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Methods for Modeling Sea-level Rise in a 4°C World<br />
The authors developed sea-level scenarios using a combination of approaches, acknowledging the fact that both physicallybased<br />
numerical ice sheet modeling and semi-empirical methods have shortcomings, but also recognizing the need to provide<br />
ice sheet loss estimates to be able to estimate regional sea-level rise. They did not attempt to characterize the full range of<br />
uncertainties, either at the low or high end. Future contributions from groundwater mining are also not included in the projections,<br />
and could account for another 10 cm (Wada et al. 2012). The scenario construction is as follows.<br />
For the upper end of the sea-level scenario construction, the<br />
authors apply a semi-empirical sea-level rise model (Rahmstorf,<br />
Perrette, and Vermeer 2011; Schaeffer et al. 2012), giving a global<br />
estimate for specific emission scenarios leading to a 2°C or 4°C<br />
increase in global mean temperature by 2100. As the semi-empirical<br />
sea-level rise models do not separately calculate the individual<br />
terms giving rise to sea-level increases, further steps are needed<br />
to characterize plausible ice sheet contributions. The authors<br />
calculate the contribution from thermal sea-level rise and from<br />
mountain glaciers and icecaps and deduct this from the total<br />
global sea-level rise and assign this difference to the ice sheets,<br />
half to Greenland and the other half to Antarctica. The resulting<br />
contributions from the ice sheets are significantly above those<br />
estimated by most process based ice sheet models and approximates<br />
the ice sheet contribution that would arise, if the rates of<br />
acceleration of loss observed since 1992 continued unchanged<br />
throughout the 21st century.<br />
For the lower end of the scenario construction, the authors use<br />
as a starting point the calculated thermal sea level-rise and the<br />
contribution from mountain glaciers and ice caps. To this, they add<br />
a surface mass balance contribution from the Greenland ice sheet<br />
(GIS; excluding ice dynamics) and assume that the Antarctic ice<br />
sheet (AIS) is in balance over the 21st century. Most AIS models<br />
project that this ice sheet would lower sea-level rise in the 21st<br />
century as it does not warm sufficiently to lose more ice than it<br />
gains because of enhanced precipitation over this period. On the<br />
other hand, observations indicate that the ice sheet is losing ice<br />
at a slowly increasing rate close to that of the Greenland ice sheet<br />
at present. Setting the AIS contribution to zero is, thus, a way of<br />
leaving open the possibility that short-term processes may have<br />
been at work over the last 20 years. This very low ice sheet contribution<br />
scenario approaches the levels of some process-based model<br />
projections, where the projected net uptake of ice by Antarctica<br />
is balanced by ice melting from Greenland over the 21st century.<br />
In the lower ice-sheet scenario (47 cm sea-level rise in the<br />
global mean), eastern Asian and northeastern American coasts<br />
both experience above-average sea-level rise, about 20 percent and<br />
15 percent, respectively above the global mean (for example, –3<br />
percent to +23 percent around New York City, 68 percent range).<br />
In the higher ice-sheet scenario (96 cm sea-level rise in the global<br />
mean), where ocean dynamic effects are relatively less significant,<br />
the eastern Asian coast clearly stands out as featuring the highest<br />
projected coastal sea-level rise of 20 percent above the global mean.<br />
In that scenario, sea-level rise is projected to be slightly below the<br />
global mean in northeast America, and 20 percent (5–33 percent,<br />
68 percent range) below the global mean along the Dutch coast<br />
(Figure A1.1, Figure 32). It is important to note the likely weakening<br />
in the Atlantic Meridional Overturning Circulation (AMOC)<br />
with increasing warming could be exacerbated by rapid ice sheet<br />
melt from Greenland. That effect, which is not included in the<br />
authors’ projections, could potentially add another 10 cm to the<br />
local sea-level rise around New York City, as currently discussed<br />
in the scientific literature (Sallenger et al. 2012; Slangen et al. 2011;<br />
Stammer, Agarwal, Herrmann, Köhl and Mechoso 2011; Yin et al.<br />
2009). Post-glacial adjustment would also add another 20 cm,<br />
albeit with large uncertainties (Slangen et al. 2011).<br />
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