Impact ofAnthropogenic Land-Cover Changes

on

1) the global radiative forcing of planet EarthEdouard Davin, Nathalie de Noblet, Pierre Friedlingstein

2) the characteristics of the El-Niño Southern Oscillation

Edouard Davin, Nathalie de Noblet, Christian Laguerre,Pascal Terray, Eric Guilyardi

Laboratoire des Sciences du Climat et de l’EnvironnementUnité mixte CEA-CNRS-UVSQ / Gif-sur-Yvette / France

3 snap-Shot experimentsfully coupled Atmosphere-Ocean (IPSL-CM4)

+ interactive seasonally varying foliage density (LAI)

200-year long simulations, analyses on last 50 years,all with pre-industrial aerosols and GHGs

PAST- pre-industrial land-cover (year 1870)PRES – present-day land-coverFUTU – Future land-cover (year 2100, SRES

A2)

Ramankutty & Foley, 1999 ; Goldewijk, 2001

IMAGE2 SRES A2 (Alcamo et al. 1998)

Change in anthropogenic land fraction (crops + pastures)

PRES - PAST FUTU - PRES

Part I:

Impact of land cover change on surface climate:

relevance of the radiative forcing conceptEdouard Davin, Nathalie de Noblet-Ducoudré,

Pierre Friedlingstein

Submitted to GRL

Question addressed in this paper :

is the radiative forcing concept

applicable to the climatic impacts

of the land-use induced land-cover

changes ?

Reminder: concept of radiative forcing

Instantaneous forcing Adjusted forcing Climatic Change

Quantity (e.g. CO2) ΔQi (change in radiative forcing, W/m-2)

ΔTs = λ * ΔQ

Radiative forcings from 1750 till 2005

175017508-9 millions km8-9 millions km22 of anthropogenic land-cover of anthropogenic land-cover

(6-7% of land areas)(6-7% of land areas)

1990199046-51 millions km46-51 millions km22 of anthropogenic land-cover of anthropogenic land-cover

(35-39% of land areas)(35-39% of land areas)

due to the sole changes in

land-surface albedo

We have computed the radiative forcingdue to land-cover changes from 1870 to

present-dayand from present-day to 2100

Simulatedclimatology

land surface modelORCHIDEE

ΔΔ albedo

ΔΔ evapotranspiration

ΔΔ water vapor content

radiative transfer schemeof our AGCM (LMDz)

ΔΔ F albedoΔΔ F water vapor

Change in land surface characteristics prior to any atmospheric feedbacks

Conversion into radiative forcings

Land cover change

Change in anthropogenic land fraction (crops + pastures)PRES - PASTPRES - PAST FUTU - PRESFUTU - PRES

Annual mean radiative forcing (albedo + water vapor)PRES – PASTPRES – PAST FUTU - PRESFUTU - PRES

Changes in the global annual radiative forcing (W/m-2)

due to albedo changes alonedue to evapotranspiration changes alone

-0,7

-0,6

-0,5

-0,4

-0,3

-0,2

-0,1

0

Preindustrial to Present Present to Future

- 0.29 W/m-2

-0.70 W/m-2

Changes in the global annual mean surface temperature (°C)

due to albedo changes alonedue to evapotranspiration changes alone

-0,14

-0,12

-0,1

-0,08

-0,06

-0,04

-0,02

0

Preindustrial to Present Present to Future

- 0.05°C

- 0.14°C

FUTU – PRESFUTU – PRESPRES - PASTPRES - PAST

ΔT = - 0.05 K ΔT = - 0.14 K

Spatial patterns of the changes in the global annual mean surface temperature (°C)

Derived climate sensitivityΔ Ts / (Δ F - Δ R)

1. Climate exhibits the same sensitivity to both historical and future land cover change.

Simulations Δ F

(W/m2)

Δ Ts

(K)

Δ Ts / (Δ F - Δ R)

K/(W.m-2)

PRES - PAST - 0.29 - 0.05 0.3

FUTU - PAST - 0.7 - 0.14 0.27

Climate sensitivity inferred from a2xCO2 experiment : 1 K/(W.m-2)

2. Climate sensitivity to land cover change is 70 % lower than the sensitivity to CO2 forcing.

2 hypothesized causes: Spatial inhomogeneity

limited poleward extent of land-cover changes limited sea-ice feedback.

but this only explains about 20% of the reduced sensitivity

Non-radiative effectsLand-cover changes applied decreased net radiation absorbed by the land-surface decreased evapotranspiration BUT increased sensible heat (due to increased Bowen Ratio)

Why is the climate sensitivityderived from land-cover changes different from

the one derived from CO2?

(figures taken from Kabat et al.: Vegetation, Water, Humans, and the Climate, IGBP BAHC)

Decreased evapotranspiration

decreased water vapor greenhouse effect

cooling of the troposphere

Increased sensible heat warming of the boundary layer

over deforested areas

Value used to quantify the climate sensitivity

Deforestation, wherever it occurs, leads to global cooling through its biophysical effects on climate

The climate sensitivity derived is the same wherever deforestation occurs the radiative concept may be applicable (similar work needs to be carried out with other models, other deforestation locations and intensities)

But the derived climate sensitivity is 70% lower than the one derived from changes in CO2.

In conclusion

Part II:

Influence of future land-use inducedland-cover change

on the characteristics ofthe El-Niño Southern Oscillation

Edouard Davin, Nathalie de Noblet-Ducoudré,Christian Laguerre, Pascal Terray, Eric Guilyardi

(Prelimirary results)

Large changes in the tropical regions

+Global oceanic cooling

Potential effects on

ENSO

Land-use induced land cover changes between present-day and year 2100

futur

PRES30 %

FUTU34 %

Observed (HADISST)30 %

First EOF of sea-surface temperature% of tropical variability explained by this

first EOF

Question addressed :

what is the impact of this

increased variance on the

frequency and/or characteristics of

the El-Niño events ?

1) No significant change in frequency

Observations

El Niños are more frequent in the model

than in reality

Year-1

Year-1

Power spectrum

Based on the following criterias:

examine anomalous SSTs (with respect to modelled climatology) in the Niño 3 box:

5°S-5°N / 150°W-90°W

Select the years exhibiting anomalous SSTs larger than 1.5* during 3 consecutive months between October and February

2) Selection of modelled El-Niñosover a 100 year-long time period

Selected eventsanomalous SSTs over a 3-year long time

period

El Niño events

Selected eventsanomalous SSTs over a 3-year long time

period

PRESFUTU

SSTs are colder at present, prior to the

event

SSTs are warmer in FUTU at the

peak of the event

PRESFUTU

El Niño compositesanomalous SSTs over a 3-year long time

period

time versus longitude diagram of anomalous SSTs

composite El Niño for each set of simulations

TIM

E

PRES FUTU

COLDER

WARMER

earlier onset of warming

similar timing of termination

Statistically significant spatial patternsof anomalous SSTs through time

PRES FUTU

Earlier warming

of the Indian ocean

Jan

uary

Ap

ril

July

Octo

berM

arc

hJu

ne

Sep

tem

berD

ecem

ber

Niño 3Indian Ocean

Comparing SST changesin the Niño 3 region to those in the Indian

OceanPRES

FUTU

~3.5 months

~3 months

Amplitude of warming in

Indian ocean almost as large

as in Pacific

What can explain the earlier onset of El Niño ?

What can explain the earlier onset of El Niño ?

hypothesis

With a deforested Amazon

Ascending motion reduced over the AmazonShift of the

main convective cell

towards the East Pacific

Reduced Walker

cirulation

What can explain the earlier onset of El Niño ?

The model shows changes in upper-level divergence

PRES

FUTU - PRES

Divergence at 200hPa

Change in divergence at 200hPa

An

nu

al m

ean

s

Reduced upper-level

divergence

decreased upper-level convergen

ce

DJF

MAM

JJA

SON

FU

TU

- P

RES

Change in divergenceat 200hPa

differences in zonal mean windbetween 200hPa and 850hPa. Winter values (DJF)

PR

ES

FU

TU

- P

RES

Weakening of the Walker

circulation

FUTU – PRESFUTU – PRES

Changes in surface temperature in global annualmean already exhibits an El Niño like pattern

Tropical deforestation reduces ascending motions above the Amazon basin reduced subsidence in the eastern side of equatorial Pacific slowing down of the Walker circulation.

The frequency of ENSO events is not modified

But the warm phases of ENSO start earlier, and the maximum temperature anomaly reached is larger.

+ Temperature changes in the Indian ocean are as large as in the eastern Pacific.

In conclusion