Coastal vulnerability to climate change-induced sea-level rise may be increased by land motion and

human factors Ballu, V., S. Calmant, J. Aucan, V. Duvat-

Magnan, B. Pelletier, M. Becker, M. Gravelle, M. Karpytchev, P. Valty, L. Testut, CK. Shum, F. Hossain, Z. Khan, P. Simeoni and T. Kanas

IPCC 5 AR5: Past and future sea level rise.

Very high emission RCP8.5 scenario

Very low emission RCP2.6 scenario

Vertical land motion and sea-level altimetric trends

1 cm/yr land uplift

1 cm/yr land subsidence

1 cm/yr « absolute » sea-level rise derived from altimetry

Velocities field is from ULR6 solution prepared for IGS2 reprocessing See www.sonel.org

Relative and « absolute » sea- level variations sea-level altimetric trends – vertical land contribution

1 cm/yr relative SLR

1 cm/yr relative SL drop

1 cm/yr « absolute » sea-level rise derived from altimetry

Velocities field is from ULR6 solution prepared for IGS2 reprocessing

Torrès Islands (Vanuatu)

Coastal vulnerability increased by tectonic vertical movements

and human factors

“possibly the world’s first community to be formally moved out of harms way because of climate change” [2005 Climate Conference in Montreal, UNEP].

Coastal flooding :

Climate related sea level rise?

Tectonic subsidence?

or a combined effect?

?

1993-2014 Regional mean sea level trend in mm/yr

AVISO/CNES/CLS data 2015

Torrès

Ground motion - Interseismic (~1cm/yr) - Co-seismic

Torres

~12cm/yr

Ground motion - Interseismic (~1cm/yr) - Co-seismic

+ Sea level variations - Seasonal + ENSO - long term

Ground motion - Interseismic (~1cm/yr) - Co-seismic

+ Sea level variations - Seasonal + ENSO - long term

=

Relative sea level change

Ballu et al., PNAS 2011

Before After

The dominant factor, in this particular case, is not the global warming…

Coastal vulnerability to SLR is exacerbated by land motion, but also by anthropogenic factors (new settlements in improper location)

Mis-identifying the causes of SLR may lead to non-optimal choices for adaptation strategies…

Gange-Brahmaputra-Meghna delta

Role of flexure due to sediment load

Visco-elastic modeling of the loading effects

Fig.2 Observed vertical land movement rates (Steckler

et al., 2012) Fig.2 Observed vertical land movement rates (Steckler

et al., 2012)

(A) Dynamics of sedimentation

(B) sediment thickness (m) deposited since 11

kyr B.P. after Goodbred and Kuehl (2000).

Fig.4 Sediment thickness (m) of the loading disks

according to Fig.3 . The underwater disks are also

loaded by the water due to sea level rise

A

B

Present-day subsidence due to eustatism and sediment loading since 11kyr

Other contributions to ground level :

• Sediment compaction • Tectonic contribution • Sediment influx

perturbation (embarkments) • Water pumping, etc…

Observed vertical land movement rates (Steckler et al., 2012)

Significant contribution of the lithospheric flexure to total subsidence

Past climate change and associated sedimentation is playing a role in today’s vulnerability to SLR.

Duvat, 2010

KIRIBATI / TARAWA An exemple of mal-adaptative pathway

KIRIBATI Tarawa

Regional MSL trend from Jan 1993 to Dec 2014 (mm/yr)

From CNES/Legos/CLS data, 2015

Are all inhabitants from low lying atolls facing the same issues?

• Not directly exposed to cyclones • No strong vertical land motion • Exposed to king tides (high astronomical tides

during La Niña periods)

Many impressive images…

Duvat, 2010

TARAWA, KIRIBATI (1.2°N)

N

Lagoon coast

Ocean coast

0 100 200 m

0.64 - 1.02 m

1.03 - 1.41 m

2.19 - 2.57 m

1.81 - 2.18 m

1.42 - 1.80 m

2.58 - 2.96 m2.97 - 3.35 m

0.25 - 0.63 m-0.14 - 0.24 m

1.10

1.7

2.30

1.40

1.8

1.64

1.91

1.88

1.14

1.66

1.941.62

2.01

2.27

1.48

3.27

1.99

1.55

2.04

3.19

0.36

1.32

1.88

1.43

1.191.73

1.851.58

1.31

2.17

0.23

0.27

1.581.36

1.54

1.48

2.24

0.76

1.91

1.94

1.541.99

1.12

1.23

2.111.31

1.96

1.89

1.75

1.52

1.67

1.96

0.97

1.15

1.07

0.34

2.30

17,567 (5.3%)

236,546 (72.1%)

72,884 (22.2%)

1,204 (0.4%) 0

50000

100000

150000

200000

250000

<1m 1-2m 2-3m >3m

Are

a m

2

Elevation Duvat et al., 2013

Very low lying land

2007

1969

Lagoon coast

Ocean coast

0 400 m200

N

Lagoon coast

Ocean coast

Mangrove

Dense coastal vegetation

Coconut plantation

Coastline Reclaimed land

Aggregate mining and dredging

Coastal defences (seawalls, groynes...)

Buildings

Based on aerial photograph (1969) and Quickbird satellite image (2007) +

Fieldwork April-May 2011

Breach

Coastal defenses, erosion, sand mining… => Maladaptative trajectories of change have increased population and assets exposure to SLR.

Instable and submersible land => uninhabited

Duvat et al., 2013

Massive population influx

Thank you

• Vulnerability to climate change-induced SLR may be increased by land vertical motion and anthropogenic factors

• Many other examples • Need local assessment for proper action/adaptation

For questions…

Bangladesh sediment loading

• The ocean loading was computed by using

the eustatic sea level rise curve from Bard

et al. (1996).

• The sediment loading spatial pattern was

built from the isopach map of Goodbred

and Kuehl (fig.3B) . Uniform spatial

distribution of sediment thickness was set

for the disks outside the isopach map

limits.

• The results presented below correspond to

the lithosphere thickness of 50 km with the

PREM averaged rigidity modulus 52.5 GPa

and the upper and lower mantle viscosities

set to 3*1020Pa*sec and 1021Pa*sec.

Sediment thickness (m) of the loading disks.

The underwater disks are loaded by the water

due to sea level rise

Past climate and induced Holocen high sedimentation rates play a significant role in the present-day subsidence in Bangladesh => increase vulnerability to SLR

Sediment loading

Sea level rise loading

Brown and Nicholls, 2015

Auerbach et al., Nature Climate Change, 2015: Elevation disparity between the natural and poldered landscapes represents ~2 cm/yr of elevation change since polder construction.

High interannual variability

40 cm

El Niño / La Niña

Same process, spatial variability

www.sonel.org

Port-Vila tide gauge / continuous GPS

1km

VILA

VANU

TGBM

PTVL

Vertical land motion and sea level altimetric trend South West Pacific Region

TONGA

HAWAI

PNG

VANUATU FIJI

TUVA

KIRI

GUAM

SEYCHELLES

REUNION

MAURICIUS

BANGLADESH

MALDIVES

Vertical land motion and sea level altimetric trend Indian Ocean Region

0 400 m200

N

Lagoon coast (1,997 m)

Ocean coast

Lagoon coast

Ocean coast (1,955m)

Swamp (2,750 m)

Total (6,702 m)

Accretion (ha) Erosion (ha) Net change (ha)

+ 0.64

+ 0.16

+ 6.70

+ 7.50

- 2.09

- 1.01

- 0.70

- 3.80

- 1.45

- 0.84

+ 5.99

+ 3.70

Land gain (natural accretion + reclamation)

Land loss (coastal erosion)

Stability

Areas exposed to flooding

Shoreline changes between 1969 and 2007

Exposure to flooding

Breach

Duvat et al., 2013

0 400 m200

N

Ocean coast

Lagoon coast

Level 1 (low exposure)

Level 2 (medium exposure)

Level 3 (high exposure)

BUILDING EXPOSURESHORELINE EVOLUTION (1969-2007)

Land gain (natural accretion + reclamation)

Land loss (coastal erosion)

Stability

Areas exposed to flooding

Level 0 (no exposure)

Level 4 (very high exposure)EXPOSURE TO FLOODING

COASTAL PROTECTION

Good condition seawall

Breach Not surveyed area

Not surveyed building

Tarawa - Eita

N

Lagoon coast

Ocean coast

0 100 200 m

0.64 - 1.02 m

1.03 - 1.41 m

2.19 - 2.57 m

1.81 - 2.18 m

1.42 - 1.80 m

2.58 - 2.96 m2.97 - 3.35 m

0.25 - 0.63 m-0.14 - 0.24 m

1.10

1.7

2.30

1.40

1.8

1.64

1.91

1.88

1.14

1.66

1.941.62

2.01

2.27

1.48

3.27

1.99

1.55

2.04

3.19

0.36

1.32

1.88

1.43

1.191.73

1.851.58

1.31

2.17

0.23

0.27

1.581.36

1.54

1.48

2.24

0.76

1.91

1.94

1.541.99

1.12

1.23

2.111.31

1.96

1.89

1.75

1.52

1.67

1.96

0.97

1.15

1.07

0.34

2.30

17,567 (5.3%)

236,546 (72.1%)

72,884 (22.2%)

1,204 (0.4%) 0

50000

100000

150000

200000

250000

<1m 1-2m 2-3m >3m

Are

a m

2

Elevation Duvat et al., 2013

Torrès islands are not unique…

Vertical land movements are not uncommon, in particular near plate tectonic boundaries.

Can vary spatially significantly.

Taylor et al. 2008, Nature Geosciences,

Solomon, M8.1 EQ, April 2007. Subarya et al., Nature, 2006

Sumatra, M9.3 EQ, Dec. 2004

Sieh et al., Science, 2008

Seismic elastic cycle

• When subsidence is sudden (earthquake), the sea-level rise should be easily attributed to ground motion.

• However, in active areas, for an EQ to happen, stress need to accumulate for decades or centuries and slow vertical motion (uplift or subsidence) can be associated with this stress accumulation => slow relative sea-level changes can occur in between earthquakes (interseismic motion).

Co-seismic deformation

Post-seismic

deformation

Long term motion

Interseismic periods: 50 to 200 years

VANUATU • Torres islands are rising by ~1mm/yr on the long term (Taylor et al. 1985).

• Long-term and short-term processes are superimposed

Typical seismic cycle in subduction zones

Sea surface height variations from satellites

- SLR well above 2-3 mm/yr…

- Not linear

- Seasonal and inter-annual

variations (El Niño/La Niña

oscillation)

1993-2014 Regional mean sea level trend in mm/yr

AVISO/CNES/CLS data 2015

Torrès

1994 2002 1998 2006 2010