Recent and future changes in ocean carbonate chemistry - Ocean Acidification - page 45

3.3 Atmospheric CO 2 emissions, sources,
and sinks during the industrial era

bles have been measured with state-of-the-art
precision for nearly three decades (Fig. 3.1). In the
central North Pacii c at station ALOHA of the
Hawaii Ocean Time-Series (HOT) program (22.75°N,
158°W) C T and A T have been observed since 1989
and the computed surface pH T (i.e. given on the
total scale) exhibits a long-term decline of 0.0019 ±
0.0002 units yr -1 ( Dore et al. 2009). The trend in direct
measurements of surface pH T , made over about half
of the period, is not signii cantly different (Table
3.1). In the western portion of the North Atlantic
gyre at the Bermuda Atlantic Time-Series Station
(BATS; 31.72°N, 64.17°W), where C T , A T , and p CO 2
have been measured since 1983, the trend in the cal-
culated pH T decline is 0.0017 ± 0.0003 units yr -1
(Bates 2007). In the eastern North Atlantic at the
European Time Series in the Canary Islands (ESTOC;
24.04°N, 15.50°W), where pH T and A T were meas-
ured from 1995 to 2004, the decline in surface in situ
pH T is 0.0018 ± 0.0003 units yr -1 (Santana-Casiano et
al. 2007; González-Dávila et al. 2010). These trends
in surface-ocean pH T are not signii cantly different
between stations. Nor do they differ statistically
from the expected decline based on the observed
trend in atmospheric CO 2 and the assumption of
air-sea equilibrium, a supposition backed up by the
similarity of measured trends in atmospheric and
oceanic p CO 2 at these stations (Table 3.1). All three
stations also exhibit reductions in [CO 3 2- ] (and thus
in the saturation state of seawater with respect to
aragonite and calcite, Ω a and Ω c respectively),
although the reduction is about 80% greater at
ESTOC.
There are also substantial subsurface trends in pH T
and other carbonate system variables based on meas-
urements both at time-series stations and along
repeated sections. At ALOHA, reductions in pH are
signii cant down to depths of at least 600 m. The maxi-
mum 1988-2009 reduction in pH occurs not at the sur-
face but at 250 m, which Dore et al. ( 2009 ) attribute to a
greater increase in C T . They suggest that this increase
is due to subduction and lateral transport of the source
waters, located at higher latitude, where the C T
increase has been larger or the location of which may
have changed, thus altering its chemical characteris-
tics (Revelle factor). At ESTOC, changes in pH T and
related variables have been measured down to well
below 1000 m (González-Dávila et al. 2010).

Over the industrial era, human activities have
released large quantities of CO 2 to the atmosphere.
By 1994, the total atmospheric release of anthropo-
genic carbon amounted to 244 ± 20 Pg C from fossil-
fuel combustion and cement production combined
with 140 ± 40 Pg C from land-use change (Sabine
et al. 2004 ; Denman et al. 2007). Out of that total, the
atmosphere retained ~43% while the ocean absorbed
~30%. The remainder was taken up by terrestrial
plants and soils. For the most recent decades, there
is some evidence that the amount of anthropogenic
CO 2 remaining in the atmosphere (airborne frac-
tion) may have increased from 40 to 45% from 1959
to 2008 (Le Quéré et al. 2009), but the debate remains
open (Knorr 2009). The ocean continues to take up a
large fraction of the anthropogenic CO 2 emitted to
the atmosphere, for example an average of 2.2 ± 0.5
Pg C yr -1 during 1990-2005, which is ~27% of
the total emissions during that time (Denman et al.
2007 ).
In 2010, the atmospheric CO 2 level reached about
390 ppmv, which is 39% more than the pre-indus-
trial concentration (280 ppmv). Half of that increase
has occurred only since 1978. The effect of this large,
rapid increase in atmospheric CO 2 is already caus-
ing measurable changes in ocean carbonate chemis-
try, both in the mixed layer, which equilibrates with
the atmospheric perturbation on a timescale of
roughly 8 months (see Box 3.1), and even in the
deep ocean, the ventilation of which typically
requires centuries.

3.4 Observed changes in ocean
carbonate chemistry during recent
decades
From basic marine carbonate chemistry, it is well
known that as atmospheric CO 2 increases, surface-
ocean p CO 2 will increase, reducing ocean pH and
[CO 3 2- ] (see Section 3.2). Trends and variability in
these ocean variables have been quantii ed and
compared with corresponding changes in atmos-
pheric CO 2 through dedicated long-term efforts to
maintain three subtropical ocean time-series sta-
tions, where surface-ocean carbonate system varia-