Ostende GLOSS course, November 2006

Measuring Long Term Sea Level Change

Philip L. Woodworth

Proudman Oceanographic Laboratory, Liverpool

plw@pol.ac.uk

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Contents

• Methods for Measuring Sea and Land Level Changes

• Problems with the Data Sets

• What We Know about How Sea and Land Levels have Changed and will Change

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Long Term Sea Level Changes

• We know from geologists that sea level has changed over many1000s of years largely as a result of the exchanges of water between the ocean and ice caps

• So we should not be too surprised if sea level is still changing

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520K BP

615K BP

712K BP

89K BP

97K BP

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Long Term Sea Level Changes

• For this talk our ‘long term’ is the last 200 years (since the invention of the ‘tide gauge’ or ‘sea level recorder’) and the next 100 years

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Local Hero

William Hutchinsonmeasured the heightsand times of highwaters at the Old Dockgates Liverpool1764-1793

These were the firstsystematic tidalmeasurements in theUK

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ClassicalFloatGauge(from about1832)

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UK Float Gaugeat Holyhead

Float gaugesare still importantand can be madeinto digital gaugeswith the use ofencoders

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Countries Share Data through International Data Banks such as

The Permanent Service for Mean Sea Level

on behalf of the International Council for Science

which is based at Proudman Oceanographic Laboratory in Liverpool

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Sea Level 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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There are only medium-length African records

Mombasa

Zanzibar

Port Louis

Simons Bay

Takoradi

Alexandria

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There are Two Main Problems with the Present ‘Global’ Sea Level Data Set

• ‘Sea Level’ measurements are relative to land level.

SOLUTION Measure Land Levels using new geodetic techniques such as Global Positioning System (GPS) and Absolute Gravity (AG)

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Land Movements can be:

Slow and monotonic e.g. Glacial Isostatic Adjustment (Post Glacial Rebound)

or

• Fast and irregular e.g. tectonics in Chile, Alaska etc.

Geodynamic models of the solid Earth exist only for GIA and even they are not perfect.

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GIA/PGR

Earthquake

Ground-water pumping

Harbour development/Sedimentation

For comparison:Hot spot (normal?)

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GIA models for VLM corrections

Peltier ICE-4G GIA model

N.B. There is nowhere on Earth which does not experience some effect of GIA

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There are Two Main Problems with the Present ‘Global’ Sea Level Data Set

• ‘Sea Level’ measurements are relative to land level.

SOLUTION Measure Land Levels using new geodetic techniques such as GPS and Absolute Gravity

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Two Main Geodetic Tools for Measuring Land Level Changes

GPS Absolute Gravity

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• The current network

of UK GPS receivers

which monitor vertical

and horizontal land

movements.

(Operated by the

University of

Nottingham with POL.)

GPS Networks (UK)

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There are Two Main Problems with the Present ‘Global’ Sea Level Data Set

• 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 The GLOSS programme to densify the existing tide gauge network, and programmes of satellite altimetry to measure sea levels from space. Also some deep ocean measurements.

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The GLOSS Programme

GLOSS Core Network (GCN) with approx. 300 stations

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Complementary Tools for Measuring Sea Level Changes

Tide Gauges Satellite Altimetry Sea Floor Systems

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We can use the PSMSL data set (in spite of its recognised deficiencies) to ask a number of questions about Past and Possible Future

Sea Level Changes

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Questions and Answers

Q. Has global sea level always been changing ?

A. Yes. Sea level has changed by over 120 m during the last 20,000 years, sometimes changing at a rate 10 * faster than the presently-observed rate of change.

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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 (the Fourth Assessment Report is in preparation)

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20th Century Sea Level Rise Estimates Region, VLM Rate ± s.e.

(mm/yr)Gornitz and Lebedeff (1987) Global, Geological 1.2 + 0.3Peltier and Tushingham (1989, 1991) Global, ICE-3G/M1 2.4 + 0.9c

Trupin and Wahr (1990) Global, ICE-3G/M1 1.7 + 0.13Nakiboglu and Lambeck (1991) Global decomposition 1.2 + 0.4Shennan and Woodworth (1992) NW Europe, Geological 1.0 + 0.15Gornitz (1995)d NA E Coast, Geological 1.5 + 0.7c

Mitrovica and Davis (1995),Davis and Mitrovica (1996) Far field, PGR Model 1.4 + 0.4c

Davis and Mitrovica (1996) NA E Coast, PGR Model 1.5 + 0.3c

Peltier (1996) NA E Coast, ICE-4G/M2 1.9 + 0.6c

Peltier and Jiang (1997) NA E Coast, Geological 2.0 + 0.6c

Peltier and Jiang (1997) Global, ICE-4G/M2 1.8 + 0.6c

Douglas (1997)d Global ICE-3G/M1 1.8 + 0.1Lambeck et al. (1998) Fennoscandia, PGR Model 1.1 + 0.2Woodworth et al. (1999) UK & N Sea, Geological 1.0 + 0.2

N.B. All these analyses use the same PSMSL data set

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Questions and Answers

Q. Do we understand why it has risen?

A. Yes. (Or at least ‘more or less’)

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Why has sea level risen?

Main driver has been the

0.6 ºC global temperature change during the past century

but there have been many contributors to the sea level change.

See IPCC TAR for a review (4AR will be published in February 2006).

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36 Melting Low Latitude Glaciers (Alaska etc.)

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-1.0 -0.5 0.5 1.0 1.5 2.00.0

0.0 1.0-1.0

mm/year

2.0 mm/year

Thermal expansion

Glaciers

Greenland (present)

Antarctica (present)

Ice sheets (long term)

Permafrost

Sedimentary deposits

TOTALTOTAL

OBSERVATIONSOBSERVATIONS

Continental waters

2020thth Century Sea Level Rise - IPCC 2001 Century Sea Level Rise - IPCC 2001

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2020ththCentury Sea Level Rise:Century Sea Level Rise:is the 1.5 mm/yr observed rateis the 1.5 mm/yr observed rate

too high?too high?OROR

are climate-related are climate-related contributions (0.7 mm/yr)contributions (0.7 mm/yr)

underestimated?underestimated?

The enigma... (Walter Munk, 2001The enigma... (Walter Munk, 2001))

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Questions and Answers

Q. Is the rate of rise increasing ?

A. Yes. On basis of study of long records spanning 18th to 20th centuries.

A. Not clear. On basis of 20th century tide gauge data alone.

A. Yes. On basis of altimeter and tide gauge from the 1990s.

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Sea level change contains an acceleration of sea level rise from the 19th to the 20th centuries probably due to climate change

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Questions and Answers

Acceleration in the1990s?

• Look at altimetry

• Look at tide gauge records.

• Look at IPCC-type models.

Answer:

• Yes

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Rate of sea level rise (Jan. 1993-Dec.2004) Rate of sea level rise (Jan. 1993-Dec.2004) 3 mm/yr 3 mm/yr

Topex/PoseidonTopex/Poseidon

Jason-1

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Sea level trends (from Anny Cazenave)

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from 177 PSMSL tide gauge records in 13 regions (Holgate and Woodworth, 2004)

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Church et al. (2004)

Church et al., 2004

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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 of 48 cm based on 35 emission scenarios and 7 AOGCMs (IPCC Third Report).

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Changes in Sea Level in the Next 100 Years

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. Will the sea level rise in the 21st century be the same everywhere?

A. No. Rises will not be the same in all parts of the world because of the readjustment of the ocean circulation to climate change, and also due the different magnitude of local vertical land movements.

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Questions and Answers

Q. How important will the 21st century changes be?

A. When MSL (or water depth) changes, and when also there are coupled changes in regional meteorology, there will be changes in storm surges, tides, waves and extreme water levels.

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It is important to keep in mind that these rising Sea Levels sooner or later lead to changes in Extreme Levels and often to local flooding. (This is not only a ‘Scientific’ exercise.)

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Floods in the Irish Sea 2002

Douglas

Ramsey

"the worst in living memory"

Many £M’s damage in few hours

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Tuvalu Floods 2002

Typical coral islandPoorinfrastructure

Erosion Plant damage

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Coastal Flood Plain

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Coastal areas at risk

• Areas below 1000-year return period level

• By 2100: the1 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. How important will the 21st century changes be?

A. When MSL (or water depth) changes, and when also there are coupled changes in regional meteorology, there will be changes in storm surges, tides, waves and extreme water levels.

A. Also there will be impacts on environmental and socio-economic infrastructure at the coast.

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Projected Coastal Megacities: 2010 (>8 million inhabitants)

Istanbul

Lagos

Lima

Buenos Aires Rio de Janeiro

MadrasKarachi

Jakarta

Calcutta

Bombay

Bangkok

Manila

Shanghai

Osaka

Tokyo

Seoul

Tianjin

Dhaka

New York

Los Angeles

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There is an enormous literature on the subject of impacts of sea level rise and especially good studies of potential impacts in Europe.

(If anyone interested I can provide further details.)

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Questions and Answers

Q. Will sea level continue rising beyond the 21st century?

A. Yes. It will continue rising for 100s of years. This is called the Sea Level Commitment i.e. a continued rising sea level 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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Watch this space for the IPCC 4AR

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Some Conclusions • Sea level has always been changing and since the last interglacial (120,000 years BP) has varied by over 120 metres – we are at an historical geological high sea level at the moment (even before any future climate change considerations)

• Sea level has risen during the last century by 10-20 cm and could rise even more during the next 100 years

We have to measure sea and land level change as well as we can and try to understand why they change.

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The End