Observations of variability and change
Direct observations of p(CO2) and pH revealed present-day conditions
of surface ocean carbonate chemistry in 12 different oceanic and coastal
systems. The open ocean mooring time series sites are located in subtropical
oligotrophic regions (WHOTS, Stratus), biologically productive subtropical
regions that experience seasonal monsoons (BOBOA) and tropical cyclones
(KEO), and subarctic regions with pronounced seasonality of physical and
biological conditions (Papa, Iceland). Annual mean Ωarag at
these sites ranged from 1.83 to 3.56; annual mean pH ranged from 7.99 to 8.12
(Figs. 1–4, Table 4). High biological productivity is a feature at each of
the four coastal mooring time series sites on the continental shelves of the
US east (Gulf of Maine, Gray's Reef) and west (CCE2, Chá bă)
coasts. Summer upwelling is another important driver of conditions at Chá
bă, located mid-shelf at 100 m bottom depth offshore of La Push,
Washington (Alford et al., 2012). While upwelling can also impact the CCE2
site located mid-shelf at 800 m bottom depth farther south in the CCE, this
subregion in particular has shown sensitivity to climatic drivers, such as
the El Niño–Southern Oscillation (ENSO; Nam et al., 2011). Seasonal
temperature and freshwater inputs impact natural variability at the two
coastal moorings in the Atlantic with the Gulf of Maine site located 10 km
from shore at 65 m bottom depth (Salisbury et al., 2009) and Gray's Reef 70
km from shore at 20 m bottom depth (Xue et al., 2016).
The Chuuk and La Parguera sites are located in coral reef ecosystems within a
semi-closed atoll lagoon in Micronesia at 23 m bottom depth and a patch reef
in the Caribbean Sea southwest of Puerto Rico at 5 m bottom depth,
respectively. Despite the more variable coastal and coral reef conditions,
the range of annual mean Ωarag at these sites was 1.97 to
3.37, less than the range observed at the more diverse set of ocean regimes
represented by the open ocean sites (Figs. 1, 5–7, Table 4). However, the
range of annual mean pH was approximately the same as the open ocean sites
from 8.01 to 8.15 (Figs. 5–7).
Descriptive statistics of Ωarag: annual mean,
annual amplitude, and 1 SD of annual anomalies from the CO2 and pH
mooring observations, from a global data synthesis of ship-based
observations (Takahashi et al., 2014), and a
biogeochemical model (Friedrich et al.,
2012). Bold values represent values larger than observed values; italicized
values represent values lower than observed values. ND signifies no data.
Open ocean
Annual
Annual
SD annual
Coastal and coral
Annual
Annual
SD annual
sites
mean
amplitude
anomalies
reef sites
mean
amplitude
anomalies
WHOTS
Chá bă
Observations
3.49
0.17
0.05
Observations
1.88
1.32
0.45
Global synthesis
3.62
0.34
Global synthesis
2.06
0.67
Model
0.25
0.09
Model
ND
ND
Stratus
CCE2
Observations
2.67
0.13
0.07
Observations
2.53
0.76
0.31
Global synthesis
2.98
0.37
Global synthesis
ND
ND
Model
0.35
0.11
Model
ND
ND
BOBOA
Gray's Reef
Observations
3.52
0.20
0.13
Observations
3.25
1.09
0.37
Global synthesis
3.59
0.24
Global synthesis
3.09
0.95
Model
0.15
0.06
Model
ND
ND
Iceland
Gulf of Maine
Observations
1.70
0.71
0.22
Observations
1.86
0.64
0.24
Global synthesis
1.77
0.64
Global synthesis
ND
ND
Model
0.45
0.16
Model
ND
ND
Papa
Chuuk K1
Observations
2.08
0.49
0.15
Observations
3.42
0.21
0.11
Global synthesis
1.83
0.55
Global synthesis
3.86
0.08
Model
0.35
0.09
Model
0.05
0.04
KEO
La Parguera
Observations
3.08
0.48
0.16
Observations
3.62
0.33
0.11
Global synthesis
3.32
0.61
Global synthesis
3.86
0.21
Model
0.35
0.06
Model
0.15
0.04
Of the open ocean time series, the moorings located in subtropical
oligotrophic regions, WHOTS and Stratus, experienced lower seasonal to
subseasonal variability in surface pH and Ωarag (Figs. 2–4).
Consistent trade winds and shallow mixed layer depth throughout the year
along with the lack of deep winter convection likely contribute to this
relatively low open ocean variability. Temporal variability was higher at the
other four open ocean mooring locations, which was likely driven by (1) more
prevalent seasonal changes in SST (on average 2 times more variable than
WHOTS and Stratus) and productivity, and (2) stochastic events such as storms
and typhoons. In general, the range of variability tended to be consistent
throughout the annual cycle at each of the open ocean sites with exceptions
of increased variability at the Iceland location in late summer and early
fall and at Papa during winter (Fig. 4). Present-day Ωarag
values were mostly > 3 year-round at the subtropical open ocean
sites except at Stratus, where Ωarag values mostly fell
between 2.5 and 3.0 (Figs. 2 and 3). Surface Ωarag conditions
were further reduced at Papa and Iceland, the subarctic sites, which range
from 1.5 to 2.5 (Fig. 4). Present-day pH observations were > 8
throughout the average year at these mooring sites except at Stratus, where
pH fell below 8 half the year from December through May (Fig. 2). Moored
observations were consistent with seasonal means from ship-based time series
observations at the WHOTS and Iceland sites (Bates et al., 2014).
Box and whisker plots of present-day monthly surface seawater
Ωarag (top) and pH (bottom) and monthly mean SST (orange
lines) at the open ocean mooring locations in subtropical oligotrophic
regions (WHOTS, Stratus). Boxes represent data between Q1 and Q3, with the
line between representing Q2 (i.e., the median). Whiskers represent 1.5 IQR,
or ∼ 2.7 SD; Eqs. 1 and 2), of the upper and lower quartiles with data
outside that range shown as outliers (open circles). Outliers here represent
natural deviations in ocean chemistry, not measurement outliers, which were
removed in the data quality control process. Estimated monthly preindustrial
Ωarag and pH variability (1.5 IQR or ∼ 2.7 SD) is shown
in gray (top) and blue (bottom) shaded areas, respectively. Shaded portions of
the pie charts indicate the percent of present-day Ωarag and
pH values falling outside the bounds of preindustrial variability for each
month. For mooring location see Fig. 1 and Table 1.
Box and whisker plots of present-day monthly surface seawater
Ωarag (top) and pH (bottom) and monthly mean SST at the open
ocean mooring locations in biologically productive subtropical regions that
experience seasonal monsoons (BOBOA) and tropical cyclones (KEO). See
detailed description of figure components in Fig. 2 caption.
Box and whisker plots of present-day monthly surface seawater
Ωarag (top) and pH (bottom) and monthly mean SST at the open
ocean mooring locations in subarctic regions with pronounced seasonality of
physical and biological conditions (Papa, Iceland). See detailed description
of figure components in Fig. 2 caption.
The seasonal cycle of surface ocean Ωarag and pH were not
always consistent with one another. Seawater Ωarag is largely
determined by variations in the concentration of the carbonate ion
(CO32-); pH is influenced by gas exchange of CO2, physical
conditions, and biological activity. Observations of surface ocean pH were
consistent with a seasonal thermodynamic response, i.e., pH decrease
(increase) with SST increase (decrease), at the four subtropical open ocean
sites and at the Papa mooring (Figs. 2–4). However, this strong relationship
was not consistent at the subarctic Iceland site. At this site, pH and SST
were positively correlated (Fig. 4), suggesting that the seasonality of surface
ocean pH was dominated by biological activity in the summer and/or winter
mixing of upwelled deep water low in temperature and pH (Takahashi et al.,
1993; Chen et al., 2007). At all open ocean sites, Ωarag was highest during
summer months, which led to the timing of low Ωarag and low pH
conditions to be anticorrelated over the annual cycle at all open ocean sites
except Iceland (Figs. 2–4). This pattern at the Iceland mooring was
consistent with seasonality of surface Ωarag and pH derived
from quarterly ship-based observations at the same site (Bates et al., 2014; Olafsson et al., 2009).
Comparisons to preindustrial bounds of variability also revealed differences
between open ocean sites. All open ocean sites experienced surface
Ωarag outside the bounds of preindustrial variability
year-round with the exception of BOBOA and Iceland. Present-day surface
Ωarag conditions still partially overlapped with
preindustrial conditions at BOBOA during the monsoon season from June
through August (Fig. 3), at Iceland during the summer to fall transition in
August and September (Fig. 4), and slightly at KEO during November and
December (Fig. 3). However, present-day surface pH observations fall
completely outside preindustrial pH conditions at all open ocean sites
year-round, except at BOBOA where there was a slight overlap of 4 % in
August (Figs. 2–4).
The coastal mooring sites experienced higher subseasonal to seasonal
variability in surface pH and Ωarag compared to the open ocean
sites (Figs. 5 and 6). Each coastal time series exhibited clear seasonal
patterns with annual amplitudes of Ωarag ranging from 0.66 to
1.32 (Table 4). Gray's Reef and Chá bă experienced the highest
subseasonal to seasonal variability in surface pH and Ωarag,
likely driven by upwelling/relaxation/downwelling dynamics that can change
rapidly at Chá bă (Alford et al., 2012; Hickey and Banas, 2003) and
high productivity and freshwater inputs in the spring and fall at Gray's Reef
(Salisbury et al., 2009; Xue et al., 2016). We also
observed the lowest pH values (7.8) and surface Ωarag values
close to undersaturation (Ωarag < 1) primarily in
the winter at Chá bă and in the spring at CCE2 (Fig. 5). These
observations of near-undersaturated conditions are consistent with other
observations and models within the northern CCE where the Chá bă
mooring resides (Harris et al., 2013; Hauri et al., 2013) and may indicate
respiration in the absence of photosynthetic uptake typical of
winter/non-bloom periods.
Box and whisker plots of present-day monthly surface seawater
Ωarag (top) and pH (bottom) and monthly mean SST at the
coastal mooring locations on the continental shelves of the US west coast
(CCE2, Chá bă). See detailed description of figure components in
Fig. 2 caption.
Box and whisker plots of present-day monthly surface seawater
Ωarag (top) and pH (bottom) and monthly mean SST at the
coastal mooring locations on the continental shelves of the US east coast
(Gulf of Maine, Gray's Reef). See detailed description of figure components
in Fig. 2 caption.
Unlike the subtropical open ocean mooring sites, seasonality of surface ocean
pH at these coastal sites showed strong influence of factors other than SST
and were not always correlated with Ωarag values. At the
moorings in the CCE, these parameters (i.e., SST, pH, and Ωarag) generally followed similar seasonal patterns, suggesting
factors other than seasonal thermodynamic response influenced surface ocean
pH (Fig. 5). However, surface ocean pH and Ωarag did not
always follow the same seasonal pattern at the Gray's Reef and Gulf of Maine
mooring sites (Fig. 6). While SST likely influenced some of the seasonal
variation in pH at these sites, biological activity and freshwater input also
influenced seasonality of the carbonate system at these US east coast
locations (Salisbury et al., 2009; Xue et al., 2016).
In general, the coastal sites experienced Ωarag outside of the
preindustrial range mainly during winter. One exception was Chá bă,
the coastal site with the highest subseasonal variability (Fig. 5). This high
subseasonal variability during spring through fall caused high month-to-month
variability in the overlap with preindustrial conditions, suggesting this
system may be on the threshold of a shift outside preindustrial conditions
during this time of the year. Observations of pH at Chá bă followed
this same pattern. In general, present-day observations of pH fell outside
preindustrial conditions more so than Ωarag at all coastal
sites (Figs. 5 and 6).
Finally, similar to the coastal moorings, the coral reef mooring sites also
experienced subseasonal to seasonal variability but not as large as within
the subtropical coastal systems (Fig. 7). Mean annual Ωarag at
the Caribbean (La Parguera) and Pacific (Chuuk) moorings was 3.62 and 3.42,
respectively, while mean pH was 8.02 and 8.01, respectively (Fig. 7,
Table 4). With the exception of low Ωarag outliers at Chuuk,
most Ωarag conditions were > 3 throughout the year
at both sites, and surface pH observations were > 7.9. The
seasonal cycle of pH and Ωarag was more pronounced at La
Parguera with relatively consistent monthly range in variability, but the
Chuuk site experienced greater variability December through April, likely
driven by local mixing during the trade winds season. Even with small
seasonal fluctuations in tropical ocean temperature, both coral mooring sites
did show patterns of pH and Ωarag seasonality associated with
SST, with lower pH and Ωarag values coinciding with slightly
warmer summer months and higher pH and Ωarag values during
winter (Fig. 7). Present-day variability at these sites did not cause
extensive overlap with preindustrial conditions. Present-day surface pH
observations fell completely outside preindustrial conditions year-round at
both coral reef sites (Fig. 7). Present-day Ωarag conditions
at La Parguera were largely outside of preindustrial bounds year-round,
while this mainly occurred during the season of lowest variability from May
to November at Chuuk (Fig. 7).
Box and whisker plots of present-day monthly surface seawater
Ωarag (top) and pH (bottom) and monthly mean SST in coral reef
ecosystems within a semi-closed atoll lagoon in Micronesia (Chuuk) and a
patch reef in the Caribbean Sea southwest of Puerto Rico (La Parguera). See
detailed description of figure components in Fig. 2 caption.
Relational plot of different modes of (a) Ωarag and (b) pH variability for each ocean acidification
mooring location. Statistics describing variability include 1 SD of monthly
anomalies (monthly mean – monthly observations), annual amplitude (maximum
monthly climatological mean – minimum monthly climatological mean), and 1 SD
of annual anomalies (annual mean – mean observations). Circles represent
open ocean mooring locations, squares denote coastal sites, and triangles denote coral
reefs.
Percent time that (a) preindustrial and
(b) present-day surface seawater Ωarag conditions
fall below biological thresholds: chronic exposure for Ostrea lurida
larvae at Ωarag < 1.4 in red, acute effect of
Crassostrea gigas larvae at Ωarag < 1.5 in
gray, chronic exposure for Mya arenaria larvae at
Ωarag < 1.6 in gold, chronic exposure for
Mytilus californianus larvae at Ωarag < 1.8
in blue, and chronic exposure for C. gigas larvae at
Ωarag < 2.0 in black. Thresholds at the Chá
bă mooring are shown as circles; thresholds at the CCE2 mooring (only for
M. californianus larvae) are shown as triangles; thresholds at the
Gulf of Maine mooring (only for M. arenaria larvae) are shown as
squares. The one acute threshold is indicated by a dashed line.
The results from these 12 mooring time series highlight the different
patterns of variability of surface ocean Ωarag and pH in both
space and time. Figure 8 compares the relative influence of subseasonal,
seasonal, and interannual variability at the mooring locations. Since the
mooring observations were well distributed throughout the year, we are
confident in the subseasonal and seasonal estimates of variability. However,
considering that most of the time series were only 2 to 5 years long, we
expect to refine the estimates of interannual variability as we obtain more
observations over the coming years. For example, ENSO is a driver of ocean
conditions, including biogeochemistry, at CCE2 (Nam et al., 2011). While
there were weak El Niño-like conditions that developed in the tropical
Pacific in 2014 (McPhaden, 2015), there were no major La Niña or El
Niño anomalies during the CCE2 time series used in this analysis (March
2012–2015). Hence, the estimate of interannual variability presented here is
likely an underestimate of the true interannual signal at this location. In
addition, this was a period of anomalously rapid warming in the Gulf of
Maine, which may have caused Ωarag to trend higher due the
reduced solubility of Ωarag in warmer waters (Mills et al.,
2013; Pershing et al., 2015). Potential variations in warming trends over time
would also impact interannual variability of Ωarag observations
in the Gulf of Maine as the time series continues.
The coastal sites generally experienced higher subseasonal to interannual
Ωarag variability than the open ocean and coral reef sites.
Relative to other patterns of variability, interannual Ωarag
variability tended to be low at all sites except for at Chá bă,
Gray's Reef, and CCE2 (Fig. 8a). The other sites tended to be equally
influenced by subseasonal and seasonal variability with the exception of the
Iceland mooring site, which was controlled more by seasonal variability over
the annual cycle (Fig. 8a); however, subseasonal variability played a large
role in August through October (Fig. 4). For pH, most mooring sites exhibited
similar patterns of variability with low interannual variability and
approximately equal influence from seasonal and subseasonal variability
(Fig. 8b). Similar to Ωarag, Chá bă, Gray's Reef, and
CCE2 were the clear outliers with the highest values of interannual pH
variability.
Biologically relevant Ωarag thresholds
Research on response of shellfish larvae living in nearshore environments of
the CCE and Gulf of Maine to changes in carbonate chemistry allowed us to
investigate when observations at the Chá bă, CCE2, and Gulf of Maine
moorings exceeded biological thresholds. Crassostrea gigas, the
Pacific oyster whose larvae are raised in hatcheries in coastal Washington
and Oregon, has shown sublethal impacts on larval development, such as shell
development and growth, when exposed to levels of
Ωarag < 2.0 (Barton et al., 2012) and acute impacts
when Ωarag < 1.5 (Waldbusser et al., 2015a, b). Other studies suggest that chronic exposure
thresholds for the larvae of Ostrea lurida, the Olympia oyster, and
Mytilus californianus, the California mussel, occur at
Ωarag < 1.4 (Hettinger et al., 2013) and
Ωarag < 1.8 (Gaylord et al., 2011), respectively.
All of these shellfish larvae, whether naturally occurring or hatchery
raised, are found in coastal environments in the region of the Chá bă
mooring and M. californianus also exist farther south in the
nearshore region of the CCE2 mooring. In addition, larvae of Mya arenaria, the soft-shell clam commercially harvested on tidal mudflats of
the western Gulf of Maine, has shown a lack of shell formation and growth in
laboratory experiments at Ωarag < 1.6 (Salisbury et
al., 2008).
Monthly climatology of Ωarag developed from the mooring
observations at Chá bă suggest that present-day Ωarag
conditions reached chronic exposure levels for C. gigas larvae
(Ωarag < 2.0) over 50 % of the time from
November to March, with nearly the entire months of December through March at
Ωarag values less than 2.0 (Fig. 9b). These present-day
conditions prevailed over more of the year compared to preindustrial times,
when the most extensive chronic exposure occurred only up to 64 % during
March (Fig. 9a). Conditions that cause acute responses in C. gigas
larvae (Ωarag < 1.5) were minimal year-round at
Chá bă except for March, when these conditions persisted in the
present day during 37 % of the month (Fig. 9b) and only 14 % of the
month during preindustrial times (Fig. 9a). A similar seasonal pattern also
existed for O. lurida larvae (Ωarag < 1.4),
when chronic exposure levels in March exceeded 27 % during the present
(Fig. 9b) compared to only 11% during preindustrial times (Fig. 9a). For
M. californianus larvae, present-day chronic exposure levels
(Ωarag < 1.8) prevailed over 40 % of the time in
January through March at Chá bă, while there was less chronic exposure
at CCE2, at 11 to 38 % of time in March through July (Fig. 9b). In both
cases, present-day exceedance of these thresholds prevailed over fewer months
and at a smaller percentage of the time during those months (Fig. 9a). For
M. arenaria, present-day Ωarag conditions exceeded
chronic exposure levels at the Gulf of Maine mooring between 11 and 31 %
of the time during December through April, with peak exposure levels in
February and March (Fig. 9b). In contrast to the CCE, which experienced
corrosive Ωarag conditions before ocean acidification, Gulf of
Maine surface water conditions did not exceed biological thresholds for
M. arenaria at any point during the year in preindustrial times
(Fig. 9a).
These observations suggest that present-day coastal Ωarag
conditions exceeded thresholds for sublethal effects on shellfish larvae in
the Gulf of Maine and during both present-day and preindustrial times at
Chá bă and CCE2. However, present-day coastal conditions surpass
these thresholds more often than preindustrial times (Fig. 9). In some
cases, unfavorable surface ocean Ωarag conditions overlap with
the spawning season. Coastal conditions of
Ωarag < 1.4 at Chá bă do not currently occur
during the May to August O. lurida larvae spawning season.
M. californianus tends to spawn year-round, and while natural
populations of C. gigas do exist in Washington coastal waters and
tend to spawn in the late summer, hatcheries raise C. gigas larvae
year-round. Mooring observations suggest that present-day chronic exposure
effects on M. californianus larvae may be more common in the winter
in the northern CCE and in the spring in the southern CCE (Fig. 9). The
summer spawning season of natural populations of C. gigas avoids
chronic and acute exposure levels during winter months; however, hatcheries
may encounter these conditions if raising larvae during this time. In the
Gulf of Maine, M. arenaria spawns when seawater temperatures reach
10 ∘C, which during the moored time series occurred in May through
November (Fig. 6). According to the Gulf of Maine mooring observations
through 2013, corrosive conditions of Ωarag < 1.6
did not occur during this spawning season (Fig. 9b). However, maximum SST
observations in April of 9.7 ∘C were at the verge of this spawning
threshold, and rapid warming in the Gulf of Maine of
0.23 ∘C yr-1 since 2004 suggests that SST as of April 2015 may have
exceeded 10 ∘C at the mooring site (Mills et al., 2013; Pershing et
al., 2015). If this warming causes M. arenaria to begin spawning in
April, larvae may become exposed to Ωarag conditions that
limit shell formation and growth (Fig. 9b).
While these observations on the continental shelf were offshore from the
inshore habitats where natural populations of shellfish and oyster hatcheries
exist, these results provide valuable information on endmember coastal
conditions that affect the nearshore regions. These monthly climatologies
suggest surface water conditions corrosive to shellfish larvae presently
exist year-round in the CCE (primarily during winter/spring) and during
winter/spring in the Gulf of Maine. For shellfish hatcheries that utilize
real-time coastal ocean acidification data and monitor conditions within
their facilities, managing the impacts of these corrosive conditions on
larvae may be possible. These climatologies may also inform the development
of experiments testing the vulnerability of shelled organisms in other
coastal regions. For example, target species may include ecologically or
economically important species that undergo critical life stages when low
Ωarag conditions persist during spring in the region around
Gray's Reef (Fig. 6). However, the coastal mooring climatologies also
illustrate that low Ωarag and low pH conditions do not always
coincide in the natural environment, and experiments testing how Ωarag, pH, and other stressors independently affect marine organisms
are necessary for understanding ocean acidification impacts under the
diversity of present-day conditions (Breitburg et al., 2015).
Comparison to models and ship-based data syntheses
Since high-frequency autonomous ocean acidification time series are
relatively new, much of our current knowledge about ocean carbonate
variability comes from ship-based observations. Due to the limitations of
ship-based oceanography, these observations can have a seasonal measurement
bias leading to errors in seasonal climatology estimates and only capture
opportunistic stochastic events, which has resulted in limited knowledge
about the influence of subseasonal processes on ocean carbonate variability.
In general, we found fairly good agreement between annual mean mooring
Ωarag observations and annual mean ship-based data syntheses,
which primarily used repeat hydrographic cruise data from the Global Data
Analysis Project (Key et al., 2004). Both the Jiang et al. (2015) and
Takahashi et al. (2014) data syntheses overestimated surface ocean
Ωarag at the Stratus mooring in the South Pacific by 0.31
(Fig. 1; Table 4). Undersampling likely contributed to this discrepancy.
Moored observations revealed the lowest Ωarag conditions
during August through October; however, ship-based observations were lacking
in this region of the Southern Hemisphere during this season. Annual mean
surface Ωarag at the four US coastal mooring sites tended to
reflect mean open ocean conditions characterized by the ship-based data
synthesis presented in Fig. 1; however, direct observations in the two coral
reef environments suggest that open ocean carbonate chemistry was modified on the
reefs and in these two cases, resulted in reduced annual mean
Ωarag compared to the ship-based data syntheses (Fig. 1;
Table 4). The ship-based data syntheses also slightly overestimated annual
mean Ωarag at WHOTS, KEO, BOBOA, Iceland, and Chá bă;
however, these overestimations were roughly within the change in
Ωarag expected between the time of the mooring observations
(typically 2010–2015) and the ship-based observations (adjusted to a
reference year of 2000 by Jiang et al., 2015 and 2005 by Takahashi et al.,
2014). Assuming a global average rate of change of surface ocean
Ωarag of -0.008 yr-1 (Bates et al., 2014), the change
over this 5–15 year period would be 0.04–0.12.
Of the 10 mooring locations with observations presented in the Takahashi et
al. (2014) data synthesis, seasonal variability was overestimated by the
ship-based observations at all open ocean sites except Iceland, and
underestimated at Iceland and the coastal and coral reef sites (Table 4).
These differences could be driven by sparse ship-based data in space and time
used to estimate climatological seasonal variability in the Takahashi et
al. (2014) synthesis. This analysis demonstrates that in addition to new
information about subseasonal variability that is not captured by ship-based
observations, moored observations can also be used to improve ship-based data
synthesis estimates of seasonal to annual Ωarag conditions in
undersampled regions such as the Southern Hemisphere, Iceland Sea, and
coastal systems.
Overall, earth system models tend to underestimate natural variability of the
carbonate system. The series of earth system models used by Friedrich et
al. (2012) underestimated both seasonal and interannual variability of
surface Ωarag at all mooring locations except for WHOTS and
Stratus, which were the sites with the lowest variability (Table 4). These
underestimations are expected at the coastal and coral sites since the models
do not capture small-scale biogeochemical processes occurring in these
environments. When Friedrich et al. (2012) extrapolated an average annual
Ωarag amplitude of ∼ 0.1 in subtropical oligotrophic
open ocean regions to coral locations, they concluded that present-day coral
conditions fell 5 times outside the preindustrial range of variability.
However, we found that actual seasonal variability was 2 to 3 times higher
than 0.1 at the Chuuk and La Parguera mooring locations (Table 4), and
present-day Ωarag conditions were only 1 to 2 times below the
preindustrial range of variability (Fig. 7). On the other hand, we found
CCSM3-based estimates of preindustrial envelop exceedance by Cooley et
al. (2009) to be conservative in some regions. They found that by 2050 all
regions will experience surface Ωarag conditions outside
preindustrial bounds of variability with emphasis in low-latitude regions.
Our present-day mooring observations suggest that not only has the shift
outside of preindustrial conditions already occurred year-round at the low-latitude coral reef sites, but also at subtropical and subarctic open ocean
sites.
Some state-of-the-art earth system models have improved the characterization
of background natural variability in the open ocean. A recent study found
that global mean surface ocean pH conditions (Ωarag not assessed)
moved outside preindustrial bounds of variability by 2008 (Mora et al,
2013), which is more consistent with the open ocean moored observations
(Figs. 2–4) compared to the CCSM3-based estimates (Cooley et al., 2009).
Newer earth systems models may still underestimate the full magnitude of
variability; however, they can illustrate the relative variability signal
between different open ocean regions (Rodgers et al., 2015). Similar to the
moored observations, Rodgers et al. (2015) showed that Ωarag
variability is higher in the North Pacific and North Atlantic regions (KEO,
Papa, Iceland) compared to subtropical and tropical regions (i.e., WHOTS,
Stratus, BOBOA).
Unlike global earth system models, some regional models are able to resolve
small-scale coastal processes and may provide better estimates of natural
variability in these dynamic systems. We found the highest levels of natural
variability at the Chá bă mooring location with an annual range of
surface ocean Ωarag of 3.3, from Ωarag values
of 1.06 to 4.36 (Fig. 5). Estimates of this range for the northern CCE
produced by a regional model were only 0.2, which led to the conclusion that
surface ocean Ωarag conditions in 2005 were already outside
the bounds of preindustrial conditions (Hauri et al., 2013). Observations at
Chá bă from 2010 to 2014 suggest conditions only fell partially
outside preindustrial variability, primarily during the lower variability
season from October to February (Fig. 5).