Climate change: certainties and uncertainties Hervé Le Treut, Laboratoire de Météorologie Dynamique (CNRS/Ecole Normale Supérieure/Ecole Polytechnique

Climate change: certainties and uncertainties

Hervé Le Treut,

Laboratoire de Météorologie Dynamique

(CNRS/Ecole Normale Supérieure/Ecole Polytechnique/ Université Paris 6)

Académie des sciences

Plan of the talk

• 1. The main greenhouse gases

The emission by human activities: recent evolution

The time scales in question: are the past climate variations relevant indicators?

How do atmospheric chemical changes affect the Earth radiative budget?

• 2. Consequences

What is already detectable?

How can we predict future evolutions? Models and scenarios.

What are the forecasts for the coming century?

During the last century, the atmospheric composition has undergone changes which are unprecedented over the last millenia

IPCC, 2001

The yearly use of energy (and associated CO2 emissions) have increased sharply since the second world war, with a direct impact on CO2 concentration.

Schilling & al + Observatoire énergie + AIE, cited by Jancovici

100 millions years = 100 000 millenia

G. Jacques, communication personnelle

Louis Agassiz

Eccentricity:~ 100 000 years

Precession of the equinoxes:~ 21 000 years

Obliquity:~ 40 000 years

MilankovitchQuaternary evolution follows slow patterns of evolution

Illustrations de S. Joussaume (éditions du CNRS,1993)

The rate of current changes is large when compared with observed evolutions over the Quaternary era.

GIEC, 20001 (CNRS/CEA)

Changes of global temperature over the last millenium are within a few tenths of a degree

Mann, IPCC, 2001

Changes in the atmospheric composition act on complex and interactive systems which had

previously reached equilibrium:

First example of a complex system: the global carbon cycle

GIEC, 1990 - CDIAC

Emission de CO2 par habitant

USA

Canada

Australie

Belgique

Allemagne

Japon

Royaume Uni

Italie

France

Ukraine

Chine

Corée du Sud

Afrique du Sud

Venezuela

Chili

Mexique

Cuba

Inde

0,00 5,00 10,00 15,00 20,00 25,00

Pays

tonne par habitant

COCO2 2 emissions per habitant emissions per habitant

0.5 1 2

Emissions de GES par kg d'aliment

0 2 4 6 8 10 12

Lait de vache

Farine

Oeufs

Poulet fermier

Cochon

Fromage pâte crue

Fromage pâte cuite

Beurre

Boeuf

Mouton

Veau

kg C

Greenhouse emission per kg of produced food (in France)Greenhouse emission per kg of produced food (in France)

Jancovici, 2002Jancovici, 2002

0 20 40 60 80 100 120

Train SNCF

Train RU

Bus

Voiture (route)

Avion (long courrier)

Voiture (ville)

Avion (court courrier)

Mo

de

de

tran

spo

rt

Emissions, gC par passager.km

Jancovici, 2002Jancovici, 2002

Greenhouse gas emission per passenger and kilometer (in France)Greenhouse gas emission per passenger and kilometer (in France)

0

50

100

150

200

250

natural anthropogenic

swamps, rice

termits

ocean

hydrates

energy

litter, wate

cows

biomass

Methane sources in millions of tons per year

Pollution:brings

Pollution: brings CH4OH

O3

Another example of a complex system: the energy cycle

GIEC, 2001

Other greenhouse

gases30%

Clouds15%

Water vapor55%

Atmospheric constituents contributing to the greenhouse effect

NaturalNatural

(155 W/m(155 W/m22))

CH4

17%

Halocarbons12%

Ozone13%

N2O5%

CO2

53%

AnthropogenicAnthropogenic

(2.8 W/m(2.8 W/m22))

Forç

ag

e r

ad

iati

f p

ar

ton

ne é

mis

eForç

ag

e r

ad

iati

f p

ar

ton

ne é

mis

e

Evolution of the greenhouse effect after Evolution of the greenhouse effect after some instantaneous emission of one ton some instantaneous emission of one ton

Année après la perturbationAnnée après la perturbation

Hauglustaine D., LSCEHauglustaine D., LSCE

Greenhouse Warming Potential

8 9005 7003 90050 000CF4

5401 7004 80012HCFC-22

4001 3003 30014HFC-134a

32 40022 20015 1003 200SF6

156296275114N2O

1 600 4 6006 30045CFC-11

5 20010 60010 200100CFC-12

7236212CH4

111150 CO2

GWP (100 years)

GWP (50 years)GWP (20 years)Time scale (years)

Greenhouse Gas

Radiative forcing of anthropogenic elements (1750 à 2000)

IPCC [2001]

Agence Européenne pour Agence Européenne pour l’Environnementl’Environnement

Are consequences already perceptible ?

Mann, IPCC, 2001

IPCC, 2001

Change in the extension of mountainglaciers

Climate modelling: an old dream which became possible in the last decades

Richardson (1922)

The weather machineAn artist view of recent climate models(L. Fairhead /LMD-CNRS)

The real world (synthetic radar image)

The « simulated planets » are now very similar to the real one

Simulation carried out using the Japanese Earth Simulator, JSTEC

Earth simulator

Atmosphere and ocean: two fluids which act as partners

Increasingly complex models: The IPSL-CM4 Earth System Model

Physics

CarbonCycle

Chemistry

Continents Atmosphere OceansIPSLCM4

Atmosphericcirculation

Ocean circulation Sea ice

Carbon

DMS

Nutrients

ChemistryGases

&

Aerosols

CO2

Land Surface

Soil and vegetation

LMDZ

Orchidée LMDZT

ORCALIM

INCA

STOMATE PISCES

Carbon

CH4, VOCs,

Aerosols

Aerosols

Marine biologyand biogeochemistry

Terrestrialbiogeochemistry

Salt

Clouds simulated by models in 1990

… and in 2000

Model evaluation: precipitations

IPCC scenarios: a wide range ofpossible futures(without taking into account specific policies, such as those

linked with Kyoto protocol)

Surface air temperature

Comit.

IPSL-CM4 coupled model

A1BA2

Control

A2 Without sulf. aerosol

B1

IPCC 2001 Report

• Changes in global mean temperature:

Unmodified for the last 20 years

Geographical disparities

Climate change for two models and two scenarios: Temperature

A2

B1

CNRM IPSL

Climate change with two models : Precipitations

A2

CNRM

IPSL

Agreement between models is scale dependent

CMIP models

Oceans: Are changes irreversible?

IPCC, 2001

Gordon (86)

Changes in sea level: A delayed effect

IPCC, 2001

Will the moderation effect of vegetation last for a long time?

Will vegetation continue to mitigate the atmospheric increase of carbon dioxide?

.CO2 absorptionwith climate change

Without climatechange

Difference of the two

Other risks are not well diagnosed: Greenland melting, methane from permafrost …

Regional approaches: PRUDENCE changes in summer precipitations

Prudence EU project

Conclusions

• Climate change will constitute one of the important sources of tension affecting the Earth during the century to come.

• A certain level of climate change is now unavoidable; the global amplitude of these changes, their rate, the nature of their impacts, however, depend on our ability to curve down greenhouse gases emissions

• The risks resulting from our unability to predict the details of future climate changes, the possibility of thresholds above which changes may be amplified, reinforce the need for immediate actions