Geoscience Reference
In-Depth Information
While there is no experimental evidence that
ocean acidii cation will affect N
2
O production
directly, the decrease in oceanic oxygen induced by
ocean acidii cation is bound to increase N
2
O pro-
duction substantially. The reduced ballast effect
could lead to higher rates of nitrii cation in low-ox-
ygen regions which would substantially enhance
N
2
O production associated with the nitrii cation
pathway (Jin and Gruber 2003). The expansion of
the anoxic regions caused by the altered C:N stoi-
chiometry is likely to accelerate N
2
O production by
the denitrii cation pathway. To date, no simulation
has been undertaken to quantify this effect, but it is
conceivable that oceanic N
2
O emissions could dou-
ble in response to a doubling of the ocean's anoxic
regions.
and denitrii cation will not become large at the glo-
bal scale. On a regional level, and from the perspec-
tive of marine organisms that depend on sufi cient
oxygen levels to live, these changes will be relevant
nevertheless—it is just that their impact on radia-
tive forcing in the atmosphere will probably not be
substantial. However, the level of coni dence in this
statement is very low. The story is different for N
2
O
because evidence is mounting that ocean acidii ca-
tion will increase its production and emission into
the atmosphere.
12.4 The ocean as a source
In addition to being a source or a sink of some major
greenhouse gases (e.g. CO
2
and N
2
O), the ocean is
also a source of climatically active trace gases to the
marine atmosphere (see Chapter 11 for detailed
information). Among them, dimethyl sulphide
(DMS) is a gaseous sulphur compound produced
by marine biota in surface seawater. Once emitted
to the atmosphere, it undergoes rapid oxidation to
produce particles that can modify the optical prop-
erties of clouds, thereby inl uencing climate. Iodo-
and bromocarbon gases are also produced in surface
seawater and can be outgassed to the atmosphere.
They represent a major source of halogens to the
marine atmosphere where their oxidation can pro-
duce reactive radicals. These radicals play a role
in the photochemical loss of tropospheric ozone
(a major greenhouse gas), but also in the regulation
of stratospheric ozone. In addition, they can con-
tribute to particle formation and modify the optical
properties of clouds, thereby affecting climate.
For more than 20 years now, it has been proposed
that marine emissions of DMS are sensitive to cli-
matic change and that the radiative budget of the
earth is in turn sensitive to modii cations of the
marine DMS source (Charlson
et al.
1987 ). Climate
model simulations suggested that a 50% decrease in
DMS emissions could result in a net increase in the
mean surface temperature of 1.6°C (Gunson
et al.
2006). Changes in the production and in the sea-to-
air l ux of DMS resulting from ocean acidii cation
could thus have a signii cant impact on climate and
hence form an indirect group 1 type feedback. A
detailed presentation of the chemistry of DMS and
12.3.5 Interactive effects—the future marine
nitrogen cycle
None of the above processes operate in isolation.
Although the degree of coupling is intensively
debated, nitrogen i xation and denitrii cation tend
to be coupled (Deutsch
et al.
2007 ; Gruber 2008 ). In
addition, the marine oxygen content is likely to
decrease substantially in response to global warm-
ing, irrespective of changes induced by ocean acidi-
i cation. So what will be the response of the system
as a whole? To date, no study has attempted to look
at these nitrogen cycle-driven feedbacks holistically
and in depth, so one can only provide a qualitative
assessment. In addition, the answer is by nature
speculative, since our understanding of how the
different processes interact with each other is poor.
The marine nitrogen cycle will probably be accel-
erated in a high-CO
2
ocean, with a substantially
elevated rate of marine nitrogen i xation and a
higher rate of (water-column) denitrii cation. This
will decrease the mean residence time of i xed nitro-
gen in the ocean. Given the enhanced rates of
sources and sinks, it is not possible to conclude any-
thing about the potential generation of imbalances
which are required to cause net changes in the oce-
anic i xed nitrogen inventory and changes in the
biological pump that could alter the air-sea CO
2
balance. It appears, however, that imbalances are
not very likely and that ocean acidii cation-induced
feedbacks to the earth system involving N
2
i xation
