Climate change: a problem with very specific time scales Hervé Le Treut Laboratoire de Météorologie Dynamique (CNRS/ Ecole Normale Supérieure /Ecole Polytechnique/Université Paris 6) Institut Pierre Simon Laplace Académie des Sciences LMD

Our planet is confronted to a situation which was never encountered in the history of human civilizations:

increased sharply after 1950 The perturbation has increased sharply after 1950 1950 1900 Energy use: oil, coal and natural gas account for most of the increase

The consequences have been anticipated by the scientists on the basis of physical models (results of IPCC 1990 and IPCC 1995, compared to subsequent measurements)

They have been detected only recently ….

They will have lasting consequences, ….

Even at a very long time scale …. Zero-emissions after 2100: Effect on temperature and sea-level rise

Adaptation to climate impacts: the necessary interactions between decision making and scientific assessment Hervé Le Treut Laboratoire de Météorologie Dynamique (CNRS/ Ecole Normale Supérieure /Ecole Polytechnique/Université Paris 6) Institut Pierre Simon Laplace Académie des Sciences LMD

Assessing the impacts of climate change is a process confronted with many difficulties: The predicted amplitude of future climate change lies within a certain uncertainty range Potential impacts are very different in nature: heat waves, droughts, cyclone intensification, ocean circulation changes, sea-level rise, modification of ecosystems and loss of biodiversity, health, economic activities The climate system is non-linear and may evolve abruptly when certain (unprecisely known) thresholds are reached The consequences of climate changes also depend on other anthropogenic factors: direct modification of the biodiversity, water management, soil use, geopolitical tensions, … A constant interplay between scientific measurements and decision making is necessary to define and revise continuously an adaptation stategy to climate change

Assessing current evolutions provides a necessary insight into future evolutions. The example of sea-level rise. Satellite measurements Space-borne instruments Observed rise: 3.3 mm/year since 1993 In situ measurements 1.8 mm/an 1993 2006 The rise is not evenly distributed! Yellow/red blue A. Cazenave (LEGOS/ Toulouse)

Assessing current evolutions provides a necessary complement to modeling approaches. A summary of current physical and biological observations by IPCC Working Group II.