when averaged globally, provides global mean sea level
(GMSL). The physical processes causing GMSL rise and
regional changes in RSL are not identical, although they are
related. The primary contributors to current GMSL rise are
the thermal expansion of sea waters, land ice loss and
freshwater mass exchange between oceans and land water
reservoirs and groundwater storage change. The recent
trends of these contributions are most likely resulted from
the climate change induced by anthropogenic greenhouse
gas emissions.
9.2.1 Ocean Warming
Analyses of in-situ ocean temperature data collected over the
past 50 years by ships and recently by Argo profiling floats
(Argo Data Management Team 2008; Roemmich et al. 2009)
reveal that ocean has been warming and increases the upper
ocean heat content (OHC). Hence, the sea level, due to the
thermal expansion of sea water, has significantly increased
since 1950 (e.g. Levitus et al. 2009; Ishii and Kimoto 2009;
Domingues et al. 2008; Church et al. 2011a). A recent study
by Cheng et al. (2017) has shown that the changes in OHC
were relatively small before about 1980; since then, OHC
has increased fairly steadily and, since 1990, has increasingly involved deeper layers of the ocean. In the climate
system, the ocean acts as a ‘buffer’ for the atmospheric
temperature by storing a large amount of heat from the
atmosphere and transporting it to deeper depths via the
ocean conveyor belt. On average, over the last 50 years,
93% of the excess heat accumulated in the climate system
because of greenhouse gas emissions has been stored in the
ocean owing its large heat capacity, the remaining 7% warm
the atmosphere and continents, and melt sea and land ice
(Levitus et al. 2012; von Schuckmann et al. 2016). Consequently, ocean warming explains about 30–40% of the
observed sea-level rise of the last few decades (e.g. Church
et al. 2011b).
9.2.2 Glaciers Melting
Apart from the global ocean thermal expansion, melting of
the continental ice storage in a warming climate is turned out
to be another factor for global mean sea-level rise. Being
very sensitive to global warming, mountain glaciers and
small ice caps have retreated worldwide during recent decades. The contribution of glacier ice melt to sea-level rise has
been estimated based on the mass balance studies of a large
number of glaciers (Meier et al. 2007; Kaser et al. 2006). In
fact, studies have shown that glaciers have accounted for
*21% of the global sea-level rise since 1993 (e.g. WCRP
2018).
9.2.3 Ice Sheets
The mass balance of the ice sheets was less known before
the 1990s due to inadequate and incomplete observations.
Different remote sensing techniques available since then
have provided important results on the changing mass of
Greenland and (west) Antarctica (e.g. Allison et al. 2009).
These data indicate that both ice sheets are currently losing
mass at an accelerated rate (e.g. Steffen et al. 2011). For the
period 1993–2003, <15% of the rate of global mean
sea-level rise was due to the melting of ice sheets (IPCC
AR4). But their contribution has increased to *40% from
2003 to 2004. The ice sheets mass loss explains *25% of
the rate of global sea-level rise during 2003–2010 (Cazenave
and Remy 2011; Church et al. 2011a). A near-complete loss
of Greenland ice sheet over a million years or more leading
to the global mean sea-level rise of about 7 m can be caused
by sustained global warming greater than a certain threshold
above pre-industrial conditions (IPCC AR5). A schematic
representation of different processes contributing to global
and regional sea-level changes is shown in Fig. 9.1.
9.2.4 Regional Sea-Level Change
Sea-level rise pattern varies substantially from region to
region. Geographical patterns of sea-level rise can result in
different processes: changes in sea-water density due to
changes in temperature and salinity (known as ‘steric’
sea-level changes) are the dominant process, especially in
the tropical oceans. Steric sea-level changes are primarily
associated with the atmosphere-ocean coupled dynamics
driven mainly by surface winds and ocean circulation. Solid
Earth’s deformation and geoid changes in response to past
and ongoing mass redistribution caused by land ice melt and
land water storage changes (known as ‘static’ factors) also
make regional changes in sea level (Stammer et al. 2013). It
was shown that the dominant contribution to observed
regional sea-level changes comes from steric effects caused
by non-uniform thermal expansion and salinity variations
(Church et al. 2013a, b; Stammer et al. 2013). Contributions
from other effects, in particular, the static factors, are little in
the present time but will become important in the future
(Milne et al. 2009).
9.3 Mean Sea-Level Change
9.3.1 Global
At the time of the last interglacial period, about
125,000 years ago, sea level was likely 4–6 m higher than it
was during the twentieth century, as polar average
9 Sea-Level Rise
177
(GMSL). The physical processes causing GMSL rise and
regional changes in RSL are not identical, although they are
related. The primary contributors to current GMSL rise are
the thermal expansion of sea waters, land ice loss and
freshwater mass exchange between oceans and land water
reservoirs and groundwater storage change. The recent
trends of these contributions are most likely resulted from
the climate change induced by anthropogenic greenhouse
gas emissions.
9.2.1 Ocean Warming
Analyses of in-situ ocean temperature data collected over the
past 50 years by ships and recently by Argo profiling floats
(Argo Data Management Team 2008; Roemmich et al. 2009)
reveal that ocean has been warming and increases the upper
ocean heat content (OHC). Hence, the sea level, due to the
thermal expansion of sea water, has significantly increased
since 1950 (e.g. Levitus et al. 2009; Ishii and Kimoto 2009;
Domingues et al. 2008; Church et al. 2011a). A recent study
by Cheng et al. (2017) has shown that the changes in OHC
were relatively small before about 1980; since then, OHC
has increased fairly steadily and, since 1990, has increasingly involved deeper layers of the ocean. In the climate
system, the ocean acts as a ‘buffer’ for the atmospheric
temperature by storing a large amount of heat from the
atmosphere and transporting it to deeper depths via the
ocean conveyor belt. On average, over the last 50 years,
93% of the excess heat accumulated in the climate system
because of greenhouse gas emissions has been stored in the
ocean owing its large heat capacity, the remaining 7% warm
the atmosphere and continents, and melt sea and land ice
(Levitus et al. 2012; von Schuckmann et al. 2016). Consequently, ocean warming explains about 30–40% of the
observed sea-level rise of the last few decades (e.g. Church
et al. 2011b).
9.2.2 Glaciers Melting
Apart from the global ocean thermal expansion, melting of
the continental ice storage in a warming climate is turned out
to be another factor for global mean sea-level rise. Being
very sensitive to global warming, mountain glaciers and
small ice caps have retreated worldwide during recent decades. The contribution of glacier ice melt to sea-level rise has
been estimated based on the mass balance studies of a large
number of glaciers (Meier et al. 2007; Kaser et al. 2006). In
fact, studies have shown that glaciers have accounted for
*21% of the global sea-level rise since 1993 (e.g. WCRP
2018).
9.2.3 Ice Sheets
The mass balance of the ice sheets was less known before
the 1990s due to inadequate and incomplete observations.
Different remote sensing techniques available since then
have provided important results on the changing mass of
Greenland and (west) Antarctica (e.g. Allison et al. 2009).
These data indicate that both ice sheets are currently losing
mass at an accelerated rate (e.g. Steffen et al. 2011). For the
period 1993–2003, <15% of the rate of global mean
sea-level rise was due to the melting of ice sheets (IPCC
AR4). But their contribution has increased to *40% from
2003 to 2004. The ice sheets mass loss explains *25% of
the rate of global sea-level rise during 2003–2010 (Cazenave
and Remy 2011; Church et al. 2011a). A near-complete loss
of Greenland ice sheet over a million years or more leading
to the global mean sea-level rise of about 7 m can be caused
by sustained global warming greater than a certain threshold
above pre-industrial conditions (IPCC AR5). A schematic
representation of different processes contributing to global
and regional sea-level changes is shown in Fig. 9.1.
9.2.4 Regional Sea-Level Change
Sea-level rise pattern varies substantially from region to
region. Geographical patterns of sea-level rise can result in
different processes: changes in sea-water density due to
changes in temperature and salinity (known as ‘steric’
sea-level changes) are the dominant process, especially in
the tropical oceans. Steric sea-level changes are primarily
associated with the atmosphere-ocean coupled dynamics
driven mainly by surface winds and ocean circulation. Solid
Earth’s deformation and geoid changes in response to past
and ongoing mass redistribution caused by land ice melt and
land water storage changes (known as ‘static’ factors) also
make regional changes in sea level (Stammer et al. 2013). It
was shown that the dominant contribution to observed
regional sea-level changes comes from steric effects caused
by non-uniform thermal expansion and salinity variations
(Church et al. 2013a, b; Stammer et al. 2013). Contributions
from other effects, in particular, the static factors, are little in
the present time but will become important in the future
(Milne et al. 2009).
9.3 Mean Sea-Level Change
9.3.1 Global
At the time of the last interglacial period, about
125,000 years ago, sea level was likely 4–6 m higher than it
was during the twentieth century, as polar average
9 Sea-Level Rise
177