Doxaran et al., 2009). As zones of complex salinity variability, nutrient dynamics, planktonic productivity, and
fish spawning and growth, ETMs are important estuarine
features forced by climate, river discharge, salinity, and
sediment transport.
Despite the complexity of processes controlling sediment, land-to-estuary sediment flux, estuaries will continue to be vulnerable to future changes in climate,
including the incidence and intensity of extreme storm
events.
Biogeochemical processes
In addition to biogeochemical changes related to nutrient
and oxygen dynamics discussed above, changes in ocean
carbonate chemistry due to the uptake of anthropogenic
CO 2 by the world’s ocean, often referred to as “ocean
acidification” (OA), pose complex, taxon-specific, and
still poorly understood impacts on marine life (Hendriks
et al., 2010; Wittmann and Pörtner, 2013; Kroeker et al.,
2013). It is estimated that mean global ocean pH has been
lowered by 0.1 pH units since $1750 and may decrease by
0.3–0.4 pH units by 2100 (Pelejero et al., 2010). For comparison, glacial-interglacial cycles of the last 400 ka may
have experienced changes of between 0.15 and 0.3 pH
units. Although anthropogenic driven pH changes cannot
be directly compared to natural events due to differing
rates and boundary conditions, paleoclimate studies show
that over multimillion year timescales, past natural acidification events had large effects on marine organisms
(Kump et al., 2009; Pelejero et al., 2010; Hönisch et al.,
2012).
Currently, the study of OA impacts on coastal marine
organisms is a growing field for corals (Hoegh-Guldberg
et al., 2007), molluscs (Talmage and Gobler, 2009;
Waldbusser et al., 2011, 2014; Gobler and Talmage,
2013), and other taxonomic groups (Ries et al., 2009;
Kroeker et al., 2010). Some case studies suggest that pH
has fallen in recent decades in some coastal systems. For
example, pH fell from $8.2 to 7.9 in the last 30 years in
Chesapeake Bay (Waldbusser et al., 2011); Feely
et al. (2010) estimate that 24–49 % of observed pH lowering in parts of Puget Sound, a deep estuary in the Pacific
NW, was due to influx of seasonal upwelled ocean water,
that is, global OA, as distinct from in situ remineralization
via respiration. Complicating the issue of causality of
observed changes in coastal pH, Pelejero et al. (2005)
found that pH variation in a southwest Pacific Ocean coral
was related to multidecadal climate variability in the
Interdecadal Pacific Oscillation. In addition, other factors,
such as reduced freshwater influx and higher salinity, may
affect estuarine pH.
Sea-level rise
Sea-level rise (SLR) is one of the most challenging yet
misunderstood concerns for estuaries and other coastal
systems. No fewer than five global and four regional
processes influence relative sea level along any particular
coast (Cronin, 2012). Global factors include thermosteric
ocean expansion (increase in ocean volume, Willis et al.,
2010), melting land-based ice from glaciers (increases
ocean mass and mean global sea level), melting parts of
the Greenland and Antarctic Ice Sheets (increases ocean
mass and sea level, Hanna et al., 2013), reservoir storage
(decreases mean sea level), and terrestrial water depletion
(increases mean sea level, Konikow, 2011). Regional processes (excluding rapid tectonic movement) include
glacio-isostatic adjustment (GIA, Peltier and Fairbanks,
2006) due to viscoelastic response of Earth’s mantle to
melting large ice sheets since the last glacial period
$20 ka (local GIA can also occur due to glacier melting),
elastic deformation of Earth’s crust due to changes in
gravity and rotation (Tamisiea and Mitrovica, 2011), local
groundwater withdrawal, and long-term thermal subsidence of the crust (typically minimal).
The contribution of each factor will vary regionally, but
nonetheless, from the standpoint of estuaries and other
coastal systems, several points deserve emphasis. Global
mean sea level has been rising at rate of 3.1 mm year
À1
over the past few decades (perhaps an acceleration over
rates averaged for the last century), mostly due to
thermosteric expansion and land ice melting. Some studies suggest that SLR is already affecting large estuaries
such as Chesapeake Bay (Hilton et al., 2008; Murphy
et al., 2011) and coastal wetlands (Cahoon et al., 2006).
In addition, although no consensus exists on future SLR,
rates are expected to increase and glacier and ice sheet
mass balance loss is likely to dominate SLR the rest of
the twenty-first century. Consequently, the modeling
study by Hong and Shen (2012) on the impacts of future
SLR on Chesapeake Bay is illustrative, finding that primary effects on salinity, stratification, circulation, nutrient
retention, and dissolved oxygen varied spatially, seasonally, and interannually. In addition, if as expected, tidal
ranges and wave heights increase, severe storms would
become an even larger concern in some estuaries (Najjar
et al., 2010). Finally, geological records show that in the
past, SLR rates reached and at times exceeded
$10–15 mm year
À1 in the absence of abrupt increase in
greenhouse gas forcing. The implication is that, although
the many factors that govern coastal ecosystem functioning cannot be oversimplified, the ability of some sensitive
systems, notably mangroves, salt marshes, and coral reefs,
to “keep up” with SL, that is, to accrete at the same rate of
SL rise, remains a major concern.
Summary
Climate changes throughout geological history have
influenced estuaries and coastal systems in a variety of
ways and over all timescales. Similarly, future climate
change will influence estuaries, perhaps at an accelerated
rate, notably through effects on salinity and temperature,
dissolved oxygen concentrations, nutrient and sediment
CLIMATE CHANGE
125
fish spawning and growth, ETMs are important estuarine
features forced by climate, river discharge, salinity, and
sediment transport.
Despite the complexity of processes controlling sediment, land-to-estuary sediment flux, estuaries will continue to be vulnerable to future changes in climate,
including the incidence and intensity of extreme storm
events.
Biogeochemical processes
In addition to biogeochemical changes related to nutrient
and oxygen dynamics discussed above, changes in ocean
carbonate chemistry due to the uptake of anthropogenic
CO 2 by the world’s ocean, often referred to as “ocean
acidification” (OA), pose complex, taxon-specific, and
still poorly understood impacts on marine life (Hendriks
et al., 2010; Wittmann and Pörtner, 2013; Kroeker et al.,
2013). It is estimated that mean global ocean pH has been
lowered by 0.1 pH units since $1750 and may decrease by
0.3–0.4 pH units by 2100 (Pelejero et al., 2010). For comparison, glacial-interglacial cycles of the last 400 ka may
have experienced changes of between 0.15 and 0.3 pH
units. Although anthropogenic driven pH changes cannot
be directly compared to natural events due to differing
rates and boundary conditions, paleoclimate studies show
that over multimillion year timescales, past natural acidification events had large effects on marine organisms
(Kump et al., 2009; Pelejero et al., 2010; Hönisch et al.,
2012).
Currently, the study of OA impacts on coastal marine
organisms is a growing field for corals (Hoegh-Guldberg
et al., 2007), molluscs (Talmage and Gobler, 2009;
Waldbusser et al., 2011, 2014; Gobler and Talmage,
2013), and other taxonomic groups (Ries et al., 2009;
Kroeker et al., 2010). Some case studies suggest that pH
has fallen in recent decades in some coastal systems. For
example, pH fell from $8.2 to 7.9 in the last 30 years in
Chesapeake Bay (Waldbusser et al., 2011); Feely
et al. (2010) estimate that 24–49 % of observed pH lowering in parts of Puget Sound, a deep estuary in the Pacific
NW, was due to influx of seasonal upwelled ocean water,
that is, global OA, as distinct from in situ remineralization
via respiration. Complicating the issue of causality of
observed changes in coastal pH, Pelejero et al. (2005)
found that pH variation in a southwest Pacific Ocean coral
was related to multidecadal climate variability in the
Interdecadal Pacific Oscillation. In addition, other factors,
such as reduced freshwater influx and higher salinity, may
affect estuarine pH.
Sea-level rise
Sea-level rise (SLR) is one of the most challenging yet
misunderstood concerns for estuaries and other coastal
systems. No fewer than five global and four regional
processes influence relative sea level along any particular
coast (Cronin, 2012). Global factors include thermosteric
ocean expansion (increase in ocean volume, Willis et al.,
2010), melting land-based ice from glaciers (increases
ocean mass and mean global sea level), melting parts of
the Greenland and Antarctic Ice Sheets (increases ocean
mass and sea level, Hanna et al., 2013), reservoir storage
(decreases mean sea level), and terrestrial water depletion
(increases mean sea level, Konikow, 2011). Regional processes (excluding rapid tectonic movement) include
glacio-isostatic adjustment (GIA, Peltier and Fairbanks,
2006) due to viscoelastic response of Earth’s mantle to
melting large ice sheets since the last glacial period
$20 ka (local GIA can also occur due to glacier melting),
elastic deformation of Earth’s crust due to changes in
gravity and rotation (Tamisiea and Mitrovica, 2011), local
groundwater withdrawal, and long-term thermal subsidence of the crust (typically minimal).
The contribution of each factor will vary regionally, but
nonetheless, from the standpoint of estuaries and other
coastal systems, several points deserve emphasis. Global
mean sea level has been rising at rate of 3.1 mm year
À1
over the past few decades (perhaps an acceleration over
rates averaged for the last century), mostly due to
thermosteric expansion and land ice melting. Some studies suggest that SLR is already affecting large estuaries
such as Chesapeake Bay (Hilton et al., 2008; Murphy
et al., 2011) and coastal wetlands (Cahoon et al., 2006).
In addition, although no consensus exists on future SLR,
rates are expected to increase and glacier and ice sheet
mass balance loss is likely to dominate SLR the rest of
the twenty-first century. Consequently, the modeling
study by Hong and Shen (2012) on the impacts of future
SLR on Chesapeake Bay is illustrative, finding that primary effects on salinity, stratification, circulation, nutrient
retention, and dissolved oxygen varied spatially, seasonally, and interannually. In addition, if as expected, tidal
ranges and wave heights increase, severe storms would
become an even larger concern in some estuaries (Najjar
et al., 2010). Finally, geological records show that in the
past, SLR rates reached and at times exceeded
$10–15 mm year
À1 in the absence of abrupt increase in
greenhouse gas forcing. The implication is that, although
the many factors that govern coastal ecosystem functioning cannot be oversimplified, the ability of some sensitive
systems, notably mangroves, salt marshes, and coral reefs,
to “keep up” with SL, that is, to accrete at the same rate of
SL rise, remains a major concern.
Summary
Climate changes throughout geological history have
influenced estuaries and coastal systems in a variety of
ways and over all timescales. Similarly, future climate
change will influence estuaries, perhaps at an accelerated
rate, notably through effects on salinity and temperature,
dissolved oxygen concentrations, nutrient and sediment
CLIMATE CHANGE
125