1 Introduction
et al.
2008) and the comprehensive carbon cycle-
climate model of the National Centre for
Atmospheric Research (NCAR), CSM1.4-carbon
( Steinacher
et al.
2009 ; Frölicher and Joos 2010 ).
The magnitude of the human perturbation of the
climate system is well documented by observations
( Solomon
et al.
2007). Carbon emissions from human
activities force the atmospheric composition, cli-
mate, and the geochemical state of the ocean towards
conditions that are unique for at least the last million
years (see Chapter 2). The current atmospheric CO
2
concentration of 390 ppmv is well above the natural
range of 172 to 300 ppmv of the past 800 000 years
( Lüthi
et al.
2008). The rate of increase in CO
2
and in
the radiative forcing from the combination of the
well-mixed greenhouse gases CO
2
, methane (CH
4
),
and nitrous oxide (N
2
O) is larger during the
Industrial Era than during any comparable period
of at least the past 16 000 years (Joos and Spahni
2008). Ocean measurements over recent decades
show that the increase in surface-ocean CO
2
is being
paralleled by a decrease in pH (Doney
et al.
2009).
Ongoing global warming is unequivocal (Solomon
et al.
2007): observational data show that global-
mean sea level is rising, ocean heat content increas-
ing, Arctic sea ice retreating, atmospheric water
vapour content increasing, and precipitation pat-
terns changing. The last decade (2000 to 2009) was,
on a global average, the warmest in the instrumental
record (
http://data.giss.nasa.gov/gistemp/).
Proxy
reconstructions suggest that recent anthropogenic
inl uences have widened the last-millennium multi-
decadal temperature range by 75% and that late 20th
century warmth exceeded peak temperatures over
the past millennium by 0.3°C (Frank
et al.
2010 ).
Ocean acidii cation caused by the uptake of carbon
dioxide (CO
2
) by the ocean is an important global
change problem (Kleypas
et al.
1999 ; Caldeira and
Wickett 2003 ; Doney
et al.
2009). Ongoing ocean acidi-
i cation is closely linked to global warming, as acidii -
cation and warming are primarily caused by continued
anthropogenic emissions of CO
2
from fossil fuel burn-
ing (Marland
et al.
2008), land use, and land-use
change (Strassmann
et al.
2007). Future ocean acidii -
cation will be determined by past and future emis-
sions of CO
2
and their redistribution within the earth
system and the ocean. Calculation of the potential
range of ocean acidii cation requires consideration of
both a plausible range of emissions scenarios and
uncertainties in earth system responses, preferably by
using results from multiple scenarios and models.
The goal of this chapter is to map out the spatio-
temporal evolution of ocean acidii cation for differ-
ent metrics and for a wide range of multigas climate
change emissions scenarios from the integrated
assessment models (Naki ´enovi ´ 2000 ; Van Vuuren
et al.
2008b). By including emissions reduction sce-
narios that are among the most stringent in the cur-
rent literature, this chapter explores the potential
benei ts of climate mitigation actions in terms of
how much ocean acidii cation can be avoided and
how much is likely to remain as a result of inertia
within the energy and climate systems. The long-
term impacts of carbon emissions are addressed
using so-called zero-emissions commitment scenar-
ios and pathways leading to stabilization of atmos-
pheric CO
2
. Discussion will primarily rely on results
from the cost-efi cient Bern2.5CC model (Plattner
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