[en] After having highlighted the main difficulties met when trying to take decisions on emission reductions (i.e. disequilibrium between short term action and long term dynamics, series of uncertainties, lack of a clear information on threats), this research aims at studying the extent and the agenda of CO₂ emission reduction efforts at a global scale. First, the author explores under various perspectives the relationship between the extent of climate change and the extent of its impacts and modes of analysis of climate policies. Then, he examines how different attitudes towards the climate risk are translated into recommendations for action which are obtained by always more complex models. In order to compare results, simulations are performed, based on optimal control models belonging to the RESPONSE family. Three different options or attitudes are then studied and assessed with RESPONSE: firstly, the uncertainty on impact and damage assessment is still too important to have confidence in present assessments; secondly, while being sceptical on damage assessment, more confidence is given to climate model results, and thirdly, an explicit representation of climate damages is used to introduce rationality issues in the debate on risks

[en] This paper explores the relationships between climate change and security. Potential threats from climate change, as a unique source of stress or together with other factors, to human security are first examined. Some of the most explicit examples illustrate this section: food security, water availability, vulnerability to extreme events and vulnerability of small islands States and coastal zones. By questioning the basic needs of some populations or at least aggravating their precariousness, such risks to human security could also raise global security concerns, which we examine in turn, along four directions: rural exodus with an impoverishment of displaced populations, local conflicts for the use of natural resources, diplomatic tensions and international conflicts, and propagation to initially-unaffected regions through migratory flows. (authors)

[en] To assess climate policies in a cost-efficiency framework with constraints on the magnitude and rate of global climate change we have built RESPONSE, an optimal control integrated assessment model. Our results show that the uncertainty about climate sensitivity leads to significant short-term mitigation efforts all the more as the arrival of information regarding this parameter is belated. There exists thus a high opportunity cost to know before 2030 the true value of this parameter, which is not totally granted so far. Given this uncertainty, a +2 deg C objective could lead to rather stringent policy recommendations for the coming decades and might prove unacceptable. Furthermore, the uncertainty about climate sensitivity magnifies the influence of the rate constraint on short-term decision, leading to rather stringent policy recommendations for the coming decades. This result is particularly robust to the choice of discount rate and to the beliefs of the decision-maker about climate sensitivity. We finally show that the uncertainty about the rate constraint is even more important for short-term decision than the uncertainty about climate sensitivity or magnitude of warming. This means that the critical rate of climate change, i.e. a transient characteristic of climate risks, matters much more than the long-term objective of climate policy, i.e. the critical magnitude of climate change. Therefore, research should be aimed at better characterising climate change risks in view to help decision-makers in agreeing on a safe guardrail to limit the rate of global warming. (author)

[en] In this analysis, the authors aims to answer the three following questions. Is the world economy really threatened by the climatic change? Must we invest massively to avoid the worst? Or shall we just manage the damages? If some economist as Nicholas Stern concludes to the emergency of the actions, other as the scientist Bjorn Lomborg prefers to devote an important part of the money to help the today inhabitants of developing countries instead of the future generations. This document takes stock on these opposite points of view. (A.L.B.)

[en] This document gathers the transparencies of the available presentations given at this colloquium on the impacts of climate change on economy, society, ecosystems and public health: 1 - images of climate changes in France in the 21. century according to CNRM and IPSL scenarios (M. Deque); 2 - climatic risks and public policy (P. Ambrosi); 3 - climatic policy and carbon sequestration by forests and agricultural practices (D. Arrouays); 4 - towards new inventories of net greenhouse gas (direct or indirect) and aerosol emissions (P. Ciais); 5 - impact on hydro-systems (P. Chevallier); 6 - impacts on health (J.P. Lacaux); 6 - climate and health (Y. Le Maho); 7 - synthesis of the colloquium (J.C. Andre). (J.S.)