Key Messages
• There is high confidence that the rate of global mean sea
level (GMSL) rise has increased. Human-caused climate
change has made a substantial contribution to the rise
since 1900.
• The GMSL has risen by 1.7 (1.5 to 1.9) mm year
−1 since
1901 and the rate of rise has accelerated to 3.3 mm
year
−1 since 1993.
• Sea-level rise in the Indian Ocean is non-uniform and the
rate of north Indian Ocean rise is 1.06–1.75 mm year
−1
from 1874 to 2004 and is 3.3 mm year
−1 in the recent
decades (1993–2015), which is comparable to the current
rate of GMSL rise.
• Indian Ocean sea-level rise is dominated by the ocean
thermal expansion, while the addition of water mass from
terrestrial ice-melting is the major contributor to the
GMSL rise.
• Interannual to decadal-scale variability in the Indian
Ocean sea level is dominated by El Niño Southern
Oscillation and Indian Ocean Dipole events.
• Relative to 1986–2005, GMSL is very likely to rise by
*26 cm by 2050 and *53 cm by 2100 for a mid-range,
mitigation scenario.
• Steric sea level along the Indian coast is likely to rise by
about 20–30 cm at the end of the twenty-first century and
the corresponding estimate for global mean steric sealevel rise is 18±5 cm (relative to 1986–2005), under
RCP4.5 (for a mid-range emission scenario, excluding
ice-melt contributions).
• Extreme sea-level events are projected to occur frequently over the tropical regions (high confidence) and
along the Indian coast (medium confidence) associated
with an increase in the mean sea level and climate
extremes.
9.1 Introduction
The global ocean plays a critical role in regulating the energy
balance of the climate system. Over 90% of the anthropogenic excess heat goes into the oceans (Church et al.
2013a, b), remaining goes into melting both terrestrial and
sea ice, and warming the atmosphere and land (Hansen et al.
2011; Church et al. 2011b; Trenberth et al. 2014). One of the
consequences of warming of the global ocean and the
melting of ice and glaciers is the rise in mean sea level.
Sea-level rise can exert significant stress on highly populated
coastal societies and low-lying island countries around the
world. Indian Ocean region is heavily populated, comprises
of many low-lying islands and coastal zones and is highly
rich in marine ecosystems. The regions in and around the
Indian Ocean are home to roughly 2.6 billion people, which
is 40% of the global population. One-third of the Indian
population and the majority of the Asian population are
located near coastal regions. Therefore, the rise in sea level
can pose a growing challenge to population, economy,
coastal infrastructures and marine ecosystems. Despite
considerable progress during recent years, major gaps
remain in our understanding of sea-level changes and their
causes, particularly at regional scales.
Changes in mean sea level are the result of the complex
interplay of a number of factors. Even though there is an
unabated rise in observed global mean sea level, the spatial
distribution of sea-level trends is not globally uniform
(Church et al. 2013a, b). Regionally, sea-level variations can
deviate considerably from the global mean. It is very likely
that in the twenty-first century and beyond, the sea-level
change will have a strong regional pattern, with some places
experiencing significant deviations from the global mean
sea-level rise (IPCC AR5). The detailed sea-level change
along coastlines can therefore potentially be far more substantial than the global mean sea-level rise. The underlying
causes of regional sea-level changes are associated with
dynamic variations in the ocean circulation as part of climate
modes of variability, changes in the wind pattern and with an
isostatic adjustment of Earth’s crust to past and ongoing
changes in polar ice masses and continental water storage
(Stammer et al. 2013). Assessment of vulnerability to rising
sea levels requires consideration of physical causes, historical evidence and projections.
This chapter reviews the physical factors driving changes
in global mean sea level (GMSL) as well as those causing
additional regional variations in relative sea level (RSL).
Geological and instrumental observations of historical
sea-level changes in the global ocean and for the RSL in the
Indian Ocean are presented here. The chapter then describes
a range of scenarios for future levels and rates of sea-level
change, for the Indian Ocean as well as for the global ocean.
Finally, an assessment of the impact of changes in sea level
on extreme water levels is discussed.
9.2 Physical Factors Contributing
to Sea-Level Rise
Sea level is measured either with respect to the surface of the
solid Earth, known as relative sea level (RSL) or a geocentric reference such as the reference ellipsoid, known as
the geocentric sea level. RSL estimates have been obtained
from tide gauges and geological records for the past few
centuries. Geocentric sea level has been measured over the
past two decades using satellite altimetry. The sea level,
176
P. Swapna et al.
• There is high confidence that the rate of global mean sea
level (GMSL) rise has increased. Human-caused climate
change has made a substantial contribution to the rise
since 1900.
• The GMSL has risen by 1.7 (1.5 to 1.9) mm year
−1 since
1901 and the rate of rise has accelerated to 3.3 mm
year
−1 since 1993.
• Sea-level rise in the Indian Ocean is non-uniform and the
rate of north Indian Ocean rise is 1.06–1.75 mm year
−1
from 1874 to 2004 and is 3.3 mm year
−1 in the recent
decades (1993–2015), which is comparable to the current
rate of GMSL rise.
• Indian Ocean sea-level rise is dominated by the ocean
thermal expansion, while the addition of water mass from
terrestrial ice-melting is the major contributor to the
GMSL rise.
• Interannual to decadal-scale variability in the Indian
Ocean sea level is dominated by El Niño Southern
Oscillation and Indian Ocean Dipole events.
• Relative to 1986–2005, GMSL is very likely to rise by
*26 cm by 2050 and *53 cm by 2100 for a mid-range,
mitigation scenario.
• Steric sea level along the Indian coast is likely to rise by
about 20–30 cm at the end of the twenty-first century and
the corresponding estimate for global mean steric sealevel rise is 18±5 cm (relative to 1986–2005), under
RCP4.5 (for a mid-range emission scenario, excluding
ice-melt contributions).
• Extreme sea-level events are projected to occur frequently over the tropical regions (high confidence) and
along the Indian coast (medium confidence) associated
with an increase in the mean sea level and climate
extremes.
9.1 Introduction
The global ocean plays a critical role in regulating the energy
balance of the climate system. Over 90% of the anthropogenic excess heat goes into the oceans (Church et al.
2013a, b), remaining goes into melting both terrestrial and
sea ice, and warming the atmosphere and land (Hansen et al.
2011; Church et al. 2011b; Trenberth et al. 2014). One of the
consequences of warming of the global ocean and the
melting of ice and glaciers is the rise in mean sea level.
Sea-level rise can exert significant stress on highly populated
coastal societies and low-lying island countries around the
world. Indian Ocean region is heavily populated, comprises
of many low-lying islands and coastal zones and is highly
rich in marine ecosystems. The regions in and around the
Indian Ocean are home to roughly 2.6 billion people, which
is 40% of the global population. One-third of the Indian
population and the majority of the Asian population are
located near coastal regions. Therefore, the rise in sea level
can pose a growing challenge to population, economy,
coastal infrastructures and marine ecosystems. Despite
considerable progress during recent years, major gaps
remain in our understanding of sea-level changes and their
causes, particularly at regional scales.
Changes in mean sea level are the result of the complex
interplay of a number of factors. Even though there is an
unabated rise in observed global mean sea level, the spatial
distribution of sea-level trends is not globally uniform
(Church et al. 2013a, b). Regionally, sea-level variations can
deviate considerably from the global mean. It is very likely
that in the twenty-first century and beyond, the sea-level
change will have a strong regional pattern, with some places
experiencing significant deviations from the global mean
sea-level rise (IPCC AR5). The detailed sea-level change
along coastlines can therefore potentially be far more substantial than the global mean sea-level rise. The underlying
causes of regional sea-level changes are associated with
dynamic variations in the ocean circulation as part of climate
modes of variability, changes in the wind pattern and with an
isostatic adjustment of Earth’s crust to past and ongoing
changes in polar ice masses and continental water storage
(Stammer et al. 2013). Assessment of vulnerability to rising
sea levels requires consideration of physical causes, historical evidence and projections.
This chapter reviews the physical factors driving changes
in global mean sea level (GMSL) as well as those causing
additional regional variations in relative sea level (RSL).
Geological and instrumental observations of historical
sea-level changes in the global ocean and for the RSL in the
Indian Ocean are presented here. The chapter then describes
a range of scenarios for future levels and rates of sea-level
change, for the Indian Ocean as well as for the global ocean.
Finally, an assessment of the impact of changes in sea level
on extreme water levels is discussed.
9.2 Physical Factors Contributing
to Sea-Level Rise
Sea level is measured either with respect to the surface of the
solid Earth, known as relative sea level (RSL) or a geocentric reference such as the reference ellipsoid, known as
the geocentric sea level. RSL estimates have been obtained
from tide gauges and geological records for the past few
centuries. Geocentric sea level has been measured over the
past two decades using satellite altimetry. The sea level,
176
P. Swapna et al.