Transcript Recent sea-level rise

Lecture 7
滄海桑田- Global warming and
Sea Level Rise
http://flood.firetree.net
CU SLR interactive wizard
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Are you knowledgeable about SLR ?
Quiz
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Sea-level rise: implications/consequences
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Coastal Erosion
• Inundation of Land
• Increased Flood and Storm Damage
• Increased salinity of estuaries and aquifers
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Cliff erosion
suspect?
Source: Tyndall Centre for Climate Change Research
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Coastal erosion and accretion
• 1 cm rise in MSL erodes
approx 1m horizontally of
beach
• SLR has a profound effect on
the rate of sedimentation
• Varying of sedimentation rates
-> changing vegetation zones
e.g. growth/shrinkage of
marshes (high tide water level
regions)
• Storm surges force large
quantities of shore-face
sediments through inlets ->
create tidal deltas/barriers
1m
0.01
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Bruun Rule (Only describes one of the processes affecting sandy beaches)
R = G(L/H)S
H = B + h*
R = shoreline recession due to a sea-level rise S
h* = depth at the offshore boundary
B = appropriate land elevation
L = active profile width between boundaries
G = inverse of the overfill ratio
Source: Nicholls, 1998
Vulnerable coastal/island regions
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Vulnerable populated regions
Mega Coastal Cities:
Populations >8 million
(over 50% of US
population (110 million)
live in coastal areas
Highly populated Delta
regions
http://www.survas.mdx.ac.uk
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Flood and storm damage
• Coastal region more susceptible to storm surges,
flooding, beach/coastal erosion
=> disruption of activities; danger to life;
infrastructure damage
• 1 m rise in MSL would enable a 15-year storm to
flood areas that today are only flooded by 100year storms
• Urban flooding: contaminated water supply;
drainage/waste systems overwhelmed
• Flood damages would increase 36-58% for a 30cm rise in sea level, and 102-200% for a 90-cm
rise
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Increased salinity in estuaries
• Saltwater will penetrate farther inland and upstream
in estuaries (i.e. estuarine salt wedge)
• Higher salinity impairs both surface water and
groundwater water supply
• Saltwater intrusion would also harm ecosystems:
• aquatic plants and animals e.g. salt marshes,
mangroves
• Higher salinity has been found to decrease seed
germination
• Flooded agricultural land takes a long time to recover
• Decline of coastal commercial fisheries
e.g. Salinity intrusion has already been cited as primary
reason for reduced oyster harvests in Delaware and
Chesapeake Bays in the USA
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Factors affecting sea surface high (SSH)
Not directly climate related:
• Tides – Periodic changes due to changing
orbital motions of earth & moon
• Storm surges - Atmospheric effects
•inverted barometer, tropical
storm/hurricane surges
•Wind-stress driven surge
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astronomical tides
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Sea level
1
0
-1
-2
0
5
10
15
Time (days)
20
25
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Inverted barometer (IB) effect
The inverse response of sea level to changes in
atmospheric pressure =>
A static reduction of 1-mb in atmospheric
pressure will cause a stationary rise of 1-cm in sea
level
980mb
1000mb
20cm
Lower Atmospheric Pressure
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storm surges
1. A deep centre of low pressure
situated over Scandinavia
produces northerly winds
2. Wind stress forces surface
waters into the “bottle-neck”
of the English Channel
3. Flow is restricted by the Straits of Dover and sea levels rise along the
adjacent coasts of East Anglia and the Netherlands
4. Other key ingredients include high Spring tides and on-shore winds15
The Thames Barrier
the major part of the tidal defenses
protecting London
Factors affecting sea surface high (SSH)
Directly climate related:
• Isostatic – Vertical movement of land
• Eustatic – changes of total sea water mass
• Steric – Thermal expansion of water volume
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Glacial/interglacial isostatic adjustment
(PGR: Post Glacial Rebound)
• The weight applied to
the crust is dispersed
throughout the
lithosphere
• The lithosphere is so
rigid that the weight is
transferred across the
crust resulting in a
peripheral depression
and “forebulge”
• Around the periphery of the ice sheet margin up to a distance of
150-180 km, depression (>100m) occurs without ice loading
This area can record relative sea level change without the complexity
of glacial erosion or deposition
• The lateral displacement of mantle material from below the centre of
ice sheet loading results in the formation of an area of slight uplift
(10 - 20 m) beyond the peripheral depression (the forebulge).
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Isostatic changes = vertical land movements
• Stockholm, Sweden (Glacial
Isostatic Adjustment)
• Nezugaseki, Japan (abrupt jump in
sea level record following
earthquake in 1964)
• Fort Phrachula Bangkok, Thailand
(sea level rise due to increased
groundwater extraction since about
1960)
• Manila, Philippines (recent deposit
from river discharges and
reclamation works)
• Honolulu, Hawaii (a site in the 'far
field' without evident strong
tectonic signals on timescales
comparable to the length of the
tide gauge record and with secular
trend 1.5 mm/year).
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(courtesy of Proudman Oceanographic Lab)
Glacial isostatic adjustment/PGR
Glacial Isostatic Adjustment (Post Glacial Rebound)
i.e. melting of high latitude glaciers from 5,000-15,000 years BP
(Proudman Oceanographic Labs)
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Eustatic changes: volumetric (mass) changes
In glaciers, ice-caps or ice-sheets:
• Gain mass by accumulation of snow (snowfall and
deposition by wind-drift), which is gradually transformed
to ice
• Lose mass (ablation) mainly by melting at the surface or
base with subsequent runoff or evaporation of the melt
water
• Net accumulation occurs at higher altitude
• Net ablation at lower altitude
• The mass balance for an individual body of ice is usually
expressed as the rate of change of the equivalent volume
of liquid water, in m3/yr; the mass balance is zero for a
steady state
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Examples of eustatic changes
Cumulative mass balance for three glaciers in different climatic regimes:
Hintereisferner (Austrian Alps),
Nigardsbreen (Norway),
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Tuyuksu (Tien Shan, Kazakhstan)
IPCC, 2001
Nigardsbreen glacier in Norway
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Nigardsbreen glacier
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Steric rise
• As oceans warm, density decreases and thus even at
constant mass the volume of the ocean increases
• Thermal expansion (or steric sea level rise) occurs at all
ocean temperatures (albeit small in the deep ocean)
• Water at higher temperature expands more for a given
heat input. Therefore, the global average expansion is
affected by the distribution of heat within the ocean
• Salinity changes within the ocean also have a significant
impact on the local density and thus local sea level, but
have little effect on global average sea level change
• The rate of climate change depends strongly on the rate
at which heat is removed from the ocean surface layers
into the ocean interior – if heat is taken up more readily,
climate change is retarded but sea level rises more
rapidly
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Measuring sea surface height
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Sea-level rise: Historic changes
• Since the Last Glacial Maximum (~20,000 years BP) MSL
has risen by over 120 m
• Between 15,000 and 6,000 years ago MSL rose rapidly at an
average rate of 10 mm/yr.
• Following last glacial period local vertical land movements
are still occurring today as a result of large transfers of
mass from the ice sheets to the ocean
• During the last 6,000 years, global MSL variations on timescales of a few hundred years (and longer) are likely to have
been less than 0.3 to 0.5 m
• During the 20th century, tide gauge data shows MSL rises
in the range 1.0 to 2.0 mm/yr (larger as compared to 19th
century)
• There is decadal variability in extreme sea levels; but no
evidence of widespread increases in extremes other than
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that associated with a change in the mean
Estimates of global sea level change over the
last 140,000 years (continuous line)
Contributions from the major ice sheets:
(i) North America, including Laurentia, Cordilleran ice, and Greenland,
(ii) Northern Europe (Fennoscandia), including the Barents region,
(iii) Antarctica (Lambeck, 1999)
Source: IPCC, 2001
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Recent sea-level rise
Local trends in sea-level (i.e. relative to local land mass)
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Recent Sea-Level Rise in South Pacific
(1991-2004)
Relative net sea-level
trend (in mm/year)
after subtracting the
effects of PGR and
the IB
Source: The South Pacific Sea Level & Climate Monitoring Project
Recent sea-level rise: Global trend
Source: http://ilrs.gsfc.nasa.gov/
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Contributing factors to Sea Level Rise
Δ h ( t) = X ( t) + g ( t) + G ( t) + A ( t) + I ( t) + p ( t) + s ( t)
X - thermal expansion (steric rise)
g - loss of mass of glaciers and ice caps (eustatic rise)
G - loss of mass of the Greenland ice sheet due to current
climate change (eustatic rise)
A - loss of mass of the Antarctic ice sheet due to current
climate change (eustatic rise)
I - loss of mass of the Greenland and Antarctic ice sheets
due to the ongoing adjustment to past climate change
(eustatic rise)
p - runoff from thawing of permafrost (eustatic rise)
s - deposition of sediment on the ocean floor
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Thawing of permafrost
• Permafrost occupies 25% of land area in the N.H.
• Estimates of ice volume in N.H. permafrost
1.1 - 3.7  1013 m3 ( 0.03 to 0.10 m of global-average sea level)
The active layer (shown in
grey) thaws each summer
and freezes each winter,
while the permafrost
layer remains below 0°C.
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Characteristics of permafrost that could change
under G.W.
• Increasing of thawing area (horizontally)
• Thickening of active layer (vertically)
Effects (IPCC TAR)
Assuming:
  permafrost vol   permafrost area;
 current warming trend continues
 50% conversion of permafrost melt available to direct
runoff into ocean
Then:
Contribution to MSL - 1990 to 2100 is 0 to 25 mm (0 to
0.23 mm/yr) as compared to - 20th century: 0 to 5 mm
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(0 to 0.05 mm/yr)
Debating the major contributing factor to sea level
rise …
IPCC:
Identified 1.5-2.0 mm yr-1 rise during 20th century
Main factor: rising surface T => steric contribution
But …
Levitus et al (2000): identified increased heat storage in oceans
-> data suggests steric contribution is only 0.5 mm/yr
IPCC estimate only 0.2 mm/yr for eustatic (volumetric) MSL rise,
i.e.
hsteric + heustatic = 0.5 + 0.2 = 0.7 mm/yr
So …
Where is the rest of the 1.5-2.0 mm yr-1 rise from?
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Debating the major contributing factor to sea level
rise …
Total
Eustatic
Steric
Salinity
The time series are spatially averaged (50ºS to 65ºN),
5-year running means computed for the upper 3000 m of the ocean
Source: Ocean Freshening, Sea Level Rising, Walter Munk,
Science 27 June 2003 300: 2041-2043
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Eustatic or steric?
Mean salinity of the global ocean has decreased, implying the addition of
fresh water mass to oceans
=> combined steric (due to temperature rise) and salinity effects
hsteric = hT + hS = 0.5 +0.05 = 0.55 mm/year
If source of freshening is due to changes in continental water storage,
there must be an eustatic contribution
But, it can NOT be counted twice as both steric and eustatic!
Consider 3 modes of ocean freshening
1.Regions where T and S steric effects cancel i.e. no density change => no
MSL
2.Melting of floating ice: will freshen ocean but cause no MSL rise
(Archimedes) => only steric rise
3. Freshwater import from continents => eustatic AND steric rise
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Eustatic or steric?
 = 1028 kg/m3
 = 28 kg/m3
hs = 0.05 mm/yr
Salinity induced rise:
heustatic = (/)hs = 36.7 hs= 1.8 mm/yr
Assuming global ocean covers an area of 3.6 108 km2
This eustatic change would require an ice melt volume of 650 km3/year
Source: Ocean Freshening, Sea Level Rising, Walter Munk, Science 27 June 2003 300: 2041-2043
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Sea ice covers:
Eustatic
an area of 107 km2  30% seasonal changes;
~ 3m thick
Total volume 30,000 km3;
seasonality reduces this volume by 0.3% or 90 km3/yr
or steric?
Estimation of sea ice thinning of approximately 4 % over the last 20
years
 60 km3/yr
a total loss of sea ice per year 150 km3/yr
 135 km3/yr of freshwater input
i.e. purely steric contribution to sea level change
=> Readjust eustatic rise estimate:
heustatic = 650 km3/year- 135 km3/yr = 515 km3/yr or 1.4 mm/yr
heustatic + hsteric = 1.4 + 0.5 = 1.9 mm/yr
Value is within range of IPCC estimate!
Source: Ocean Freshening, Sea Level Rising, Walter Munk, Science 27 June 2003 300: 2041-2043
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IPCC, 2001:
Global average sea
level changes from
thermal expansion
AOGCM
experiments with
observed
concentrations of
GHGs in 20th
century;
then, following
IS92a scenario for
21st century;
predicted future changes
shaded region shows the
bounds of uncertainty
associated with land ice
changes, permafrost
changes and sediment
deposition for the groups
of models showing
largest/smallest sea level
change
(including the direct
effect of sulphate
aerosols)
Estimated rate of Mean Sea Level (MSL) rise:
5  2-9 mm/yr i.e. 2 – 5 times the rate experienced
over the past century
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Updating estimate in IPCC AR4
S. Rahmstorf, Science
315, 368 (2007).
M. Vermeer, S.
Rahmstorf, Proc. Natl.
Acad. Sci. U.S.A.
106, 21527 (2009).
A. Grinsted, J. C. Moore,
S. Jevrejeva, Clim. Dyn.
34, 461
(2009).
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Updating estimates in recent works
Source: Has the IPCC underestimated the risk of sea level
rise? Stefan Rahmstorf, Nature, 2010
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Interactions between ice sheets, ocean, and atmosphere affect the balance of mass of the
Greenland and Antarctic ice sheets. The dynamic response of the ocean may bring warmer waters
in contact with marine glaciers, leading to the decay of ice shelves. Rapid changes at the boundary
of the ice sheets can be communicated far into the interior of the ice sheets by ice streams, leading
to unloading of the continent and changes in the global gravitational field and thus sea level.
Changes in atmospheric temperatures and circulation may bring more precipitation to the Antarctic,
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offsetting ice loss at the boundaries. (source: Regional Sea-Level Projection, Willis and Church, Science, 2012)
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Summary
• Sea Level Rise has massive global implications on the
natural world and human society
• Major climate-related causes of sea level rise:
• Isostatic - PGR
• Eustatic – Volumetric
• Steric – Temperature
•IPCC likely underestimates the risk of SLR
•Interaction of processes still not well understood
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