Sea Level Rise in the Coastal Waters of Washington State
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Sea Level Rise in the Coastal Waters of Washington State A report by the University of Washington Climate Impacts Group and the Washington Department of Ecology Prepared by Philip Mote, Alexander Petersen, Spencer Reeder, Hugh Shipman, and Lara Whitely Binder January 2008
CONTENTS
1. Background................................................................................................................................ 4
2. Observed rates of global sea level rise....................................................................................... 4
3. Sea level rise projections ........................................................................................................... 4
3.1 Thermal expansion............................................................................................................... 4
3.2 Cryospheric contribution......................................................................................................5
3.3 Local atmospheric circulation.............................................................................................. 6
3.4 Local tectonic movement .....................................................................................................7
4. Synthesis: Summary and calculation of SLR projections ............................................................9
5. Unknowns and additional considerations................................................................................. 10
References ......................................................................................................................................11
The authors would like to thank the following persons who reviewed this report: Jim Simmonds, Supervi-
sor for Water Quality and Quantity, and Angela Grout, Oceanographer, for King County, Washington; Dr.
Ian Joughin, University of Washington Polar Science Center; Ray Weldon, Professor, University of Ore-
gon; and Susan Tonkin, Coastal Engineer, Moffatt & Nichol. In addition, we wish to thank Dr. Tim Mel-
bourne, Associate Professor, Andrew Miner, Researcher, and Marcelo Santillan, Scientific Programmer
and GPS Data Analyst, Central Washington University, for providing Figure 8.
Note: Units less than centimeters (cm) are not converted into English units. Centimeter conversions to
inches are rounded to the nearest whole unit for ease of reading.
This publication is partially funded by the Joint Institute for the Study of the Atmosphere and Ocean
(JISAO) under NOAA Cooperative Agreement No. NA17RJ1232, Contribution # 1474.
Photo: Whidbey Island near Fort Casey. Photo by Philip Mote.
2Summary. Local sea level rise (SLR) is produced by the Sea Level Rise Summary Figure:
combined effects of global sea level rise and local factors 50”
such as vertical land deformation (e.g., tectonic movement,
isostatic rebound) and seasonal ocean elevation changes due
to atmospheric circulation effects. In this document we re-
view available projections of these factors for the coastal wa- 40”
ters of Washington and provide low, medium, and high esti-
mates of local SLR for 2050 and 2100.
The fourth Assessment Report of the Intergovernmental Panel 30”
on Climate Change (IPCC) projects global SLR over the
course of this century to be between 18 and 38 cm (7-15”) for
their lowest emissions scenario, and between 26 and 59 cm 20”
(10-23”) for their highest emissions scenario. Based on the
current science, our “medium” estimate of 21st century
SLR in Washington is that in Puget Sound, local SLR will 13”
closely match global SLR. On the northwest Olympic 10”
Peninsula, very little relative SLR will be apparent due to 6”
rates of local tectonic uplift that currently exceed pro- 6”
jected rates of global SLR. On the central and southern 3”
Washington coast, the number of continuous monitoring sites
2050 2100
with sufficiently long data records is small, adding to the un-
certainty of SLR estimates for this region. Available data
points suggest, however, that uplift is occurring in this region, Projected sea level rise in Washington’s wa-
but at rates lower than that observed on the NW Olympic Penin- ters relative to 1980-99, in inches. Shading
roughly indicates likelihood.
sula.
The application of SLR estimates in decision making will depend on location, time frame, and risk toler-
ance. For decisions with long timelines and low risk tolerance, such as coastal development and public
infrastructure, users should consider low-probability high-impact estimates that take into account, among
other things, the potential for higher rates of SLR driven by recent observations of rapid ice loss in Green-
land and Antarctica, which though observed were not factored into the IPCC’s latest global SLR esti-
mates. Combining the IPCC high emissions scenario with 1) higher estimates of ice loss from Green-
land and Antarctica, 2) seasonal changes in atmospheric circulation in the Pacific, and 3) vertical
land deformation, a low-probability high-impact estimate of local SLR for the Puget Sound Basin is
55 cm (22”) by 2050 and 128 cm (50”) by 2100. Low-probability, high impact estimates are smaller for
the central and southern Washington coast (45 cm [18”] by 2050 and 108 cm [43”] by 2100), and even
lower for the NW Olympic Peninsula (35 cm [14”] by 2050 and 88 cm [35”] by 2100) due to tectonic up-
lift (see Table III). The authors intend to continue investigating the factors contributing to local SLR and
will provide updates to this report.
3Observations: Oceanic Climate Change and Sea Level Chapter 5
The TAR chapter on sea level change provided
1. Bac kground
estimates of climate and other anthropogenic
Sea level torise
contributions (SLR) is sea
20th-century increasingly being
level rise, based
considered
mostly onby private(Church
models and public et al.,entities
2001).when The
making
sum ofdecisions about the ranged
these contributions placement fromand –0.8pro-
to
tection
2.2 mmofyrstructures
–1, with a mean near value
shorelines.
of 0.7 The
mm yr Cli-
–1,
mate
and aImpacts
large partGroup (CIG) at University
of this uncertainty was due to the of
lack of information
Washington on anthropogenic
has recently received land water
inquiries
change.
from For observed
several 20th-century
municipalities, sea level rise,
consultants, and
based on tide gauge records,
private citizens concerning the likely Church et al.rates
(2001) of
adopted as a best estimate a value in the range of 1
SLR at specific locations in the
to 2 mm yr , which was more than twice as large as
–1 waters of
Washington State during the 21st century.
the TAR’s estimate of climate-related contributions. This
document is intended
It thus appeared to address
that either those questions
the processes causing
and
sea to provide
level rise hadguidance on the useorof
been underestimated theSLR
rate
of sea level
projections. rise observed with tide gauges was
biased towards higher values.
Since the TAR, a number of new results have
2. Observed rates of global sea level rise Figure 5.13. Annual averages of the global mean sea level (mm). The red curve shows reconstructed
been published. The global coverage of satellite
Global since
altimetry estimatesthe of SLR1990s
early (Figure 1) can be derived
(TOPography sea levelFigure 1. 1870
fields since Annual averages
(updated of the
from Church andglobal meanthe
White, 2006); seablue
level
curve(mm).
shows The
coastalred
tide
curve shows
gauge measurements reconstructed
since sea level
1950 (from Holgate fields since
and Woodworth, 2004)1870,
and thethe
blackblue
curvecurve
is based
EXperiment (TOPEX)/Poseidon and
by considering tide gauge records in combination with Jason) has on satellite altimetry (Leuliette
shows coastal tideetgauge
al., 2004). The red and bluesince
measurements curves1950,
are deviations
and the from their averages
black curve
improved
models or the estimate
actual of global seaoflevel
measurements rise and
Earth’s local tec-for 1961 to 1990, and the black curve is the deviation from the average of the red curve for the period
is based on satellite altimetry. Error bars show 90% confidence inter-
has revealed
tonic movement. the The
complexaveragegeographical
rate of globalpatterns 1993 to 2001. Error bars show 90% confidence intervals.
SLR for vals. Figure 5.13 from IPCC (2007).
of sea level change in open oceans. Near-global
1961-2003 is 1.8 ± 0.5 mm/yr (IPCC SPM, 2007).
ocean temperature data for the last 50 years have
Satellite altimetry measurements by the TOPEX/
been recently made available, allowing the first observationally a global rise of 1.8 ± 0.3 mm yr–1 during 1950 to 2000, and
Poseidon and Jason 1 satellites covering the years 1993- mainly to improvements in data collection tech-
based estimate of the thermal expansion contribution to sea Church and White (2006) determined a change of 1.7 ±
2003
levelprovide
rise in pasta value
decades. of 3.1 ± 0.7 years,
For recent mm/yrbetter(IPCC 2007, of niques.
estimates 0.3 mm Foryr–1the for1993-2003 period,Changes
the 20th century. the largest
in globaltermsea level
Nerem
the landet ice
al. 2006).
contribution to sea level are available from various (and the largest increase from the previous
as derived from analyses of tide gauges are displayed era) is thein Figure
observations
Table I shows of glaciers, ice caps and
the estimated ice sheets. of various
contribution thermal expansion term.
5.13. Considering the above results, and allowing for the
In thistosection,
processes observed weSLR summarise
during thosethe current
two time knowledge
periods. of ongoing higher trend in recent years shown by altimetry (see
present-day
The agreement seabetween
level rise.the Thesumobservational results areand
of contributions assessed,
the 3. Section
Sea level 5.5.2.2),
risewe assess the rate for 1961 to 2003 as 1.8 ±
projections
followed by our current interpretation of these observations in 0.5 mm yr–1 and for the 20th century as 1.7 ± 0.5 mm yr–1.
observed change in SLR is substantially better for the Four
terms of climate change and other processes, and ending with a Whilemain driverspublished
the recently of localestimates
SLR areof(1) sea global
level rise over
1993-2003
discussion period
of the seathan forbudget
level the 1961-2003 period, and the
(Section 5.5.6). SLRthe (Table
last decadesII andremainFigurewithin
3) driven by the
the range thermal
of the TAR values
difference between the sum and the observed change is no (i.e., 1–2 of mm
expansion theyrocean;
–1), there is an increasing opinion that the best
(2) global SLR driven by the
longer
5.5.2 statistically
Observations significant.
of SeaThis Levelconvergence
Changes is due estimate
melting oflies closer to 2ice;
land-based mm(3) yr–1local
than dynamical
to 1 mm yr–1 . The lower
SLR
driven by changes in wind, which push coastal watersregional
bound reported in the TAR resulted from local and
Table I. SLR contributions SeainLevel
mm/yr, from IPCC
Tide2007 (Table studies;
5.5.2.1 20th-Century Rise from Gauges toward or local
awayand from regional
shore; rates
and (4) maylocal
differ from the global
dynamical
5.3). See also Figure 2. mean, as discussed below (see Section 5.5.2.5).
SLR driven by local movement of the land itself, due
Table 11.9 of the TAR listed several estimates for global and A critical issue concerns how the records are adjusted for
regional 20th-century sea level trends based on the Permanent primarily vertical to tectonic forces.
movements of the landWe now upondiscuss
which eachthe tide of gauges
Source
Service for Mean Sea 1961-2003 Level (PSMSL) 1993-2003
data set (Woodworth these factors.and
are located Changes relatedTrends
of the oceans. to the instorage
tide gaugeof sur-records are
and Player,
Thermal 2003). The0.42
expansion concerns
± 0.12about 1.6 geographical
± 0.5 bias in face water in
corrected forreservoirs
GIA using and aquifers
models, but not areforestimated
other landtomotions.
the PSMSL data set remain, with most long sea level records beThe GIA correction
substantially smallerranges
thanfrom
theabout
other1terms
mm yrand –1 (or more) near
thus
Glaciers
stemming and ice the
from capsNH,0.5 and± 0.18
most from 0.77 ± 0.22 coastlines aretonot
continental former ice sheets to a few tenths of a millimetre per year in
discussed.
rather than ocean interiors. Based on a small number (~25) of the far field (e.g., Peltier, 2001); the error in tide-gauge based
Greenland
high-quality icetide
sheetgauge0.05 ± 0.12
records from 0.21
stable± 0.07
land regions, the global average sea level change resulting from GIA is assessed
3.1 Thermal expansion
rate of seaice
Antarctic level
sheetrise has0.14been± estimated
0.41 0.21 as 1.8 mm yr–1 for the
± 0.35 as 0.15 mm yr–1. The TAR mentioned the developing geodetic
past 70 years (Douglas, 2001; Peltier, 2001), and Miller and The ocean(especially
technologies has absorbed roughly
the Global 80% of
Positioning the GPS)
System;
Douglas (2004) find a range
Sum 1.1 ± of0.51.5 to 2.02.8 mm± 0.7yr–1 for the 20th heating that hold of the theclimate
promisesystem associated
of measuring rateswith
of rising
vertical land
century from 9 stable tide gauge sites. Holgate and Woodworth greenhouse movement gases at tideduring
gauges,the no past
matter ~50if those
years movements
(IPCC are
Observed
(2004) estimated a rate1.8 ± 0.5
of 1.7 ± 0.4 mm 3.1 yr±–10.7
sea level change SPM due 2007),
to GIA leading
or to other geological processes.
to substantial ocean warming. Although there
averaged along the global coastline during the period 1948 to Because has been some model
seawater expands validation,
slightly especially
when warmed, for GIA the models,
Difference
2002, based on data from 0.7177± 0.7stations0.3 ± 1.0into 13 regions.
divided systematic problems with such techniques, including short data
volume of the ocean has increased and the ocean is
Church et al. (2004) (discussed further below) determined spans, have yet to be fully resolved.
expected to continue expanding as a result of pro-
jected increases in 21st century global temperature.
410
4Observations: Oceanic Climate Change and Sea Level
feet), substantially higher than the IPCC projections.
dence While caution must be used in extrapolating a linear
udget relationship so far beyond the 20th century variability
her is used to derive it, Rahmstorf's findings provide a sci-
ted or entific basis for considering much higher rates of sea
level rise than the current IPCC projections.
Table II. Sea level rise contributions 2090-99 minus 1980-
99, expressed in mm/yr for comparison with Table I. Re-
formatted from IPCC (2007) Table 10.7.
hange
2003
been Source B1 A1FI
ur and Thermal expansion 1.7± 0.7 2.9±1.2
5.5.5),
2001), Glaciers and ice caps 1.05±0.35 1.25±0.45
nown Figure 2. Estimates of the various contributions to the budget of the
Figure 5.21. Estimates of the various contributions to the budget of the global mean Greenland ice sheet 0.3±0.2 0.7±0.5
e (see global mean sea level change (upper four entries), the sum of these
sea level change (upper four entries), the sum of these contributions and the observed
ratecontributions and and
of rise (middle two), the the
observed
observedrate
rateof rise the
minus (middle
sum oftwo), and the ob-
contributions Antarctic ice sheet -0.6±0.4 -0.85±0.55
served rate minus the sum of contributions (lower), all for
(lower), all for 1961 to 2003 (blue) and 1993 to 2003 (brown). The bars represent1961 tothe
only Sum 2.8±1.0 4.25±1.65
90%2003
error (blue, topthe
range. For barsum,
in each pair)
the error hasand
been1993 to 2003
calculated (brown,
as the square bottom
root of thebar).
r and sumThe
of squared errors of the
bars represent thecontributions.
5-95% error Likewise
range.theFig
errors of the
5.21 fromsumIPCC
and the
(2007).
puting observed rate have been combined to obtain the error for the difference.
Sum (meters per century) 0.28±0.10 0.425±0.165
ceanic
estern This fact, when combined with the long timescale of
tor of outweighed oceanbythermal
negative terms (especially
expansion, has significant impoundment).
long-term impli-We
n nine conclude that the budget has not yet been closed
cations for SLR. Ocean thermal expansion will continue satisfactorily. 3.2 Cryospheric contribution
el rise Given forthe large temporal
~1000 variability intemperature
yr after atmospheric the rate of sea level riseas
stabilizes Melting of global ice (the cryosphere) provides
water evaluated the slow circulation of the deep ocean gradually5.17),
from tide gauges (Section 5.5.2.4 and Figure brings another substantial contribution to global SLR.
at the the budget older cold water into contact with the new conditions.The
is rather problematic on decadal time scales. Melting of glaciers and ice caps is presently, and is
ice is thermostericThe contribution
IPCC generated has smaller
a rangevariability
of scenarios(though still
of socioeco- projected to remain, the largest cryospheric contribu-
verage substantial; Section 5.5.3) and there is only moderate
nomic change during the 21st century, which in turn lead temporal tion to SLR. However, several independent meas-
e. The correlationto a between
range ofthe thermosteric
projected rate and
temperature andtheSLRtidechanges.
gauge urements of Greenland and Antarctic mass balance
erence rate. The difference between them has to be explained by ocean using lasers and gravity measurements indicate that
These scenarios range from the low B1 scenario, in which
cially mass change. Because the thermosteric and climate-driven land both Greenland and Antarctica have recently (2002-
carbon dioxide rises to roughly double its pre-industrial
been water contributions are negatively correlated (Section 5.5.5.3.), 2006) been substantial contributors to global SLR
concentration by 2100, to the high A1FI scenario, in
which carbon dioxide reaches 3.5 times its preindustrial (IPCC 2007, pp. 363-366; Zwally et al. 2006, radar
concentration. altimetry; Thomas et al. 2006, laser altimetry; Veli-
an sea level change for 1961 to 2003 and 1993 to 2003 compared with the observed rate of rise.
ise. A GIA correction has been Projected thermalfrom
applied to observations expansion
tide gauges andfor altimetry.
the 21st century
For the sum, cogna and Wahr 2005, 2006, satellite gravity meas-
of the contributions. Theranges
thermosteric
from sea17±7
level changes are for thefor
cm (7”±3”) 0 toIPCC’s
3,000 m layer
low ofemissions urements). In stark contrast to these observations, the
B1 scenario to 29±12 cm (11”±5”) for the IPCC’s high IPCC projections (Figure 3 and Table II) assume that
emissions A1FI scenario (see Table II and Figures 3 and Antarctica alone and the sum of contributions by
Sea Level Rise 4). (mm
Overall,yr–1)thermal expansion accounts for about one- Greenland and Antarctica will (with 95% confidence)
2003 1993–2003
half of projected 21st century SLR. Reference tend to offset, not add to, sea level throughout the
A recent paper (Rahmstorf 2007) noted a strong rela- 21st century as increased precipitation in Antarctica
0.12 tionship between
1.6 ± 0.5observed global temperature Section 5.5.3and rate of increases the mass balance of the continent. In effect,
SLR per unit of time. Using a linear relaxation model the IPCC has dismissed recent observations of sub-
0.18
(i.e., SLR 0.77 ± 0.22
equilibrates to a change in Section 4.5
temperature over a stantial SLR contribution from Greenland and Ant-
0.12 long period), 0.21 Rahmstorf
± 0.07 used the 20th century
Section 4.6.2relation- arctica as nothing more than a brief excursion away
ship together with future scenarios of temperature change from the true long-term mass balance.
0.41 0.21 ± 0.35 Section 4.6.2
from IPCC to infer that 21st century SLR from thermal Several physical processes appear to be contrib-
0.5 expansion alone could be in the range 0.5-1.4 m (1.6-4.6
2.8 ± 0.7 uting to the recent large contributions from Green-
land. These include basal melting, ice flow accelera-
0.5 Section 5.5.2.1
3.1 ± 0.7 Section 5.5.2.2
0.7 0.3 ± 1.0
5Chapter 10 Global Climate Projections
report.
during the 21st This
century factor
is largeris than
illustrated
central in Figure 3 as
“scaled-up
estimate of 1.6 mm ice yrsheet discharge”
–1 for 1993 to 2003 or “dynamical im-
(Section
balance”, 5.5.3). and
Likewise,it was in all scenarios the
estimated at levels substantially
average rate of mass loss by G&IC during
smaller than recent
the 21st century is greater than the central observations would suggest.
Global Climate Projections Chapter 10
Furthermore,
estimate of 0.77 mmityrwas –1 for based
1993 to on 2003a poorly understood
(Section 4.5.2). By the end of the century,
relationship in the 1993-2003 period between a
a large fraction of the present global G&IC
10.6 Sea Level Change in the massglobal
scenariosis projectedtemperature
SRES toB1,have A1B anomaly
beenandlost A2 0.63°C in
(see,respectively
e.g., (1.1°F)
the AOGCMand
21st Century possible
Table
ensemble Theice-sheet
4.3). (the G&IC
widthprojections
of thedynamicalare rather
range contribution
is affected by the to sea
different
insensitive to the scenario because the main
level
numbers rise
of modelsof 0.32mm/yr
under
uncertainties come from the G&IC model.
each (IPCC
scenario). 2007,
The Appendix
acceleration is
caused
10.A.5).
Further by accelerations
theWe increased
willinargue climatic
ice flowbelow warming.
of thethat for Results are shown
the very high
10.6.1 Global Average Sea Level Rise Due to for
kind allrecently
estimate SRESof marker
SLR, inthese
observed scenarios
some factors in Table
Greenland 10.7more
warrant (see Appendix
careful
Thermal Expansion 10.A
outlet for methods).
glaciers and WestInAntarctic
the AOGCM ice streams ensemble, under any given
attention.
could increase
SRES scenario,thethere ice is sheet
some contributions
correlation of the global average
substantially, but quantitative projections
As seawater warms up, it expands, increasing the volume temperature change across models with thermal expansion
cannot be made with confidence (see Section
of the global ocean and producing thermosteric sea level rise and 3.3
its rate
10.6.4.2).
Local
Theofland
atmospheric
change,
ice sum suggesting circulation
in Table 10.7 that the spread in thermal
(see Section 5.5.3). Global average thermal expansion
Figure 10.33. Projections and uncertainties (5 to 95% ranges) of global average sea level rise and its can be expansion
includesThe for that
presence
the effect scenario is
of a changes
of dynamical caused
northward in both by the
wind spread
along thein
calculated
components in 2090 to directly
2099 (relativefrom
to 1980simulated
to 1999) for the
Figure 3. Projections and uncertainties (5 to 95% ranges) of global changes in
six SRES marker ocean temperature.
scenarios. The projected surface
the ice warming
sheets that and
can beby model-dependent
simulated with a ocean heat uptake
sea level rise assumes that the part of the present-day ice sheet mass imbalance that is due to recent ice outer coastsheet
continental ice
plays a (Sectionsignificant role in local sea level
flow Results
average
accelerationseawillare
level
persistavailable
rise andItfrom
unchanged. not17
its components
does AOGCMs
include in 2090
the contribution for the
shown 21st
tofrom
2099 century
(relative
scaled-up ice sheet efficiency (Raper model et al., 2002; Table 10.6.4.2).8.2) and the distribution of
forwhich
to 1980
discharge, SRES an scenarios
tois1999) theA1B,
for possibility.
alternative isA2
sixItSRES alsoand B1that(Figure
marker
possible present10.31),
scenarios.
the Thecontinuing
imbalance projected
might be transient, It on
addedalso seasonal
includes
heat within and interannual
a scenario-independent
the ocean (Russell timescales.
term et al., 2000).The wind-
in which case the projected sea level rise is reduced by 0.02 m. It must be emphasized that we cannot assess ofdriven
0.32 ± 0.35 mm yr–1 (0.035
enhancement of± sea
0.039levelm in occurs because the
sea level
from rise assumes
simulations ofthatthe the
20th part of
century.the present-day
One ensemble ice sheet
member
the likelihood of any of these three alternatives, which are presented as illustrative. The state of understanding
mass
110 years). This is the central estimate for
imbalance
preventswas used
a best that
estimate iseach
forfrom due to
beingmodel
made.recent
andice flow acceleration
scenario. The time series will persist
are rather northward
10.6.2
1993 Local
to 2003 wind,
of Sea
the sea common
Level
level Change duringDue
contribution winter months (and
to Change in
unchanged.
smooth compared It does notwith include
global theaverage
contribution shown from
temperature scaled-
time series, even
from Oceanprevalent
more
the Antarctic Density
Ice Sheet, plusand halfDynamics
during of Elthat Niño events) com-
up ice
Inbecause sheet thermal
all scenarios, discharge, which
rate of is
expansion
the average rise anduring
alternative
reflects heat
the possibility.
21ststorage
century in fromIt isentire
the also
Greenland (Sections
possible that the present imbalance might be transient, in which bines 4.6.2.2with and the5.5.5.2).
effects We of take Earth’s
this as rotation to push
is very likely to
ocean, exceed
being the 1961 to 2003proportional
approximately average rate ofto 1.8the
± 0.5 an estimate
time integral of of the partTheof the present ice sheet
geographical massof
pattern imbalance
mean sea level relative to the
case
mm yr the projected
(see Section sea level
5.5.2.1). rise
The central is reduced
estimate by 0.02
of the rate m. It must be
that is due to recent ocean
ice flow water toward shore, elevating
andaspectsea level. The
(theacceleration (Section 4.6.3.2),
–1
temperature change (Gregory et al., 2001). geoid dynamic topography) is an of the dynamical
ofemphasized
sea level rise during that we2090 cannotto 2099assess
is 3.8the mmlikelihood
yr–1 under A1B, of any ofassume thesethat this contribution
result is that
will mean
persist wintertime
unchanged. sea level is roughly 50
During
threeexceeds
which the2000
alternatives, toestimate
which
central 2020 ofunder
3.1 mmscenario
are presented as SRES
yr–1illustrative.
for 1993 A1Bstate
to The Wein ofalso
balance
the evaluate relating the ocean’s density structure and its circulation,
the contribution of rapid dynamical
cm (20”)
which higher than
are maintained by summer
air-sea sea level on Washing-
2003 ensemble
(see Sectionofprevents
understanding AOGCMs,
5.5.2.2). Thea besttheestimate
1993 rate
to 2003 of rate
thermal
from may being expansion
have made.
a is 1.3under
From
changes ± two alternative assumptions (see, e.g., Alleyfluxes et of heat, freshwater
0.7(2007).
contribution
IPCC mmofyrabout –1, and1 mm is not
yr–1 significantly
from internallydifferent generatedunder or A2 or B1. First,and
al., 2005b). theton’s
present coasts
momentum. imbalance and Over estuaries
might much
be a rapid (Figure
of the ocean
short- 5), and during El
on multi-annual
Thisforced
naturally rate is morevariability
decadal than twice (seethe observationally
Sections 5.5.2.4 and derived rate
term adjustment, timeNiño
which events,
scales,
will a good
diminish sea level
during can decades.
approximation
coming be elevated by asofmuch
to the pattern as
dynamic
9.5.2). ofThese
0.42sources
± 0.12ofmm variability are not predictable
yr–1 during 1961 to 2003. and notIt is We take to
similar an e-folding time of change
topography 100 years, is on thebybasis
given of an sea level change, which
the steric
includedtion, in and other nonlinear
the projections; iceratedynamics. For example, idealisedafter
an additional 30 cm (12”) for several months at a
the rate of 1.6 ± 0.5 the mmactual
yr–1 during during any future
1993 to 2003 (see Section model study (Payne et al., 2004). This assumption
can time be (Ruggiero
calculated et straightforwardly
al. 2005). from local temperature
decade might
the Larsen-B therefore be more
icebeshelf or less
(eastthan the
of of projected
theprevious rate
Antarctic reduces
peninsula) the sea level rise in Table 10.7 by 0.02 m. Second,
5.5.3), whichAlthoughmay larger than andthat sea leveldecades and salinity
partly imbalance change (Gregory et climate
al., 2001; Lowe and Gregory,
by a similar amount.
disintegrated in 2002,
simulated
numerous
observed
glaciers feeding
the present
theperhaps
ice 2006). Givenbe the
might strength
a response of this
to recent effect locally, it is im-
because
rise agree of natural
reasonably well forcing
for 1993 and to 2003,internal variability
the observed rise (seechange,
Sections through In muchorof
oceanic the world,
surface warming salinity
(Section changes are as important
for 1961shelf
5.5.2.4, accelerated
to 2003 5.5.3is notand with the
9.5.2).
satisfactorily removal
Inexplained
particular, ofmany
(Section the back-pressure
of the 10.6.4.2).
9.5.2), AOGCM No asportant
ofmodels temperature
are available to for
consider
changes inthe
such a link, so wepossible
determining
assume the future
patternchanges
of dynamicin
as theexperiments
sum
the of observationally
do not estimated
include components
the influence
ice shelf. IPCC projections of future SLR included the is 0.7
of ±
Mt.0.7 that
Pinatubo, the imbalance
the atmospheric
might
topography scale up circulation
with
change global
in the over
average
future, the
surface
and North
their Pacific.
contributions Fig-
can
mm yr –1 less than the observed rate of rise (Section 5.5.6). This
omission of which may reduce the projected rate temperature
of thermal change,
be which
ure 6
opposed we take
shows as athe
(Landerer measure et of
estimatesthe2007;
al., magnitude
of sea
and level
as in thechange
past, as a
Section
possibility
indicates a deficiency ofincontinued
current scientific rapid ice loss through
understanding of sea these proc-
of climate change (see Appendix 10.A). This assumption adds
expansion during the early 21st century. 5.5.4.1).
result Lowe
of and Gregory
changes in (2006) show
atmospheric that in the and
circulation UKMO- in
level esses,
change and butmaythey implywere not discussed
an underestimate in the widely
in projections. 0.1 to 0.2 readm to the estimated upper bound for sea level rise
For anDuring
average model 2080 (the to central
2100,estimate
the rate for eachof scenario),
thermal expansion dependingison theHadCM3 ocean (Table
scenario AOGCM,
density, 10.7). changes
averaged
During 2090 inover
heat fluxes
18
to 2099, are thefor
models cause
the of many
mod-
summary
projected for
to be policymakers,
1.9B1 ± 1.0, 2.9 ±in 1.4 only deep within the IPCC of the large-scale features ofbalances
sea level change, but freshwater
the scenario spread (from to A1FI) sea and level3.8rise±is1.3onlymm yr –1 under
the rate of scaled-up antarctic discharge roughly
erate IPCC A1B emissions scenario. For the coast of the
0.02 m by the middle of the century. This is small because of the increased rate of antarctic accumulation (SMB). The central
time-integrating effect of sea level rise, on which the divergence estimate for the increased antarctic discharge under the SRES
among the scenarios has had little effect by then. By 2090 to scenario A1FI is about 1.3 mm yr–1, a factor of 5 to 10 greater
2099 it is 0.15 m. than in recent years, and similar to the order-of-magnitude
In all scenarios, the central estimate for thermal expansion upper limit of Section 10.6.4.2. It must be emphasized that we
by the end of the century is 70 to 75% of the central estimate for cannot assess the likelihood of any of these three alternatives,
the sea level rise. In all scenarios, the average rate of expansion which are presented as illustrative. The state of understanding
prevents a best estimate from being made.
821
Figure 4. Projected global average sea level rise (m) due to thermal expansion during the 21st century relative to 1980 to 1999
under emissions scenarios A1B, A2, and B1. Colored curves refer to different global climate models. From IPCC (2007).
6 the 21st century relative to 1980 to 1999 under SRES scenarios A1B, A2 and B1.
Figure 10.31. Projected global average sea level rise (m) due to thermal expansion during
See Table 8.1 for model descriptions.
812Chapter 10 Global Climate Project
flux change dominates the North Atlantic and momentum flux (2001). The largest spatial correlation coefficient between
change has a signature in the north(1898-2000) rangeandfrom
and low-latitude Pacific pair1.04
is 0.75,mm/yr
but only to 25%2.80
of correlation coefficients exc
Monthly Mean Sea Level the Southern Ocean. mm/yr (ibid).
0.5. To identify common features, an ensemble mean (Fig
3.4 Results are available for local sea level change due Linear
to ocean trends
10.32) isareexamined.
influencedThere arebyonly limited areas where
density and circulation change fromfluctuation
AOGCMs inin the annual
multi- and ensemble
model decadal meanrates
change of
exceeds the inter-model stand
3.2 La Push: 0.48m model ensemble for the 20th century and the 21st century. deviation, unlike for surface air temperature change (Sect
global
There is substantial spatial variability in allsea level
models (i.e.,rise
sea as10.3.2.1).
well as variations in
3.0 the
level change is not uniform), and as rate of
the geographical local
pattern of vertical land etmovement
Like Church al. (2001) and Gregory et al. (2001), Fig
climate change intensifies, the spatial standard deviation of local 10.32 shows smaller than average sea level rise in the South
sea level change increases (Church (VLM).
et al., 2001; Gregory et al., Ocean and larger than average in the Arctic, the former possi
2.8 2001). Suzuki et al. (2005) show that, in their high-resolution due to wind stress change (Landerer et al., 2007) or
Deducing the contribution of local VLM
m
model, enhanced eddy activity contributes to this increase, but thermal expansivity (Lowe and Gregory, 2006) and the la
2.6 formerly
across models there is no significant correlationrequired
of the spatiala model of Earth's
due to freshening. Anothercrustal
obvious feature is a narrow band
standard deviation with model spatial resolution. This section pronounced sea level rise stretching across the southern Atla
Toke Point: 0.62m movement. Recently,
evaluates sea level change between 1980 to 1999 and 2080 to
direct measurements of
and Indian Oceans and discernible in the southern Pacific. T
2.4
2099 projected by 16 models forced seawithlevel
SRES from
scenariosatellites,
A1B. couldand of landwithmove-
be associated a southward shift in the circumpo
(Other scenarios are qualitatively similar, but fewer models front (Suzuki et al., 2005) or subduction of warm anoma
2.2 ment from global positioning
are available.) The ratio of spatial standard deviation to global
system (GPS)
in the region of formation of sub antarctic mode water (Ba
average thermal expansion varies sensors,
among models, have
but isimproved
mostly et our understanding
al., 2002). In the zonal mean, ofthere are maxima of sea le
2.0
within the range 0.3 to 0.4. The model median spatial standard rise in 30°S to 45°S and 30°N to 45°N. Similar indications
2005 Jan Apr Jul Oct 2006 Jan Apr Jul Oct 2007 Jan Apr Jul Oct these two contributions
deviation of thermal expansion is 0.08 m, which is about 25%
to tide gage meas-
present in the altimetric and thermosteric patterns of sea le
of the central estimate of global urements of sea
average sea level level. change for 1993 to 2003 (Figure 5.15). The model projecti
rise during
Figure 5. Monthly mean sea level in meters for January 2005
the 21st through
century under A1B (Table 10.7).
September 2007 at La Push and Toke Point (Willapa Bay), Washington.
The geographical
Crustal deformation do
patterns of sea level change from different
not share other aspects of the observed pattern of sea le
associated with plate
rise, such as in the western Pacific, which could be related
Monthly average values for La Push and Toke Point aremodels
shownare not
asgenerally
cir- similar tectonics and isostatic
in detail, although they have rebound
interannual(adjustments
variability. to
more similarity than those analysed in the TAR by Church et al.
cles and crosses, respectively. Figure source: Climate Impacts Group, the disappearance of the great ice sheets) pro-
University of Washington.
western North America, the sum of these contri-
butions in the annual mean is about 2-3 cm
(about 1”) below the global average.
CIG has analyzed over 30 scenarios from
global climate models (Mote et al. 2007) and the
mean changes in wintertime northward wind are
indeed minimal. Consequently, we subtract 1 and
2 cm (less than 1”) from the “very low” SLR
estimates for 2050 and 2100, respectively, and
consider this component to be negligible for the
“medium” SLR estimate. However, several
models produce increases in northward wind in
wintertime of sufficient strength to add as much
as 15 cm (6”) to mean sea level for 2050-2099
compared with 1950-1999, so for the “very high”
Figure 10.32. Local sea level change (m) due to ocean density and circulation change relative to the global average (i.e., positive values indicate
SLR estimate we add 15 cm (6”). Figure 6. Local sea level change (m) due to ocean density and circulation
greater local sea level change than global) during the 21st century, calculated as the difference between averages for 2080 to 2099 and 1980 to
change relative to the global average (i.e., positive values indicate greater local
1999, as an ensemble mean over 16 AOGCMs forced with the SRES A1B scenario. Stippling denotes regions where the magnitude of the multi-model
ensemble mean divided by the multi-model standard deviation exceeds 1.0.
sea level change than global) during the 21st century, calculated as the differ-
3.4 Local tectonic movement ence between averages for 2080 to 2099 and 1980 to 1999, as an ensemble
Direct measurements of sea level at tide mean over 16 AOGCMs forced with the SRES A1B scenario. Stippling de-
gauges are difficult to interpret because tide gauges notes regions where the magnitude of the multi-model ensemble mean divided
by the multi-model standard deviation exceeds 1.0. From IPCC (2007).
record the difference between local sea level and
local land level, with interannual variability and
duces local vertical land movement. Western Wash-
measurement uncertainty clouding the picture. Differ-
ington sits on the edge of the North American conti-
ences in rates of sea level rise can be substantial. For
nental plate, under which the Juan de Fuca oceanic
example, the linear trend in sea level for 1973-2000 was
plate is subducting. This subduction tends to produce
2.82±1.05 mm/yr at Toke Point (Willapa Bay, southern
uplift in the western extent of the region over time
coast) and 1.39±0.94 mm/yr at Cherry Point (near Bel-
(although historically, large subduction zone earth-
lingham; Zervas 2001). Without additional evidence it is
quakes of magnitude > 8.0 in the region have resulted
difficult to separate sea level rise from local land level
in sudden land subsidence of 1 meter (3.3 ft) or more
change, which itself could be caused by a variety of fac-
[Leonard et al. 2004, Jacoby et al. 1997]).
tors including tectonic movement or soil compaction.
Trends also change over time: 50-year trends at Seattle
7Figure 8. GPS derived current annual vertical deformation rates
Figure 7. Vertical land movements, from Verdonck (2006). (mm/year), from Pacific Northwest Geodetic Array (stations indi-
cated by symbols), Central Washington University, November
2007, www.geodesy.org
An earlier analysis of records in the Pacific North-
west (Holdahl et al. 1989) suggested that south Puget uplift at >2 mm/yr. Reliable estimates of VLM for the
Sound was subsiding at a rate of approximately 2 mm/yr central and southern Washington coast are not avail-
and the northwest Olympic Peninsula was rising at a able due to sparse data, but are estimated to be on the
comparable rate, while VLM on most of the Washington order of 0-2 mm of uplift per year.
coast and the rest of Puget Sound was mostly less than 1 The Puget Sound basin seems to be the least
mm/yr. Another study by Mitchell et al. (1994) found consistent. Based on current analysis we do not be-
little VLM in Puget Sound, but similar VLM for the coast lieve we can justify factoring VLM into the “very
as those of Holdahl et al. (1989). low” and “medium” SLR estimates for Puget Sound.
More recently, Verdonck (2006) recalculated VLM However, for the upper or “very high” SLR estimate
and again found uplift, but at a rate as high as 3.5 mm/yr (high impact, low-probability) for the Puget Sound
on the north and northwest part of Olympic Peninsula, basin, we assume subsidence of 10 cm (4”) by 2050
only small movement in central and southern Puget and 20 cm (8”) by 2100 on the basis of the Verdonck
Sound, and some strong local subsidence on the central (2006) data set. Rates of tectonic uplift were incorpo-
Washington coast (Figure 7). However, ongoing GPS rated into the SLR estimates for the northwest corner
measurements at Pacific Beach, WA suggest uplift in this of the Olympic Peninsula and the “very low” and
region of the outer coast of 1.8 mm/yr. Recent analysis of “medium” estimates for central and southern Wash-
continuous GPS monitoring sites comprising the Pacific ington coast. Again, because of the characteristics of
Northwest Geodetic Array (PANGA) by staff at Central the “very high” SLR estimate, VLM along the central
Washington University support the conclusion of general and southern coast is removed to reflect a scenario of
uplift occurring along most of the outer coast with the zero or negligible uplift in this region.
greatest uplift (>3mm/yr) located in the northwest corner Local areas of subsidence due to sediment com-
of the Olympic Peninsula and with uplift dropping off to paction in estuaries and coastal basins as well as
near zero in the central Puget Sound (Figure 8). subsidence in terrain overlying areas that have expe-
Thus, it appears that the method of analysis and the rienced significant groundwater extraction are not
time period studied lead to different estimates of VLM, considered in this report, but could very well domi-
except in the northwest corner of the Olympic Peninsula nate smaller scale relative SLR and its variability
where all three studies, and current observations, agree on throughout the region.
8in the B1 scenario level off by 2100. Consequently,
4. Synthesis: Summary and calculation of SLR the SLR profile is approximately linear (Figure 4), so
projections the values in 2050 are half those in 2100.
Three important questions need to be considered in the For the end-of-century “medium” SLR estimate,
use of SLR estimates in decision making: we use the average of the six central values from the
1) what is the location of interest? six IPCC scenarios (34 cm or 13”). The value for
2) what time horizon should be considered?, and 2050 is somewhat below half of this value owing to
3) what risk level is acceptable? the acceleration of SLR in all scenarios except B1
As indicated by Sections 3.3 and 3.4, location is important (Figure 4), with a low of 39% for A2 and a high of
as rates of SLR vary depending on oceanographic condi- 50% for B1 and a mean of 45%. The atmospheric
tions and on local VLM. contribution is approximately zero. For the VLM
Time horizon is very important and will be defined by term, we take the uplift value of 3mm/yr (translates
the nature of the decision being made; decisions with long to a SLR of –12” per century) for the NW Olympic
life spans or long-term implications should be based on Peninsula and 0.5 mm/yr (translates to a SLR of –2”
longer-term estimates of sea level rise. Note that time per century) for the central and southern coast. For
horizon is not just a function of the lifespan of a specific the Puget Sound basin, we again assume no change.
structure. The choice of time horizon should take into For the end-of-century “very high” SLR estimate,
account the overall “footprint” of the decision, i.e., the we start with the A1FI 95% value of 59 cm (23”) by
committed long-term use of the site once it is developed. 2100 but allow the possibility that the recent
For some factors that contribute to local SLR, changes cryospheric contributions could continue and even
will probably be linear with time so the 2050 value will increase in the 21st century. Although it is difficult to
be half the 2100 value. However, this is not the case for quantify the importance of such processes over the
the most important term, global SLR: in most scenarios span of the 21st century, we take as a starting point
the rate of global SLR increases over time (the curve is the calculation in IPCC 2007 (Appendix 10.A.5).
concave upward or accelerating). Hence, it is inappropri- They presumed a linear relationship between global
ate to estimate SLR in 2050 simply by halving an estimate temperature anomalies (0.63°C) and enhanced ice
of change that applies to the year 2100. sheet loss from these dynamical processes (0.32 mm/
Finally, risk tolerance determines whether the medium yr), and arrived at an estimate of 0-17 cm (0-7”) for
or a less likely but higher (or lower) impact estimate is the 21st century SLR. However, observations cannot
used. Risk tolerance will vary from community to com- constrain their estimate of 0.32 mm/yr within a factor
munity, person to person, and project to project. of two. For example, one could posit a situation in
We now attempt to combine the factors in the above which the difference between observed SLR and the
discussion to construct estimates of SLR for the NW sum of known terms during 1993-2003 (Table I) is
Olympic Peninsula, the central and southern Washington entirely due to these processes; this gives an upper
coast, and Puget Sound for 2050 and 2100 (Table III). We estimate of 1.3mm/yr, roughly a factor of 4 larger
stress that (1) these calculations have not formally than their estimate. Likewise, there are small uncer-
quantified the probabilities, (2) SLR cannot be esti- tainties in the estimated global temperature anomaly
mated accurately at specific locations, and (3) these used in this ratio. Since an error of a factor of two in
numbers are for advisory purposes and are not actual this ratio is plausible, we take that as a rough estimate
predictions. of the upper limit of ice sheet contributions, adding
For the end-of-century “very low” SLR estimate, we 34 cm (13”) for 2100.
use the 5% value of the B1 SLR scenario, namely 18 cm The atmospheric contribution in all areas is 15 cm
(7”) by 2100. The atmospheric component is assumed to (6”) by 2100 and 7 cm (3”) for 2050.
be the same for all three areas and contributes –2 cm (less For the VLM term in our “very high” SLR esti-
than –1”). For local contributions from VLM we take the mate, we use an uplift value of 2 mm/yr (SLR about
low end of the various estimates discussed above: uplift in –8” per century) at the NW Olympic Peninsula. For
the NW Olympic Peninsula of 4 mm/yr (translates to a the central and southern Washington coast, we as-
local SLR of –16” per century) and no uplift for Puget sume zero VLM. For the Puget Sound region, subsi-
Sound. Uplift for the central and southern Washington dence of 2 mm/year (SLR about 8” per century) is
coast is estimated at 1 mm/year (translates to a SLR of used.
about –4” per century). Furthermore, global temperatures
95. Unknowns and additional considerations pheric dynamical factors, whereas a more rigorous
We reiterate that the four factors discussed here are not analysis would use the SLR output of the global
well quantified. Future contributions to SLR from Green- models directly.
land and Antarctica are very uncertain. The rates of VLM Finally, our analysis has focused on the slow
at specific locations are generally poorly understood and change in mean sea level. Societal and ecological
it is impossible to estimate the uncertainty associated with impacts will be driven at least as much by the se-
using measurements of VLM in the recent past to predict quence of extreme events as by the slow change in
changes over the next century. Additionally, we have not the mean. That is, a coastal inundation event could be
developed a formal framework to quantify the probabili- produced either by our “very high” sea level plus a
ties of our “very high” or “very low” SLR estimates. moderate high tide and storm surge, or by our “very
As additional studies of these subjects are published, a low” sea level plus an exceptionally high tide and
thorough assessment of the state of science would be war- storm surge. Whether such an event occurs in 2009
ranted, along with a more careful quantification of prob- or 2099 depends as much on the random confluence
abilities and uncertainties. We have assumed independent of events as on the background change in sea level
probabilities in combining estimates of global SLR driven by anthropogenic global climate change.
(which the IPCC made using a combination of global
climate models and simpler models) and local atmos-
Table III. Calculation of very low, medium, and very high estimates of Washington sea level change for 2050 and
2100, in cm (and, for totals, inches). VLM and and Total (the sum of factors used to calculate the total relative SLR
value) are reported for NW Olympic Peninsula, the central and southern Washington coast, and Puget Sound.
Negative VLM values represent vertical uplift of the land and a negative Total represents an apparent or relative sea
level drop. Both the very low and very high SLR estimates are considered low probability scenarios.
SLR
Components 2050 2100
Estimate
Central & Central &
NW Olympic NW Olympic
Southern Puget Sound Southern Puget Sound
Peninsula Peninsula
Coast Coast
Global SLR 9 cm 18 cm
Very Low Atm. Dynamics -1 cm - 2 cm
VLM -20 cm - 5cm 0 cm - 40 cm -10 cm 0 cm
Total -12 cm (-5”) 3 cm (1”) 8 cm (3”) -24 cm (-9”) 6 cm (2”) 16 cm (6”)
Global SLR 15 cm 34 cm
Medium Atm. Dynamics 0 cm 0 cm
VLM - 15 cm - 2.5 cm 0 cm -30 cm - 5 cm 0 cm
Total 0 cm (0”) 12.5 cm (5”) 15 cm (6”) 4 cm (2”) 29 cm (11”) 34 cm (13”)
Global SLR 38 cm 93 cm
Very High Atm. Dynamics 7 cm 15 cm
VLM -10 cm 0 cm 10 cm - 20 cm 0 cm 20 cm
Total 35 cm (14”) 45 cm (18”) 55 cm (22”) 88 cm (35”) 108 cm (43”) 128 cm (50”)
10References
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