paleoecological reconstructions, obtained from geochemical, physical, or biological proxies recovered from sediment cores (Cronin and Walker, 2006; Gooday et al.,
2009). Paleo-reconstructions provide direct evidence for
past climate impacts and prehistorical baseline conditions
for ecosystem restoration, impact assessment, and
planning.
Identifying climate impacts on modern estuaries is
complicated by multiple environmental stresses from
a wide range of local and regional anthropogenic activity
such as land use changes associated with urbanization,
agriculture, and other activities (Willard and Cronin,
2007; Canuel et al., 2010). These factors can make the
attribution of observed changes in estuarine environments
to specific causes very challenging.
This chapter summarizes climate impacts on estuaries
during the mid-Holocene to late Holocene interglacial
period (the last $7,000 years), which is the period since
postglacial sea-level rise stabilized and modern coastal
systems took their modern form. Thus, this chapter applies
to both prehistorical natural climate variability and climate
change since the onset of the Anthropocene, sometimes
defined as the period since the industrial revolution beginning $1750–1800 CE (Common Era) (Gale and Hoare,
2012). Climate impacts can be grouped into five broad,
interconnected categories: regional precipitation, sediment processes, temperature (global and regional), biogeochemical processes, and sea-level rise.
Regional precipitation
Climate change has direct impacts on estuaries through its
effects on regional rainfall patterns. Seasonal and/or mean
annual precipitation in watersheds is often highly correlated with river discharge into estuaries, which in turn
affects salinity patterns and circulation. For example, in
a partially mixed, microtidal estuary like Chesapeake
Bay, river discharge, along with wind and tidal forcing,
affects buoyancy-driven circulation, stratification, the
development of a pycnocline, and oxygen exchange
between upper and deeper layers (Schubel and Pritchard,
1986).
As a consequence, river discharge affects nutrient
influx, phytoplankton blooms, dissolved oxygen, water
quality, and ecosystem functioning such that excess
nutrient loading coupled with greater river discharge has
led to estuarine eutrophication on a global scale
(Diaz and Rosenberg, 2008; Kemp et al., 2009; Howarth
et al., 2011).
Internal modes of climate variability
The term “internal modes of climate variability” is often
used to refer to climate changes that are not forced by radiative forcing from GHGs and solar and volcanic activities
but rather interactions between the atmosphere, oceans,
and ice sheets. The most widely recognized climate patterns are called the El Niño-Southern Oscillation
(ENSO), North Atlantic Oscillation (NAO), Pacific
Decadal Oscillation (PDO), and Atlantic Multidecadal
Oscillation (AMO). Many studies have demonstrated
a strong connection between internal modes of climate
variability over interannual to multidecadal timescales
and estuarine circulation, salinity, and dissolved oxygen
(DO). Using a global dataset, Gilbert et al. (2010) could
identify a secular pattern of decreasing DO between
1976 and 2000 that was more evident in coastal regions
than in the open ocean. However, they also stressed that
when interpreting the twentieth century patterns of oxygen
concentrations, decadal climate variability can impose
large-amplitude oscillations larger than the overall linear
trend (see Garcia et al., 2005). Some examples of climate
variability impacting regional rainfall, river discharge,
estuarine salinity, and, in some cases, nutrient flux include
studies of the PDO (Xu et al., 2012), the NAO (Cronin
et al., 2005; Prasad et al., 2010), ENSO (Swart et al.,
1996; Schmidt et al., 2001; Cronin et al., 2002), and the
AMO (Enfield et al., 2001).
There are also well-established links between climate
variability and marine biological systems (Mantua et al.,
1997, Drinkwater et al., 2003; Pershing et al., 2005;
Greene and Pershing, 2007). Cloern et al. (2010) showed
that biological communities in San Francisco Bay are sensitive to ocean currents, temperatures, and coastal upwelling connected to PDO variability and North Pacific gyre
circulation. Paerl et al. (2013) showed that climate-driven
changes in river discharge to North Carolina estuaries
altered the composition and biomass of phytoplankton
communities. ENSO- and NAO-connected climate variability also influences outbreaks of infectious diseases
on a global scale (Lafferty, 2009; Morand et al., 2013)
and, in particular, viruses, bacteria, and infectious disease
outbreaks in coastal waters (Lipp et al., 2001; Rose et al.,
2001).
Two specific aspects of climate that deserve attention
are extended droughts or wet periods and extreme events
such as tropical cyclones. Evidence from tree-rings,
corals, sediments, molluscan isotopes, and speleothems
shows that droughts are an inherent part of Holocene climate. Quantitative reconstructions of precipitation show
that North America (Cook et al. 2014) and Europe
(Büntgen et al., 2010) have experienced decadal,
continent-scale droughts over the past millennium. Multiple paleo-reconstructions based on several proxies show
that droughts frequently affected mid-Atlantic climate
and Chesapeake Bay watershed (Stahle et al., 1998;
Cronin et al., 2005, Saenger et al., 2006; Harding et al.,
2010). Precipitation changes over centennial timescales
also affected coastal systems, such as changes in runoff
and productivity in Chilean fjords during the latter part
of the Little Ice Age from $1600 to the 1800s
(Rebolledo et al., 2008).
Although specific weather events cannot be directly
linked to climate change, there is nonetheless concern that
changing climate might increase the frequency and
CLIMATE CHANGE
123
2009). Paleo-reconstructions provide direct evidence for
past climate impacts and prehistorical baseline conditions
for ecosystem restoration, impact assessment, and
planning.
Identifying climate impacts on modern estuaries is
complicated by multiple environmental stresses from
a wide range of local and regional anthropogenic activity
such as land use changes associated with urbanization,
agriculture, and other activities (Willard and Cronin,
2007; Canuel et al., 2010). These factors can make the
attribution of observed changes in estuarine environments
to specific causes very challenging.
This chapter summarizes climate impacts on estuaries
during the mid-Holocene to late Holocene interglacial
period (the last $7,000 years), which is the period since
postglacial sea-level rise stabilized and modern coastal
systems took their modern form. Thus, this chapter applies
to both prehistorical natural climate variability and climate
change since the onset of the Anthropocene, sometimes
defined as the period since the industrial revolution beginning $1750–1800 CE (Common Era) (Gale and Hoare,
2012). Climate impacts can be grouped into five broad,
interconnected categories: regional precipitation, sediment processes, temperature (global and regional), biogeochemical processes, and sea-level rise.
Regional precipitation
Climate change has direct impacts on estuaries through its
effects on regional rainfall patterns. Seasonal and/or mean
annual precipitation in watersheds is often highly correlated with river discharge into estuaries, which in turn
affects salinity patterns and circulation. For example, in
a partially mixed, microtidal estuary like Chesapeake
Bay, river discharge, along with wind and tidal forcing,
affects buoyancy-driven circulation, stratification, the
development of a pycnocline, and oxygen exchange
between upper and deeper layers (Schubel and Pritchard,
1986).
As a consequence, river discharge affects nutrient
influx, phytoplankton blooms, dissolved oxygen, water
quality, and ecosystem functioning such that excess
nutrient loading coupled with greater river discharge has
led to estuarine eutrophication on a global scale
(Diaz and Rosenberg, 2008; Kemp et al., 2009; Howarth
et al., 2011).
Internal modes of climate variability
The term “internal modes of climate variability” is often
used to refer to climate changes that are not forced by radiative forcing from GHGs and solar and volcanic activities
but rather interactions between the atmosphere, oceans,
and ice sheets. The most widely recognized climate patterns are called the El Niño-Southern Oscillation
(ENSO), North Atlantic Oscillation (NAO), Pacific
Decadal Oscillation (PDO), and Atlantic Multidecadal
Oscillation (AMO). Many studies have demonstrated
a strong connection between internal modes of climate
variability over interannual to multidecadal timescales
and estuarine circulation, salinity, and dissolved oxygen
(DO). Using a global dataset, Gilbert et al. (2010) could
identify a secular pattern of decreasing DO between
1976 and 2000 that was more evident in coastal regions
than in the open ocean. However, they also stressed that
when interpreting the twentieth century patterns of oxygen
concentrations, decadal climate variability can impose
large-amplitude oscillations larger than the overall linear
trend (see Garcia et al., 2005). Some examples of climate
variability impacting regional rainfall, river discharge,
estuarine salinity, and, in some cases, nutrient flux include
studies of the PDO (Xu et al., 2012), the NAO (Cronin
et al., 2005; Prasad et al., 2010), ENSO (Swart et al.,
1996; Schmidt et al., 2001; Cronin et al., 2002), and the
AMO (Enfield et al., 2001).
There are also well-established links between climate
variability and marine biological systems (Mantua et al.,
1997, Drinkwater et al., 2003; Pershing et al., 2005;
Greene and Pershing, 2007). Cloern et al. (2010) showed
that biological communities in San Francisco Bay are sensitive to ocean currents, temperatures, and coastal upwelling connected to PDO variability and North Pacific gyre
circulation. Paerl et al. (2013) showed that climate-driven
changes in river discharge to North Carolina estuaries
altered the composition and biomass of phytoplankton
communities. ENSO- and NAO-connected climate variability also influences outbreaks of infectious diseases
on a global scale (Lafferty, 2009; Morand et al., 2013)
and, in particular, viruses, bacteria, and infectious disease
outbreaks in coastal waters (Lipp et al., 2001; Rose et al.,
2001).
Two specific aspects of climate that deserve attention
are extended droughts or wet periods and extreme events
such as tropical cyclones. Evidence from tree-rings,
corals, sediments, molluscan isotopes, and speleothems
shows that droughts are an inherent part of Holocene climate. Quantitative reconstructions of precipitation show
that North America (Cook et al. 2014) and Europe
(Büntgen et al., 2010) have experienced decadal,
continent-scale droughts over the past millennium. Multiple paleo-reconstructions based on several proxies show
that droughts frequently affected mid-Atlantic climate
and Chesapeake Bay watershed (Stahle et al., 1998;
Cronin et al., 2005, Saenger et al., 2006; Harding et al.,
2010). Precipitation changes over centennial timescales
also affected coastal systems, such as changes in runoff
and productivity in Chilean fjords during the latter part
of the Little Ice Age from $1600 to the 1800s
(Rebolledo et al., 2008).
Although specific weather events cannot be directly
linked to climate change, there is nonetheless concern that
changing climate might increase the frequency and
CLIMATE CHANGE
123