Download Factors affecting sea level rise

Sea Level Rise
SOEE3410: Lecture 15
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Sea-level rise: implications
• Coastal Erosion
• Inundation of Land
• Increased Flood and Storm Damage
• Increased salinity of estuaries and aquifers
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Coastal erosion and accretion
• 1 cm rise in MSL erodes
approx 1m horizontally of beach
• Sea level rise has a profound effect
on rate of sedimentation
• Varying of sedimentation rates
-> changing vegetation zones
e.g. growth/shrinkage of marshes
1m
0.1
• Storm surges force large quantities
of shore-face sediments through
inlets -> create tidal deltas/barriers
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Vulnerable populated regions
Large Coastal Cities:
Populations >8 million
(over 50% of US population live in
coastal areas, >110 million)
Highly populated Delta regions:
Vulnerable to MSL rise
http://www.survas.mdx.ac.uk
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
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 100-year storms
• Urban flooding: contaminated water supply; drainage/waste systems
overwhelmed
• Flood damages would increase 36-58% for a 30-cm rise in sea level,
and increase 102-200% for sea level rise greater than 90 cm
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
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 human 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 from saline
water
• 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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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Factors affecting sea level
Not directly climate related:
• Tides – Periodic changes due to changing orbital motions of earth &
moon
• Storm surges - Atmospheric effects
• inverse barometer, tropical storm/hurricane surges
• Wind-stress driven surge
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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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Factors affecting sea level: astronomical tides
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Sea level
1
0
-1
-2
0
5
10
15
Time (days)
20
25
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Factors affecting sea level: atmospheric variations
Inverse barometer effect
The inverse response of sea level to changes in atmospheric pressure.
A static reduction of 1.005 mb in atmospheric pressure will cause
a stationary rise of 1 cm in sea level
Low Atmospheric Pressure
980mb
1000mb
20cm
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
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Factors affecting sea level: 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 winds
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Glacial isostatic adjustment/PGR
• 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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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Factors affecting sea level: isostatic changes
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 PGR '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).
(courtesy of Proudman Oceanographic Lab)
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Factors affecting sea level: glacial isostatic adjustment/PGR
Glacial Isostatic Adjustment (Post Glacial Rebound)
i.e. melting of high latitude glaciers from 5000-15000 years BP
(Proudman Oceanographic Labs)
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Factors affecting sea level rise: Eustatic changes
Eustatic changes = volumetric (mass) changes
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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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Factors affecting sea level rise: 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 or under greater pressure (at greater depth)
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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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Measuring sea level
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Sea-level rise: Historic changes
Past changes in sea level (From IPCC Analysis, 1998, 2001; Pugh, 2004)
• Since the Last Glacial Maximum (~20,000 years BP) MSL has risen by over 120 m
at locations far from present and former ice sheets
• 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 time-scales 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 (more than during 19th century)
• There is decadal variability in extreme sea levels but no evidence of widespread
increases in extremes other than that associated with a change in the mean
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Evidence of recent sea-level rise
Local trends in sea-level (i.e. relative to local land mass)
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Evidence of recent sea-level rise II
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Evidence of eustatic changes in sea-level
Cumulative mass balance for three glaciers in different climatic regimes:
Hintereisferner (Austrian Alps),
Nigardsbreen (Norway),
Tuyuksu (Tien Shan, Kazakhstan)
IPCC, 2001
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Evidence of eustatic changes in sea-level II
Estimates of global sea level change over the last 140,000 years (continuous line)
and contributions to this change from the major ice sheets:
(i) North America, including Laurentia, Cordilleran ice, and Greenland,
(ii) Northern Europe (Fennoscandia), including the Barents region,
(iii) Antarctica (From Lambeck, 1999)
Source: IPCC, 2001
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Sea level rise: contributing factors
Δ h ( t) = X ( t) + g ( t) + G ( t) + A ( t) + I ( t) + p ( t) + s ( t)
The components of MSL rise are due to:
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
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
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Contributing factors: permafrost
• Permafrost occupies 25% of land area in the northern hemisphere
• Estimates of ice volume in northern hemisphere 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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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Possible climate induced changes: permafrost
Characteristics that could change:
• Area of permafrost
• Thickening of the active layer (layer of seasonally thawed ground
above permafrost)
Guestimated Effects (IPCC)
Assuming:
  permafrost vol   permafrost area;
 present warming trends
 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 (0 to 0.05 mm/yr)
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
MSL: predicted changes
IPCC, 2001:
Global average sea
level changes from
thermal expansion
AOGCM experiments
with observed
concentrations of GHGs
in 20th century;
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
then, following IS92a
scenario for 21st
century;
(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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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
The major climate-related contributing factor to sea level rise is …
IPCC:
Identified 1.5-2.0 mm yr-1 rise during 20th century
Main factor was 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
Where is the rest of the 1.5-2.0 mm yr-1 rise from?
IPCC estimate only 0.2 mm/yr for eustatic (volumetric) MSL rise
i.e.
steric (hsteric) height + (heustatic) = 0.5 + 0.2 = 0.7 mm/yr
So …
Does this mean that the IPCC estimates are actually wrong?
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
The major climate-related contributing factor to sea level rise is …
Total
Eustatic
Temperature
Salinity
The time series are spatially averaged (50ºS to 65ºN),
5-year running means computed for the upper 3000 m of the ocean
Ocean Freshening, Sea Level Rising, Walter Munk, Science 27 June 2003 300: 2041-2043
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
The major climate-related contributing factor to sea level rise is …
Eustatic or steric?
Ocean Freshening, Sea Level Rising, Walter Munk, Science 27 June 2003 300: 2041-2043
Mean salinity of the global ocean has decreased slightly between 1954 - 1997,
implying the addition of fresh water mass to oceans
=> combined steric rise due to temperature and salinity
hsteric = hT + hS = 0.5 +0.05 = 0.55 mm/year
If source of freshening is melting ice sheets and changes in continental water
storage, there must be a eustatic contribution
But, it must not be counted twice i.e. 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
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
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The major climate-related contributing factor to sea level rise is …
Ocean Freshening, Sea Level Rising, Walter Munk, Science 27 June 2003 300: 2041-2043
 = 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
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
The major climate-related contributing factor to sea level rise is …
Ocean Freshening, Sea Level Rising, Walter Munk, Science 27 June 2003 300: 2041-2043
Sea ice covers:
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
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!
Munk: Probably not the solution but gives insight to problem!
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
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Future improvements to observational methods
GPS is being used to fix the position of tide gauge bench marks to an
accuracy  10 mm
Will allow local MSL to be corrected for all local land movement
Allow decoupling of earth movement from tide gauge measurements
Buddy checking – compare records of MSL between different
localities (~ few 100 km)
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Summary - sea level rise
• 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
• Interaction of processes still not well understood
• Global estimates of MSL will improve as satellite and GPS database
and time series increases
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ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics