Heat

Focus: Sea-level Rise Projections
Projecting sea-level rise as a consequence of climate change is one of the most difficult, complex, and controversial scientific
problems. Process-based approaches dominate—i.e the use of numeric models that represent the physical processes at
play—and are usually used to project future climate changes such as air, temperature, and precipitation. In the case of Greenland
and Antarctic ice sheets however, uncertainties in the scientific understanding about the response to global warming lead
to less confidence in the application of ice sheet models to sea-level rise projections for the current century. On the other hand,
semi-empirical approaches, which have begun to be used in recent years and take into account the observed relation between
past sea level rise and global mean temperature to project future sea level rise, have their own limitations and challenges.
It is now understood that, in addition to global rise in sea levels,
a number of factors, such as the respective contribution of the ice
sheets or ocean dynamics, will affect what could happen in any
particular location. Making estimates of regional sea-level rise,
therefore, requires having to make estimates of the loss of ice on
Greenland and Antarctica and from mountain glaciers and ice caps.
Furthermore, there is at present an unquantifiable risk of
nonlinear responses from the West Antarctic Ice Sheet and possibly
from other components of Greenland and Antarctica. In the
1970s, Mercer hypothesized that global warming could trigger
the collapse of the West Antarctic Ice Sheet, which is separated
from the East Antarctic Ice Sheet by a mountain range. The West
Antarctic Ice Sheet is grounded mainly below sea level, with the
deepest points far inland, and has the potential to raise eustatic
global sea level by about 3.3 m (Bamber, Riva, Vermeersen, and
LeBrocq 2009). This estimate takes into account that the reverse
bedslope could trigger instability of the ice sheet, leading to an
unhalted retreat. Since the first discussion of a possible collapse of
the West Antarctic Ice Sheet because of this so-called Marine Ice
Sheet Instability (Weertman 1974) induced by global anthropogenic
greenhouse gas concentrations (Hughes 1973; Mercer 1968, 1978),
the question of if and how this might happen has been debated.
In their review of the issue in 2011, Joughin and Alley conclude
that the possibility of a collapse of the West Antarctic Ice Sheet
cannot be discarded, although it remains unclear how likely such
a collapse is and at what rate it would contribute to sea-level rise.
A range of approaches have been used to estimate the regional
consequences of projected sea-level rise with both a small and
a substantial ice sheet contribution over the 21st century (see
Appendix 1 and Table 2 for a summary).
Using a semi-empirical model indicates that scenarios that
approach 4°C warming by 2100 (2090–2099) lead to median estimates
of sea-level rise of nearly 1 m above 1980–1999 levels on this
time frame (Table 2). Several meters of further future sea-level rise
would very likely be committed to under these scenarios (Schaeffer
et al. 2012). In this scenario, as described in Appendix 1, the
Antarctic and Greenland Ice Sheets (AIS and GIS) contributions
to the total rise are assumed to be around 26 cm each over this
time period. Applying the lower ice-sheet scenario assumption,
the total rise is approximately 50 cm, the AIS and GIS contributions
to the total rise 0 and around 3 cm, respectively (Table 2).
Process-based modeling considerations at the very high end of
physically plausible ice-sheet melt, not used in this report, suggest
that sea-level rise of as much as 2 m by 2100 might be possible at
maximum (Pfeffer et al. 2008).
For a 2°C warming by 2100 (2090–99), the median estimate
of sea-level rise from the semi-empirical model is about 79 cm
above 1980–99 levels. In this case, the AIS and GIS contributions
to the total rise are assumed to be around 23 cm each. Applying
the lower ice-sheet scenario assumption, the median estimate of
the total rise is about 34 cm, with the AIS and GIS contributing
0 and around 2 cm respectively (Table 2).
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