Monitoring and Predicting Long Term Global Sea and Land Level Changes

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Monitoring and Predicting Long Term Global Sea
and Land Level Changes

• Philip L. Woodworth
• Permanent Service for Mean Sea Level
• Proudman Oceanographic Laboratory
• psmsl_at_pol.ac.uk

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Contents
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National Sea Level Networks

• The UK A Class Network is an example of a
  national tide gauge network which contributes
  data to the Permanent Service for Mean Sea Level
  (PSMSL) international data bank - see
  http//www.pol.ac.uk/psmsl/

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PSMSL Data Coverage

• Map shows the global coverage of PSMSL data

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Changes in Last 100 Years

• Past 100 years
• Most PSMSL records show evidence for rising sea
  levels during the past century
• IPCC Third Assessment Report concluded that
  there has been a global rise of approximately
  10-20 cm during the past 100 years

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UK MSL change

• UK mean sea level (MSL) is rising, consistent
  with the global picture
• Plot shows MSL "relative" (to the land) as
  measured by tide gauges
• Corrected for local land movements, the
  "absolute" MSL trend has been about 1mm/y
  10cm over past century
• Compare to IPCC prediction of 47cm in 21st
  century

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Two Problems with Present Global Sea Level Data
Set

• (1) Sea Level measurements are relative to land
  level. SOLUTION ? Measure Land Levels using new
  geodetic techniques such as GPS and Absolute
  Gravity
• (2) Uneven geographical distribution the PSMSL
  data set is under-represented in Africa,
  Antarctica etc. and there are no long term
  records from the deep ocean. SOLUTIONS ? IOC
  GLOSS programme to densify the existing tide
  gauge network, and programmes of satellite
  altimetry to measure sea levels from space

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UK GPS Network

• Map shows current network of UK GPS receivers
  which monitor vertical and horizontal land
  movements. (Operated by the University of
  Nottingham in collaboration with DEFRA and POL.)

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Land movements

• In addition to GPS measurements etc. we can use
  geological data and geodynamic models to estimate
  vertical land movements (Shennan, 1989 estimates
  shown here, in mm/yr)
• Land subsidence/uplift can result from
• post-glacial rebound
• water extraction
• sediment compaction
• earthquakes etc. Not all of these processes can
  be modelled.

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Space and deep ocean data
Deep ocean sea level recorders
Satellite Altimetry
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Why Then Do We Need Tide Gauges Now in the Age
of Altimetry?

• Principle of continuity, relative low cost of
• gauges (altimetry has been operational during
  only the last decade)
• Long records for secular trend/acceleration
• studies need to be continued (e.g. for input
  to IPCC)
• Higher frequency sampling important in straits
• and other areas
• High latitude regions of ice coverage cannot be
  monitored by altimetry
• Altimeter calibration
• Coastal applications

Acoustic Gauge in Australia
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Questions and Answers
After all this monitoring there must be many
scientific questions and answers
Q. Has global sea level risen during the 20th
century ? A. Yes. By 10-20 cm. (There are many
references - see the IPCC Third Assessment Report
for a review)
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Questions and Answers
Q. Do we understand why it has risen? A. Yes.
(More or less)
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Why has sea level risen?
Main driver has been the 0.6 º global temperature
change during the past century, but there have
been many contributors to the sea level change
(numbers are approximate in cm) Thermal
expansion (5), Glaciers/ice caps (3), Greenland
(0.5), Antarctica (-1), Ice sheets ongoing since
last glacial max. (2.5), Permafrost (0.3),
Sediment deposition (0.3), Terrestrial storage
(-3.5) . (See IPCC TAR for a review)
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Questions and Answers
Q. Is the rate of rise increasing ? A. No. From
20th century data alone. A. Yes. From 18th-20th
century data.
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Increasing rate of rise

• 6 of the longest sea level records from Northern
  Europe showing a small acceleration of sea level
  change into the 20th century

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Questions and answers
Q. How much might sea level rise in the 21st
century? A. 9-88 cm with central value 48 cm from
35 emission scenarios and 7 AOGCMs (IPCC Third
Report). Predictions relatively insensitive to
emission scenarios over next few decades. (N.B.
rises will not be the same in all parts of the
world because of the readjustment of the ocean
circulation to climate changes.)
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Long Term Changes in Sea Level

• Next 100 years
• a rise between 9 and 88 cm
• a central value of 48 cm
• a rate of approx. 2.2 - 4.4
• times that of the past
• 100 years (IPCC TAR)

Projected sea level rise, IPCC 2001
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Questions and Answers
Q. How important will the 21st century changes
be? A. MSL changes coupled to changing
meteorology, surges, tides, waves (water depth
changes) gt Extreme level analyses.
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Extreme Level Analysis

• This shows the return period of levels being
  exceeded at Lowestoft (a typical east coast port)
  at present (solid line) and with a 50 cm rise in
  MSL (dotted line). The 50 cm causes return
  periods (for a given level above ODN) to be
  reduced by about a factor of 10.

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Coastal areas at risk

• Areas below 1000-year return period level
• By 2100 the
• 1 in 1000 year flood level (shown here in red)
  may become a 1 in 100 year
  level

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Questions and Answers
Q. Will sea level continue rising beyond the 21st
century? A. Yes, for 100s of years (the Sea
Level Commitment) as the lower levels of the
ocean warm.
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Sea Level Commitment

• Scenarios of long term sea level change beyond
  the 21st century (from IPCC).
• Rises of 1-4 m might be expected over several 100
  years.

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• SUMMARY MEASUREMENTS
• TIDE GAUGES
• The PSMSL data set is the basis of understanding
  that sea level has risen during the past 100
  years.
• NEW GEODETIC TECHNIQUES
• Developments in new geodetic techniques (GPS,
  DORIS, Absolute Gravity) are progressing for
  monitoring vertical land movements. This will
  eventually provide estimates of absolute sea
  level change.

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SUMMARY CONTINUED EARTH OBSERVATION
TECHNIQUES (1) Satellite radar altimetry has the
potential to provide truly-global sea level
change estimates, rather than at coastlines as
for tide gauges. However, gauges continue to be
required for continuity, local coastal use and
for altimeter calibration. (2)Space gravity will
provide precise measurements of the geoid and of
temporal gravity variations ? a range of
applications to sea level science. Challenge is
to combine the several measurement techniques
into one global monitoring system.
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• SUMMARY SEA LEVEL TRENDS
• In 20th century, global sea level rose by 10-20
  cm, with a slow acceleration between the 19th and
  20th centuries.
• In 21st century, the rise could be of order 50 cm
  (IPCC TAR). (However, note the importance of
  interannual variability and of the role of
  extremes)

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SUMMARY UK CONTRIBUTIONS TO THIS FIELD UK
(POL/NERC) hosts the PSMSL global sea level data
bank. Funds monitoring in UK and South Atlantic,
Gibraltar etc. BAS (NERC) expertise in
glaciology Hadley Centre (Met Office) is a
world centre for modelling sea level change due
to climate change Tyndall Centre (NERC/hosted
by UEA) for collaborative studies of impacts of
climate change DEFRA plays major role in
supporting UK sea/land level monitoring and
science POL will lead the formation of a UK
National Tidal Sea Level Facility in 2002