Transcript No Slide Title

Monitoring and Predicting Long Term
Global Sea and Land Level Changes
Philip L. Woodworth
Permanent Service for Mean Sea Level
Proudman Oceanographic Laboratory
[email protected]
www.pol.ac.uk
Contents
How Do We Measure Sea/Land Level
Changes?
How Much has Sea Level been
Changing in Recent Years and Why?
How Much will it Change in the Near
Future? And Why?
What are we (UK) doing in this field?
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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
4
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
5
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
Satellite Altimetry
Deep ocean sea
level recorders
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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
Acoustic Gauge in
Australia
• High latitude regions of ice coverage
cannot be monitored by altimetry
• Altimeter calibration
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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) ==> Extreme level analyses.
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Extreme Level Analysis
Lowestoft
3.6
3.4
Height above O.D.N . (m)
• 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.
3.2
3.0
2.8
2.6
2.4
2.2
2.0
1
10
100
1000
10000
Return Period
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Coastal areas at risk
• Areas below 1000year 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 trulyglobal 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
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