Geoscience Reference
In-Depth Information
Box 12.1 The concept of feedback and ocean acidii cation
A feedback is an interaction mechanism in which the
result of an initial process drives changes in a second
process that in turn influences the initial one. A
positive feedback intensifies the original process, while
a negative feedback reduces it. In the climate system,
one of the main positive feedbacks is the tendency of
warming to increase the quantity of water vapour in
the atmosphere, and hence the greenhouse effect and
earth's warming itself. Ocean acidification, a direct
consequence of increasing levels of atmospheric CO
2
,
interacts with biogeochemical processes, alters air-sea
exchange of CO
2
, and hence atmospheric CO
2
. If
ocean acidification leads to an increase (decrease)
in atmospheric CO
2
then this represents a positive
(negative) feedback. In this chapter, the notion of
feedback is extended to include the effect of ocean
acidification on climate not only through changes in
atmospheric CO
2
(CO
2
-acidification feedback), but
also through changes in other atmospheric constitu-
ents (e.g. nitrous oxide and dimethyl sulphide)
which do not represent, strictly speaking, feedbacks
on CO
2
.
proportional to the Revelle buffer factor, i.e. the
larger this factor, the lower the capacity of the ocean
to take up additional CO
2
. Ocean acidii cation
increases the Revelle buffer factor, causing a drastic
decrease in the capacity of ocean water to take up
CO
2
from the atmosphere, leading to a decrease in
the rate of uptake and a transient accumulation of
CO
2
in the atmosphere.
The majority of the indirect feedbacks are those
affecting the ocean's biological pumps (both organic
and carbonate). Two groups of indirect feedbacks
can be identii ed: group 1, in which ocean acidi-
i cation affects the biological pumps directly, and
group 2, in which ocean acidii cation affects a par-
ticular ocean biogeochemical process, which in turn
alters the biological pumps.
The stimulation of marine photosynthesis by
increased levels of CO
2
( Rost
et al.
2008 ) is an exam-
ple of an indirect group 1 feedback. However, the
extent of CO
2
fertilization depends on the physio-
logical characteristics of individual phytoplankton
groups. It is higher in organisms with an inefi cient
carbon acquisition pathway. A second indirect
effect of the i rst group is the change in calcii ca-
tion (Fabry
et al.
2008, but see also Iglesias-
Rodriguez
et al.
2008). An example of an indirect
effect of the second group is the reported enhance-
ment of dinitrogen (N
2
) i xation by cyanobacteria
at elevated
p
CO
2
concentrations (Hutchins
et al.
2009). This process represents a major source of
reactive nitrogen (N) to oligotrophic tropical and
subtropical areas, and given the N-limited nature
of these areas, has the potential to substantially
increase primary production.
Over recent years, an increasing number of stud-
ies have addressed the effects of ocean acidii ca-
tion on isolated processes based on i rst-order
chemical principles (e.g. trace metal speciation) or
controlled process studies (e.g. N
2
i xation, calcii -
cation, photosynthesis). While these studies pro-
vided new and valuable insights into, for example,
the vulnerability of specii c processes in response
to ocean acidii cation, they do not allow us to
apprehend impacts at the scale of the marine bio-
geochemical cycle. Scaling up from the level of
physiological processes to that of organisms and
ecosystems is not straightforward and challenges
global biogeochemical modelling efforts. It is fur-
ther complicated by the fact that ocean acidii ca-
tion does not occur in isolation, but in synergy
with ocean warming and related changes in the
physical environment that might amplify or allevi-
ate its impacts (e.g. Brewer and Peltzer 2009). All
these changes taken together will alter the parti-
tioning of climate-relevant gases between the
ocean and the atmosphere.
This chapter presents a synthesis of our under-
standing of impacts of ocean acidii cation on marine
biogeochemical cycles, including its interaction
with climate change and feedbacks to the earth sys-
tem. It starts with the discussion of the marine car-
bon cycle, an area for which experimental and
modelling studies allow a i rst-order evaluation of
impacts and feedbacks, moves to the nitrogen cycle
and ends with atmospherically active trace gases.
Impacts are discussed together with associated
feedbacks and, when possible, taking into account
climate change.
