Climate change impacts and vulnerability in Europe 2016

Changes in the climate system
the ice sheet dynamically suggests that the threshold
could be as low as about 1 °C above pre-industrial
levels (Robinson et al., 2012). The complete loss of the
Greenland ice sheet is not inevitable because it has
a long time scale. Complete melting would take tens
of millennia if near the threshold and a millennium
or more for temperatures a few degrees above the
threshold (Robinson et al., 2012; Church et al., 2013;
Applegate et al., 2015).
The uncertainties around future ice discharge from
Antarctica, and the associated sea level rise, are
larger than for Greenland. However, mass loss of the
Antarctic ice sheet has a greater impact on the sea
level in the northern hemisphere than a comparable
loss of the Greenland ice sheet, owing to gravitational
forces. A comprehensive analysis applying various
climate, ocean and ice sheet models estimates that
the additional ice loss for the 21st century is 7 cm
(90 % range: 0–23 cm) of global sea level equivalent
for a low emissions scenario (RCP2.6) and 9 cm (90 %
range: 1–37 cm) for a high emissions scenario (RCP8.5)
(Levermann et al., 2014). By 2100, the rise of global sea
level will be clearly influenced by the development of
the Antarctic ice sheet. A recent study suggests that the
Antarctic ice sheet has the potential to contribute more
than a metre to sea level rise by 2100 and more than
15 metres by 2500, if emissions continue unabated
(DeConto and Pollard, 2016).
Several studies that were published after the release
of the IPCC AR5 suggest that melting of the West
Antarctic Ice Sheet (WAIS) has been accelerating
recently and that a WAIS collapse is already inevitable
and irreversible. There are also indications of instability
in some parts of the much larger East Antarctic Ice
Sheet. These new results suggest that the global mean
sea level contribution from Antarctica alone could be
several metres on a time scale of a few centuries to
a millennium (Favier et al., 2014; Gunter et al., 2014;
Joughin et al., 2014; McMillan et al., 2014; Mengel and
Levermann, 2014; Mouginot et al., 2014; Rignot et al.,
2014; Holland et al., 2015).
The long-term development of the ice sheets is hugely
important in determining the consequences of climate
change. Amplifying feedback mechanisms, including
slowdown of meridional overturning circulation, may
accelerate ice sheet mass loss (Hansen et al., 2015).
A coupled ice sheet–ice shelf model suggests that,
if atmospheric warming exceeds 1.5 to 2 °C above
present, the major Antarctic ice shelves would collapse,
which would trigger a centennial- to millennial-scale
response of the Antarctic ice sheet and cause an
unstoppable contribution to sea level rise (Golledge
et al., 2015). Although current estimates of sea level
rise by 2100 suggest that they will fall in a range of
some tens of centimetres (Clark et al., 2015), collapsing
ice sheets could, in the long term, result in a faster
and greater rise in sea level than currently assumed,
underlining the urgency of climate change mitigation
(EASAC, 2015; Golledge et al., 2015; Hansen et al.,
2015). The uncertainties in the long-term projections
are significant, however. Assumptions concerning,
for example, bedrock uplift and sea surface drop
associated with ice sheet retreat have a significant
effect on the results with respect to sea level rise
(Gomez et al., 2015; Konrad et al., 2015).
98 Climate change, impacts and vulnerability in Europe 2016 | An indicator-based report