Climate change impacts and vulnerability in Europe 2016

Climate change impacts on environmental systems
4.2.2 Global and European sea level
Key messages
• Global mean sea level has risen by 19.5 cm from 1901 to 2015, at an average rate of 1.7 mm/year, but with significant
decadal variation. The rate of sea level rise since 1993, when satellite measurements have been available, has been
higher, at around 3 mm/year. Global mean sea level in 2015 was the highest yearly average over the record and ~ 70 mm
higher than in 1993.
• Evidence for a predominant role of anthropogenic climate change in the observed global mean sea level rise and for an
acceleration during recent decades has strengthened since the publication of the IPCC AR5.
• Most coastal regions in Europe have experienced an increase in absolute sea level and in sea level relative to land, but
there is significant regional variation.
• Extreme high coastal water levels have increased at most locations along the European coastline. This increase appears to
be predominantly due to increases in mean local sea level rather than to changes in storm activity.
• Global mean sea level rise during the 21st century will very likely occur at a higher rate than during the period 1971–2010.
Process-based models considered in the IPCC AR5 project a rise in sea level over the 21st century that is likely in the range
of 0.26–0.54 m for a low emissions scenario (RCP2.6) and 0.45–0.81 m for a high emissions scenario (RCP8.5). However,
several recent studies suggest substantially higher values. Several national assessments, expert assessments and recent
model‐based studies have suggested an upper bound for 21st century global mean sea level rise in the range of 1.5–2.0 m.
• Available process-based models project that global mean sea level rise by 2300 will be less than 1 m for greenhouse
gas concentrations that peak and decline and 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, these models are likely to systematically
underestimate the sea level contribution from Antarctica, and some recent studies suggest substantially higher rates of
sea level rise in the coming centuries.
• The rise in sea level relative to land along most European coasts is projected to be similar to the global average, with the
exception of the northern Baltic Sea and the northern Atlantic Coast, which are experiencing considerable land rise as a
consequence of post-glacial rebound.
• Projected increases in extreme high coastal water levels are likely to mostly be the result of increases in local relative
mean sea level in most locations. However, recent studies suggest that increases in the meteorologically driven surge
component can also play a substantial role, in particular along the northern European coastline.
Relevance
Sea level is an important indicator of climate
change because it can have significant impacts on
settlements, infrastructure, people and natural
systems. It acts on time scales much longer than those
of indicators that are closely related to near‐surface
temperature change. Even if greenhouse gas
concentrations were stabilised immediately, sea level
would continue to rise for many centuries (IPCC, 2013,
2014).
Changes in global mean sea level (GMSL) result from
a combination of several physical processes. Thermal
expansion of the oceans occurs as a result of warming
ocean water. Additional water is added to the ocean
from a net melting of glaciers and small ice caps, and
from the disintegration of the large Greenland and
Antarctic ice sheets. Further contributions may come
from changes in the storage of liquid water on land,
in either natural reservoirs such as groundwater or
man‐made reservoirs.
The locally experienced changes in sea level differ
from global average changes for various reasons
(Church et al., 2013, FAQ 13.1). First, changes in water
density are not expected to be spatially uniform,
and the spatial pattern also depends on changes
in large-scale ocean circulation. Second, changes in
the gravity field, for instance as water moves from
melting ice on land to the ocean, also varies across
regions. Finally, at any particular location, there may
be a vertical movement of the land in either direction,
for example due to the ongoing effects of post-glacial
rebound (also known as glacial isostatic adjustment),
which is particularly strong in northern Europe, to
124 Climate change, impacts and vulnerability in Europe 2016 | An indicator-based report