Geoscience Reference
In-Depth Information
Table 15.1
Continued
Statement
Level of
evidence
Level of
coni dence
Challenges
Chapter(s)
Policy and socio-economic aspects
There will be socio-economic
consequences
Limited
?
Quantifying the monetary value of the goods and
services that oceans provide and assessing how
these may be impacted by ocean acidii cation
13
An ocean acidii cation threshold
that must not be exceeded
can be dei ned
Limited
?
Initiate and sustain an international effort to
compile the increasing number of data being
published in order to dei ned threshold(s).
Investigate the need to consider thresholds based
on geographical location, species, and ecosystems
to advise decision-makers
13
saturation state of the ocean with respect to CaCO
3
,
as two parameters of the carbonate system are
required to determine the seawater carbonate chem-
istry (see Box 1.1 in Chapter 1). There have been
periods in earth's history during which the ocean
had a lower pH than today (see Chapters 2 and 4),
for instance at the end of the Permian 251 Myr ago,
at the Palaeocene-Eocene Thermal Maximum
(PETM) 55 Myr ago, and during the deglaciations
that are characteristic of the Pleistocene epoch
(which started ~1.8 Myr ago and ended 10 kyr ago,
with the beginning of the Holocene). These events
were a consequence of perturbations of the carbon
cycle of different sizes, origins, and rates. The aver-
age saturation state with respect to CaCO
3
of sur-
face waters was still favourable to calcii ers despite
high CO
2
levels, due to higher concentrations of cal-
cium and/or higher total alkalinity than today. In
other words, pH and saturation state were decou-
pled during these events whereas both are declin-
ing together in the Anthropocene (the geological
epoch that serves to mark the recent extent of
human activities that have had a signii cant global
impact on the earth's ecosystems).
All past ocean acidii cation events were accompa-
nied by global warming, stronger stratii cation of the
water column, and a decrease in the oxygenation of
the deep sea. Attribution of the biological responses
to one or several of these environmental factors is
therefore difi cult. It is important to realize that the
climatic conditions prior to each of the events were
very different from today (initial CO
2
, ocean temper-
ature, and chemistry). However, the most important
difference between all previous geological events
compared with that of the Anthropocene is the rate
at which the human-induced carbon perturbation is
proceeding (see Section 15.2.1.2).
Even though there is no perfect analogue to the
present carbon perturbation, one should expect that
the consequences of anthropogenic ocean acidii ca-
tion can only be worse than those recorded in the
geological records, simply because the rate of change
is unprecedented in the earth's history and marine
ecosystems as we know them today have mainly
evolved during a time of low atmospheric CO
2
and
well-buffered seawater. The most pressing challenge
remains to i nd two independent carbonate chemis-
try proxies that will allow us to reconstruct the ocean
carbonate chemistry during earth's history.
15.2.1.2 Ocean acidii cation is in progress
It is known with a very high level of coni dence that
ocean acidii cation has been in progress since the
beginning of the Industrial Revolution. The evi-
dence is robust and comes from modelling but also
from time-series stations and repeat measurements
(see Chapter 3). Despite the short duration of the
time series, the relatively short time interval
between repeat measurements and a large seasonal
variability in some sites, the decrease in pH and car-
bonate ion concentration and the increase in the
concentration of dissolved inorganic carbon and
p
CO
2
are statistically signii cant. The decline in pH
ranges from -0.0017 to -0.0019 units yr
-1
in the sur-
face waters of the open-ocean stations. The evidence
is not as extensive in marginal seas, mostly because
