Longer-term projections opposite is true. At this stage there is no agreed-upon
pattern for the longer-term regional distribution of pro-
For the next few decades, the rate of sea-level rise jected sea-level rise. There are, however, several features
is partly locked in by past emissions, and will not be that are common to most model projections – for exam-
strongly dependent on 21st century greenhouse gas ple a maximum in sea-level rise in the Arctic Ocean and
emission. However, sea-level projections closer to and a minimum sea-level rise in the Southern Ocean south
beyond 2100 are critically dependent on future green- of the Antarctic Circumpolar Current
34
.
house gas emissions, with both ocean thermal expan-
sion and the ice sheets potentially contributing metres In addition, past and ongoing transfers of mass from
of sea level rise over centuries for higher greenhouse the ice sheets to the oceans result in changes in the
gas emissions. gravitational field and vertical land movements and thus
changes in the height of the ocean relative to the land
35–37
.
For example, in the case of the Greenland Ice Sheet, These large-scale changes, plus local tectonic move-
if global average temperatures cross a point that is ments, affect the regional impact of sea-level rise.
estimated to be in the range of 1.9°C to 4.6°C above
pre-industrial values
32
, this will lead to surface melting Withdrawal of groundwater and drainage of suscepti-
exceeding precipitation. The inevitable consequence ble soils can cause significant subsidence. Subsidence
of this is an ongoing shrinking of the Greenland Ice of several metres during the 20th century has been ob-
Sheet over a period of centuries and millennia
15
. This served for a number of coastal megacities
38
. Reduced
conclusion is consistent with the observation that glo- sediment inputs to deltas are an additional factor which
bal sea level in the last interglacial, when temperatures causes loss of land elevation relative to sea level
39
.
were in this range, was several metres higher than it is
today. This threshold (of melting exceeding precipita-
tion) could potentially be crossed late in the 21st cen- Extreme events
tury. In addition, dynamic responses of the Greenland
and West Antarctic Ice Sheets could lead to a signifi- Sea-level rise will be felt both through changes in mean
cantly more rapid rate of sea-level rise than from sur- sea level, and, perhaps more importantly, through
face melting alone. changes in extreme sea-level events. Even if there are
no changes in extreme weather conditions (for example,
increases in tropical cyclone intensity), sea-level rise will
Regional patterns of sea-level rise result in extreme sea levels of a given value being ex-
ceeded more frequently.
For the period 1993 to the present, there is a clear pat-
tern of regional distribution of sea-level change that is This change in the frequency of extreme events has al-
also reflected in patterns of ocean heat storage
33
. This ready been observed at many locations
40–43
(Figure 6C.7).
pattern primarily reflects interannual climate variabil- The increase in frequency of extreme events will depend
ity associated with the El Niño/La Niña cycle. During on local conditions, but events that currently occur once
El Niño years sea level rises in the eastern Pacific and every 100 years could occur as frequently as once every
falls in the western Pacific whereas in La Niña years, the few years by 2100.
CHAPTER 6C ICE AND SEA-LEVEL CHANGE 163
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