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 CO 2 emissions) have increased sharply since the second world war, with a direct impact on CO 2 concentration. Schilling & al + Observatoire énergie + AIE, cited by Jancovici

100 millions years = millenia G. Jacques, communication personnelle

Louis Agassiz

Eccentricity: ~ years Precession of the equinoxes: ~ years Obliquity: ~ years Milankovitch Quaternary 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, (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, CDIAC

CO 2 emissions per habitant CO 2 emissions per habitant

Greenhouse emission per kg of produced food (in France) Jancovici, 2002

Greenhouse gas emission per passenger and kilometer (in France)

Methane sources in millions of tons per year

Pollution: brings Pollution: brings CH4 OH O3O3

Another example of a complex system: the energy cycle GIEC, 2001

Atmospheric constituents contributing to the greenhouse effect Natural (155 W/m 2 ) Anthropogenic (2.8 W/m 2 )

Forçage radiatif par tonne émise Evolution of the greenhouse effect after some instantaneous emission of one ton Année après la perturbation Hauglustaine D., LSCE

Greenhouse Warming Potential CF HCFC HFC-134a SF N2ON2O CFC CFC CH CO 2 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 l’Environnement

Are consequences already perceptible ? Mann, IPCC, 2001

IPCC, 2001 Change in the extension of mountain glaciers

Climate modelling: an old dream which became possible in the last decades Richardson (1922) The weather machine An 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 Carbon Cycle Chemistry ContinentsAtmosphereOceans IPSLCM4 Atmospheric circulation Ocean circulation Sea ice Carbon DMS Nutrients Chemistry Gases & Aerosols CO 2 Land Surface Soil and vegetation LMD Z OrchidéeLMD ZT ORCALIM INCA STOMATE PISCES Carbon CH 4, VOCs, Aerosols Marine biology and biogeochemistry Terrestrial biogeochemistry Salt

Clouds simulated by models in 1990 … and in 2000

Model evaluation: precipitations

IPCC scenarios: a wide range of possible futures (without taking into account specific policies, such as those linked with Kyoto protocol)

Surface air temperature Comit. IPSL-CM4 coupled model A1B A2 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 CNRMIPSL

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?. CO 2 absorption with climate change Without climate change 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