© Yann Arthus-Bertrand / Altitude

Sea Level ChangeSea Level ChangeMark A. Merrifield

Lead Author, AR5, Chapter 13

Third Order Draft Chapter 13 IPCC WGI Fifth Assessment Report

Chapter 13: Sea Level Change

Coordinating Lead Authors: John A. Church (Australia), Peter U. Clark (USA) Lead Authors: Anny Cazenave (France), Jonathan M. Gregory (UK), Svetlana Jevrejeva (UK), Anders Levermann (Germany), Mark A. Merrifield (USA), Glenn A. Milne (Canada), R. Steven Nerem (USA), Patrick D. Nunn (Australia), Antony J. Payne (UK), W. Tad Pfeffer (USA), Detlef Stammer (Germany), Alakkat S. Unnikrishnan (India) Contributing Authors: David Bahr (USA), Jason E. Box (USA), David H. Bromwich (USA), Mark Carson (Germany), William Collins (UK), Xavier Fettweis (Belgium), Piers Forster (UK), Alex Gardner (USA), W. Roland Gehrels (UK), Rianne Giesen (Netherlands), Peter J. Gleckler (USA), Peter Good (UK), Rune Grand Graversen (Sweden), Ralf Greve (Japan), Stephen Griffies (USA), Edward Hanna (UK), Mark Hemer (Australia), Regine Hock (USA), Simon J. Holgate (UK), John Hunter (Australia), Philippe Huybrechts (Belgium), Gregory Johnson (USA), Ian Joughin (USA), Georg Kaser (Austria), Caroline Katsman (Netherlands), Leonard Konikow (USA), Gerhard Krinner (France), Anne Le Brocq (UK), Jan Lenaerts (Netherlands), Stefan Ligtenberg (Netherlands), Christopher M. Little (USA), Ben Marzeion (Austria), Kathleen L. McInnes (Australia), Sebastian H. Mernild (USA), Didier Monselesan (Australia), Ruth Mottram (Denmark), Tavi Murray (UK), Gunnar Myhre (Norway), J.P. Nicholas (USA), Faezeh Nick (Norway), David Pollard (USA),Valentina Radi (Canada), Jamie Rae (UK), Markku Rummukainen (Sweden), Christian Schoof (Canada), Aimée Slangen (Netherlands), Jan H. van Angelen (Netherlands), Willem Jan van de Berg (Netherlands), Michiel van den Broeke (Netherlands), Miren Vizcaíno (Netherlands), Yoshihide Wada (Netherlands), Neil J. White (Australia), R.Winkelmann (Germany),!Jianjun Yin (USA), Masakazu Yoshimori (Japan), Kirsten Zickfeld (Canada) Review Editors: Jean Jouzel (France), Roderik van de Wal (Netherlands), Philip L. Woodworth (UK), Cunde Xiao (China)

AR5 advancements:• incorporation of ice dynamics in projections• assessment of regional sea level change

Fig. 13.1

Fig. 13.1

Global mean sea level (GMSL) – the instrumental record Tide gaugesPaleo recordsSatellite altimetry

Between 1993 and 2010, the rate was very likely higher at 3.2 [2.8 3.6] mm yr–1.

It is virtually certain that the rate of global mean sea level rise has increased from the 19th to the 20th century.

It is very likely that the mean rate was 1.7 [1.5 1.9] mm yr–1

between 1901 and 2010 for a total sea level rise of 0.19 [0.17 0.21] m.

Fig. 13.3e

Text from the report highlighted in blue.

Contributions to global mean sea level rise (GMSLR)

Observations since 1971 indicate that thermal expansion and glaciers (excluding the glaciers in Antarctica) explain 75% of the observed rise (high confidence).

The contribution of the Greenland and Antarctic ice sheets has increased since the early 1990s, partly from increased outflow induced by warming of the immediately adjacent ocean.

Since 1993, when observations of all sea level components are available, the sum of contributions equals the observed global mean sea level rise within uncertainties (high confidence).

Contributions to global mean sea level rise (GMSLR)

Since 1993, when observations of all sea level components are available, the sum of contributions equals the observed global mean sea level rise within uncertainties (high confidence).

5 7 9

11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59 61 63

1993-2010

Data from Tab. 13.1 Fig. 13.6

Models of global mean sea level rise (GMSLR)

Process-based model calculations of contributions to past sea level change from ocean thermal expansion, glacier mass loss and Greenland ice-sheet surface mass balance are consistent with available observational estimates over recent decades.

Fig. 13.7a,b

Process-based model projections of global mean sea level

The process-based projections of GMSL rise for each RCP scenario are based on results from 21 CMIP5 AOGCMs from which projections of surface air temperature (SAT) change and thermal expansion are available

68

10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58

Fig. 13.8

Process-based model projections of global mean sea level

Changes in glacier and ice-sheet SMB are calculated from the global mean SAT projections using parameterisations derived from the results of process-based models of these components.

Possible ice-sheet dynamical changes by 2100 are assessed from the published literature, which as yet provides only a partial basis for making projections related to particular scenarios; therefore they are treated as independent of scenario, except that a higher rate of change is used for Greenland ice-sheet outflow under RCP8.5.

FAQ13.2, Fig. 1

ResultsProcess-based model projections of global mean sea level

Fig. 13.10

ResultsProcess-based model projections of global mean sea level

For the period 2081 to 2100, compared to 1986 to 2005, global mean sea level rise is likely (medium confidence) to be in the 5-95% range of process-based models, which give 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.

Rapid increase in ice sheet outflow

Accelerated outflow

Based on current understanding, only the collapse of marine-based sectors of the Antarctic Ice Sheet, if initiated, could cause global mean sea level to rise substantially above the likely range during the 21st century.

Grounded ice sheet

Floating ice shelf

Box 13.2

Bedrock

Icebergs

This potential additional contribution cannot be precisely quantified but there is medium confidence that it would not exceed more than several tenths of a meter of sea level rise during the 21st century.

GM

SL

rise

(m) i

n 20

81-2

100

rela

tive

to 1

986-

2005

Despite their successful calibration and evaluation against the observed 20th century sea level record, there is low agreement in their projections and no consensus in the scientific community about the reliability of SEM projections, and there is low confidence in their projections

Semi-empirical model projections of global mean sea level

RCP8.5

Like

lyra

nge

The colours indicate different types of RCP-derived input data

Process-based projection

Fig. 13.12d

Projections of regional sea level change

Glaciers

Ice-sheet SMB0.0

-0.4

0.4

m

0.8

RCP4.5

2081-2100 relative to 1986-2000Regional sea level changes result from ocean dynamical processes, movements of the sea floor, and changes in gravity due to water mass-redistribution (land ice and other terrestrial water storage).

About 70% of the coastlines worldwide are projected to experience sea level change within 20% of the global mean sea level change.

Frac

tion

of to

tal c

oast

line

excl

udin

gG

reen

land

and

Ant

arct

ica

Fig. 13.20b

Fig. 13.22b

Sea level projections beyond 2100It is virtually certain that global mean sea level rise will continue beyond 2100, with sea level rise due to thermal expansion to continue for many centuries. Longer term sea level rise depends on future emissions.

The few available process-based models indicate global mean sea level rise by 2300 to be less than 1 m for greenhouse gas concentrations that peak and decline and do not exceed 500 ppm CO2-equivalent but 1–3 m for concentrations above 700 ppm CO2-equivalent (medium confidence).

The available evidence indicates that global warming greater than a certain threshold would lead to the near-complete loss of the Greenland Ice Sheet over a millennium or more, causing a global mean sea level rise of about 7 m. Studies with fixed ice sheet topography indicate the threshold is greater than 2 °C but less than 4 °C of global mean surface temperature rise with respect to preindustrial.

Fig. SPM.7a

Extreme sea level projections

It is very likely that there will be a significant increase in the occurrence of future sea level extremes by 2050 and 2100. This increase will primarily be the result of an increase in mean sea level (high confidence), with the frequency of a particular sea level extreme increasing by an order of magnitude or more in some regions by the end of the 21st century.

The estimated multiplication factor (shown at tide gauge locations by red circles and triangles), by which the frequency of flooding events of a given height increase using regional projections of MSL for the RCP 4.5 scenario.

Fig. 13.25b

Wind wave projections

In general, there is low confidence in region-specific wind wave projections due to the low confidencein tropical and extratropical storm projections, and to the challenge of downscaling future wind fields from coarse-resolution climate models.

Fig. 13.26a-c

~2075-2100 compared with ~1980-2009

Percentage difference in a) annual, b) January-March, and c) July-September mean significant wave height.

0

1

2

3

Feet

abo

ve p

rese

nt s

ea le

vel

Fig. 13.27

SummaryImproved observations of recent sea level change –models show reasonable agreement

21st century projections include assessment of ice sheet dynamics

Levels above likely range possible if marine-based sectors of Antarctic Ice Sheet collapse – likely will add no more than several tenths of a meter during 21st century

Regional sea level projections

Sea level assessments beyond 2100