Climate change impacts and vulnerability in Europe 2016

Climate change impacts on environmental systems
Projections: global mean sea level
Currently, there are two main approaches to projecting
future sea level. Process-based models represent the
most important known physical processes explicitly,
whereas empirical statistical models look at the
relationship between temperature (or radiative forcing)
and sea level that has been observed in the past and
extrapolate it into the future. A significant recent step
forwards in projecting future sea levels is the improved
understanding of the contributing factors to recently
observed sea level rise, which has increased confidence
in the use of process-based models for projecting the
future (Church et al., 2013).
The IPCC AR5 concludes that the rate of GMSL rise
during the 21st century will very likely exceed the rate
observed in the period 1971–2010 for all emissions
scenarios. Process-based models estimate the rise
in GMSL for the period 2081–2100, compared with
1986–2005, to be likely in the range of 0.26–0.54 m
for RCP2.6, 0.32–0.62 m for RCP4.5, 0.33–0.62 m for
RCP6.0 and 0.45–0.81 m for RCP8.5. For RCP8.5, the
rise in GMSL by 2100 is projected to be in the range
of 0.53–0.97 m, with a rate during 2081–2100 of
7–15 mm/year. Based on current understanding, only
a collapse of marine‐based sectors of the Antarctic Ice
Sheet (i.e. those areas where the bed lies well below
Figure 4.8
Projections of global mean sea level
rise and its contributing factors
sea level and the edges flow into floating ice shelves)
could cause GMSL to rise substantially (by up to several
tenths of a metre) above the likely range projected for
the 21st century, but the evidence is insufficient for
estimating the likelihood of such a collapse (Figure 4.8).
Projections of sea level rise from recent empirical
statistical models calibrated with newly available sea
level reconstructions align with those from current
process-based models (Kopp et al., 2016; Mengel et al.,
2016).
Some recent studies highlight the contributions to
recent sea level rise from ice sheet melting that were
not included in the process-based models underlying
the AR5 estimates above (e.g. Khan et al., 2014;
DeConto and Pollard, 2016; Fürst et al., 2016; Hansen
et al., 2016) (see also Section 3.3.3). A broad expert
assessment of future sea level rise resulted in higher
estimates than those from the process-based models
reviewed in the AR5. The best expert estimates for sea
level rise during the 21st century were 0.4–0.6 m for the
low forcing scenario (RCP2.6) and 0.7–1.2 m for the high
forcing scenario (RCP8.5) (Horton et al., 2014). Similar
results were derived in other recent studies (Kopp et al.,
2014; Johansson et al., 2014). A study that combined
the model-based assessments from the AR5 with
updated models and probabilistic expert assessment
of the sea level contributions from the Greenland and
Antarctic ice sheets suggests an upper limit for GMSL
rise during the 21st century of 180 cm, which has a 5 %
probability of being exceeded under the RCP8.5 forcing
scenario (Jevrejeva, Grinsted et al., 2014). Various
national reports have also used values in the range of
1.5–2.0 m as upper estimates for GMSL rise during the
21st century. One recent study suggests that ice sheet
melting in Antarctica and Greenland could occur much
faster than previously assumed, which would result in
sea level rise of several metres during the 21st century
(Hansen et al., 2016), but these findings have been
controversial. These high-end scenarios are somewhat
speculative, but their consideration is nevertheless
important for long‐term coastal risk management, in
particular in densely populated coastal zones (Hinkel
et al., 2015).
Note:
Source:
This figure shows projections for global mean sea level
rise and its contributing factors in 2081–2100 relative to
1986–2005 from process-based models for the four RCPs
and the emissions scenario SRES A1B used in the IPCC AR4.
The grey boxes show the median of the model projections
(central bar) and the likely range, which comprises twothirds
of the model projections. The coloured bars and
boxes show estimates for the different contributions to
global mean sea level rise. For further information, see the
source document.
Church et al., 2013 (Figure 13.10). @ 2013 Intergovernmental
Panel on Climate Change. Reproduced with permission.
Sea level rise will continue far beyond 2100. Based
on a limited number of available simulations with
process-based models, the AR5 suggests that the
GMSL rise by 2300 will be less than 1 m for greenhouse
gas concentrations that do not exceed 500 ppm
CO 2 ‐equivalent, but will be in the range of 1 m to
more than 3 m for concentrations above 700 ppm
CO 2 ‐equivalent. However, the AR5 also considered
it likely that the underlying models systematically
underestimate Antarctica's future contribution to sea
level rise (Jevrejeva et al., 2012; Schaeffer et al., 2012;
Church et al., 2013). Estimates for GMSL rise beyond
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