Communicating uncertainties of future coastal impacts for decision making

As sea-level rises, coastal hazards and risks such as

extreme flooding or erosion are changing. For accurate

impact assessments, several factors must be considered,

such as the variability of sea-level rise and storm surge

patterns. We proceed to a global sensitivity analysis of

future shoreline changes, in order to provide quantitative

insight into the relative importance of contributing

uncertainties over the coming decades. module

Gonéri Le Cozannet1,2, Jeremy Rohmer1, Anny Cazenave3, Déborah Idier1, Franck Lavigne2, Carlos Oliveros1

1: BRGM, Orléans (France); 2: Univ Paris 1 / LGP (France); 3: LEGOS/CNES, Toulouse (France);

Critical questions from decision makers

Conclusions: what information can be provided to support

decision making

Key messages for decision makers:

• The relative ranking of the sources of uncertainties change over the time

• Local coastal processes are the most important during the 1st part of the 21st century,

whereas uncertainties of future sea-level rise scenarios largely dominate beyond 2080

- Communication facilitated by a rigorous approach toward uncertainties

- Limitation: variance based measure

For decision makers concerned with adaptation to climate change in coastal areas, this

approach provides quantitative insight into three key issues related to: (1) the

timeliness of coastal adaptation planning (2) the identification of periods by which rising

sea-levels cause rapid obsolescence of regular adaptation measures (3) the

constraints imposed by different future climate change scenarios for long-term

adaptation planning.

Method: modeling future sea-level rise

Study funded by BRGM with additional support

from the ADAPT-MED project (CIRCLE-2)

Model Future

coastal

impacts

Regional to local coastal parameters Global, regional

and local sea-

level changes

Principle: separate the

variance of the model

outcome into several

terms, corresponding

to the effects of input

parameters and their

interactions

Research question: applying the Global Sensitivity Analysis requires a probability

density function of future sea-level rise

Rapid melting of

ice-sheets Likely range

(IPCC)

Global sea-level rise by 2100 (RCP 8,5 scenario)

Median value

(IPCC) ? ? PD

F (

1/m

)

- Skewed and bounded distribution

→ Beta distribution

- Uncertainties on actual values of

« low » and « high-end » scenarios:

use of conservative values.

RCP 2.6 RCP 4.5

RCP 6.0 RCP 8.5

This probabilistic interpretation

of IPCC scenarios enables

undertaking a variance-based

global sensitivity analysis of

future climate change impacts

Probabilistic Sea-level rise scenarios for the 21st century

IPCC, 2013

Can we provide a quantitative insight into these uncertainties

to support decision making? (and finally coastal adaptation)

What are the potential benefits of climate change mitigation for

coastal areas?

Answering these questions is difficult due to large uncertainties in

future coastal impacts

Can we quantify coastal impacts of climate change for different

scenarios?

Coastal risks prevention is already being managed. When should

specific measures be undertaken to adapt to climate change in

coastal areas?

Examples from national and regional adaptation plans in France since 2008

Method: Global Sensitivity Analysis

Sobol’ (2001);

Saltelli et al.,

(2008).

Shoreline change scenarios uncertainties Ranking the importance of sources of uncertainties

Contribution of a given input parameter alone to the

variance of the model outcome

Expected proportion of the variance of Ft that would

be removed if Xi was known

Si

Defining research priorities

What parameters can be fixed to average values without much

impacts to the variance of the outcome ?

See figure above for description of each parameter below

Contribution of Xi and all its

interractions with other parameters to

the variance of Ft STi

RCP 2.6 RCP 4.5

RCP 6.0 RCP 8.5

Sh

ore

lin

e c

han

ges

(m

/year)

S

ho

reli

ne c

han

ges

(m

/year)

Simulations

suggest the

emergence of

an

observable

shift toward

shoreline

retreats

driven by

sea-level rise

by ~2060.

Uncertainties

are very large

Beach slopes

Climate

change

scenarios

Sea-level rise

Interactions

Storms

Longshore effects

1st part of the

21st century:

growing

importance of

sea-level rise

uncertainties

2nd part of the

21st century:

growing

importance of

climate change

scenarios

Norm

aliz

ed f

irst ord

er

index

Climate

change

scenario

Global

sea-level

rise

Sea-level

variability

Aeolian

processes

Yearly

probability

of storms

Dune

retreat after

a storm

Waves

assymetry Low or high-

energy coast

Human

actions

Longshore

processes Randomness

of storms

Beach

slopes

Processes involved in shoreline changes