Geoscience Reference
In-Depth Information
CHAPTER 12
Marion Gehlen, Nicolas Gruber, Reidun Gangstø, Laurent Bopp,
and Andreas Oschlies
12.1 Introduction
feedbacks are those which directly affect radiative
forcing in the atmosphere by altering the air-sea
l ux of radiatively active substances. Indirect feed-
backs are those that i rst alter a biogeochemical
process in the ocean, and through this change then
affect the air-sea l ux and ultimately the radiative
forcing in the atmosphere. For example, when
ocean acidii cation alters the production and export
of organic matter by the biological pump, then this
is an indirect feedback. This is because a change in
the biological pump alters radiative forcing in the
atmosphere indirectly by i rst changing the near-
surface concentrations of dissolved inorganic car-
bon and total alkalinity. These changes will in turn
affect the air-sea l ux of CO
2
. The magnitude of the
indirect feedbacks associated with biological
pumps depends on at least three elements: (1) the
magnitude of the impact that ocean acidii cation
has on a particular aspect of the ocean's biological
pumps, (2) how these changes in the biological
pumps affect the ocean's carbonate chemistry, and
(3) how these changes in carbonate chemistry affect
the air-sea CO
2
l ux. For example, the response of
the air-sea CO
2
l ux to a given change in the export
of organic carbon from near-surface waters may
vary by up to a factor of 10, depending on where
and how this change occurs (e.g. Jin
et al.
2008 ;
Oschlies 2009 ).
By far the most important direct feedback is that
associated with ocean acidii cation changing the
buffer (Revelle) factor of the ocean. As discussed in
detail in Chapter 3, the ocean's capacity to hold
additional CO
2
from the atmosphere is inversely
By the year 2008, the ocean had taken up approxi-
mately 140 Gt carbon corresponding to about a third
of the total anthropogenic CO
2
emitted to the atmos-
phere since the onset of industrialization (Khatiwala
et al.
2009). As the weak acid CO
2
invades the ocean,
it triggers changes in ocean carbonate chemistry
and ocean pH (see Chapter 1). The pH of modern
ocean surface waters is already 0.1 units lower than
in pre-industrial times and a decrease by 0.4 units is
projected by the year 2100 in response to a business-
as-usual emission pathway (Caldeira and Wickett
2003). These changes in ocean carbonate chemistry
are likely to affect major ocean biogeochemical
cycles, either through direct pH effects or indirect
impacts on the structure and functioning of marine
ecosystems. This chapter addresses the potential
biogeochemical consequences of ocean acidii cation
and associated feedbacks to the earth system, with
focus on the alteration of element l uxes at the scale
of the global ocean. The view taken here is on how
the different effects interact and ultimately alter the
atmospheric concentration of radiatively active
substances, i.e. primarily greenhouse gases such as
CO
2
and nitrous oxide (N
2
O).
Changes in carbonate chemistry have the poten-
tial for interacting with ocean biogeochemical
cycles and creating feedbacks to climate in a myr-
iad of ways (Box 12.1). In order to provide some
structure to the discussion, direct and indirect feed-
backs of ocean acidii cation on the earth system are
distinguished ( Table 12.1 and Fig. 12.1 ). Direct
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