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7.3.1 Past Sea Level Rise and Twenty-First Century
Projections
The International Panel on Climate Change fifth Assessment Report (IPCC 5AR)
mentions how surface temperatures have oscillated for millions of years following
glacial cycles. This in turn has influenced sea levels, which have risen and fallen
according to such variations in temperature (because of thermal expansion of the
oceans and the melting or accumulation of water in polar caps). During most of the
twentieth century the global mean sea level rose by around 1.7 mm per year on average, though this intensified to 3 mm per year towards the end of the century (IPCC
5AR). The IPCC 5AR estimates that sea levels could rise by between 26 and 82 cm
by 2100, substantially higher than the 18–59 cm projections that had been given by
IPCC 4AR. So-called “semi-empirical methods” (see IPCC 5AR) such as those by
Vermeer and Rahmstorf (2009) provide more onerous predictions, indicating sea
level rise for 1990–2100 could be in the 0.75–1.9 m range.
There is little doubt that climate change – and consequently sea level rise, as it is
greatly affected by global temperatures – is mostly being driven by the release of
greenshouse gases into the atmosphere. Current world efforts to reduce greenhouse
gas emissions, centred around the United Nations Framework Convention for
Climate Change (UNFCCC), have not yet convincingly managed to halt their
increase, despite the signing of the Paris Agreement in 2015. However, even if emissions were to reduce, the IPCC 4AR points how “if actions are taken to reduce the
emissions, the fate of the trace gas concentrations will depend on the relative
changes not only of emissions but also of its removal processes” (Bindoff et al.
2007). This means that it could potentially take a very long time for the Earth to
revert to its current condition. As CO 2 emissions continue unabated, global temperatures will inevitably continue to rise unless drastic action is taken to curtail them.
Such effects can very well lead to the flooding of low-lying deltaic areas such as the
Mekong delta (see Nguyen et al. 2013; Takagi et al. 2014; Nobuoka and Murakami
2011) or atoll islands (Yamamoto and Esteban 2014), unless significant adaptation
measures are implemented.
The IPCC 5AR discusses the long-term climate change and commitment up to
the year 2500. Essentially, if greenhouse gas concentrations rise to between 500 and
700 ppm CO 2 , sea level rise could exceed 1.5 m by the year 2300; or if concentrations were to exceed 700 ppm CO 2 sea level rise could surpass 3 m by 2300, reaching almost 7 m by 2500. Essentially, the most optimistic scenarios related to sea
level rise require a positive outcome of UNFCCC efforts and negotiations. The
entire Earth climate system, however, exhibits a certain lag, due to the thermal inertia of the oceans. The oceans will gradually absorb heat from the atmosphere, and
this will lead to the heating of the top layers, gradually extending deeper into the
ocean. Even when air temperatures stop increasing, the heat absorbed by the oceans
7 Time-Scale in Framing Disaster Risk Reduction in Sustainability
7.3.1 Past Sea Level Rise and Twenty-First Century
Projections
The International Panel on Climate Change fifth Assessment Report (IPCC 5AR)
mentions how surface temperatures have oscillated for millions of years following
glacial cycles. This in turn has influenced sea levels, which have risen and fallen
according to such variations in temperature (because of thermal expansion of the
oceans and the melting or accumulation of water in polar caps). During most of the
twentieth century the global mean sea level rose by around 1.7 mm per year on average, though this intensified to 3 mm per year towards the end of the century (IPCC
5AR). The IPCC 5AR estimates that sea levels could rise by between 26 and 82 cm
by 2100, substantially higher than the 18–59 cm projections that had been given by
IPCC 4AR. So-called “semi-empirical methods” (see IPCC 5AR) such as those by
Vermeer and Rahmstorf (2009) provide more onerous predictions, indicating sea
level rise for 1990–2100 could be in the 0.75–1.9 m range.
There is little doubt that climate change – and consequently sea level rise, as it is
greatly affected by global temperatures – is mostly being driven by the release of
greenshouse gases into the atmosphere. Current world efforts to reduce greenhouse
gas emissions, centred around the United Nations Framework Convention for
Climate Change (UNFCCC), have not yet convincingly managed to halt their
increase, despite the signing of the Paris Agreement in 2015. However, even if emissions were to reduce, the IPCC 4AR points how “if actions are taken to reduce the
emissions, the fate of the trace gas concentrations will depend on the relative
changes not only of emissions but also of its removal processes” (Bindoff et al.
2007). This means that it could potentially take a very long time for the Earth to
revert to its current condition. As CO 2 emissions continue unabated, global temperatures will inevitably continue to rise unless drastic action is taken to curtail them.
Such effects can very well lead to the flooding of low-lying deltaic areas such as the
Mekong delta (see Nguyen et al. 2013; Takagi et al. 2014; Nobuoka and Murakami
2011) or atoll islands (Yamamoto and Esteban 2014), unless significant adaptation
measures are implemented.
The IPCC 5AR discusses the long-term climate change and commitment up to
the year 2500. Essentially, if greenhouse gas concentrations rise to between 500 and
700 ppm CO 2 , sea level rise could exceed 1.5 m by the year 2300; or if concentrations were to exceed 700 ppm CO 2 sea level rise could surpass 3 m by 2300, reaching almost 7 m by 2500. Essentially, the most optimistic scenarios related to sea
level rise require a positive outcome of UNFCCC efforts and negotiations. The
entire Earth climate system, however, exhibits a certain lag, due to the thermal inertia of the oceans. The oceans will gradually absorb heat from the atmosphere, and
this will lead to the heating of the top layers, gradually extending deeper into the
ocean. Even when air temperatures stop increasing, the heat absorbed by the oceans
7 Time-Scale in Framing Disaster Risk Reduction in Sustainability