BAMOS Vol 31 Special Issue October 2018 Bulletin Vol 31 Special Issue 01 2018 | Page 20

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BAMOS
Special Issue
Achievements and Challenges in
Understanding Contemporary Sea-
level Change
John A. Church
Climate Change Research Centre, University of New South Wales
Today, tens of millions of people live within a metre of high tide
level, and more people are moving towards the coast in both
the developed and developing world. As a result, sea‐level rise
has the potential to have major impacts.
Historical sea-level change
Sea level has waxed and waned by over 100 m during the ice
age cycles of the last million years. Satellite altimeter missions in
combination with global in situ observations indicate the rate of
rise was between 1.0 and 1.7 mm/yr from 1900 to 1990, an order
of magnitude larger than rates over corresponding periods over
the previous two millennia when our coastal society developed.
The acceleration of sea-level rise, now also seen in the modern
satellite record since 1993, has resulted in a rate of rise over the
last decade of well over 3 mm/yr.
Understanding 20th Century sea-level rise
The major contributions to 20th century sea‐level rise were
ocean thermal expansion and the loss of mass from glaciers.
Improved upper ocean temperature observations have resulted
in improved agreement with model simulations of both the
increase in global ocean heat content (and thermal expansion)
and its variability associated with volcanic eruptions. However,
the deep ocean, continental shelves and the polar regions are
inadequately observed leading to ongoing uncertainties in
simulations and projections with global implications. Estimates
of glacier contributions since the late 19th century have
converged and comparison of observations and simulations
indicate a significant anthropogenic component in the latter
half of the 20th century.
Increased surface melting in Greenland and increased glacier
discharge in both Greenland and Antarctica have resulted in
increased (and accelerating) ice sheet contributions since 1993
when satellite observations became available. There were also
significant contributions from Greenland in the early half of the
20th century but little is known of the Antarctic contributions
over this period. As yet there are no realistic model simulations
of the total ice-sheet contributions for the 20th century as a
whole.
The storage of water in terrestrial reservoirs and the depletion
of ground water in aquifers also contributed to sea-level change
during the 20th century. Natural climate variations contribute
to interannual variations of sea level.
It is now possible to close the global mean sea-level budget using
observations since 1960 and a combination of observations and
models since 1900. These contributions explain a significant
portion of the regional and decadal variability at coastal tide
gauges around the world (Meyssignac et al. 2017, Figure 1,
shown on the next page). An implication of our improved
understanding is that anthropogenic climate forcing was
responsible for the dominant contribution to sea-level rise since
1970 (Slangen et al. 2016).
Projections of Future Sea-level Change and
Future Challenges
The IPCC AR5 projections (Church et al. 2013) of the likely (66%
probability) global mean sea-level rise in 2100 (compared
to 1996) range from 0.28–0.61 m for a strong greenhouse
gas mitigation (RCP2.6) to 0.53–0.98 m for business-as-usual
emissions (RCP8.5). Sea-level rise will continue for many
centuries after 2100 and for unmitigated emissions would be at
least several metres.
While there has been significant progress, significant and
important challenges remain. Critically important is the
changing structure of the oceans and the role of the oceans in
the future of the ice sheets of Antarctica and Greenland. The
AR5 indicated the possibility of an additional several tenths
of a metre rise from Antarctica by 2100. A number of studies
since the AR5 indicate a potential Antarctic contribution near
the lower end of this additional allowance. One recent study
(DeConto and Pollard, 2016) indicates an Antarctic contribution
of up to about 1m for RCP8.5 but only a small contribution for
RCP2.6, highlighting the value of strong mitigation. Narrowing
the uncertainty of the Antarctic contribution is a pressing
challenge. Another is narrowing the warming threshold leading
to a virtually complete melting of the Greenland Ice Sheet.
Other challenges include improved attribution of sea-level
rise (and ocean warming) by more accurately quantifying the
separate contributions of greenhouse gases and aerosols,
thus allowing constrained projections. Finer resolution climate
models are required for accurately assessing ocean-atmosphere-
ice sheet interactions and for evaluating the coastal impacts of
sea-level rise.