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Restoration programs need to increasingly address both the restitution of biodiversity and ecosystem services and the preparation of habitats for future climate change. One option to adapt habitats to climate change in the temperate zone is the translocation of southern populations to compensate for climate change effects - an option known as assisted migration (AM). Although AM is widely criticized for endangered species, forest managers are more confident that tree populations can be translocated with success because of previous experiences within native ranges. Here, we contend that translocations of tree populations are also subject to uncertainties, and we extract lessons for future programs of AM within species ranges from a well-documented failed case of population translocation of Pinus pinaster Ait. in Europe. The failure of these translocations originated from the unawareness of several unpredictable ecological and social events: cryptic maladaptation of the introduced populations, underestimation of climate variability differences between the source and target sites and complexity in the management schemes, postponing decisions that could have been undertaken earlier. Under the no-analog conditions that are expected with climate change, management decisions need to be made with incomplete data, implying that a certain degree of maladaptation should always be expected when restoring plant populations from local or external seed sources.

The main purpose of this proposal was to provide the necessary tools to address a particular case ofadaptation and mitigation of forests to climate change in Western Europe: the translocation of treepopulation to compensate for climate change (Assisted Migration; AM).Tree populations have been moved with different purposes before, and we started analyzing theseparticular cases in Europe. In particular, we analyzed a well-document failed case of populationtranslocation of Pinus pinaster Ait from the Iberian peninsula to southern France, which entailed anenormous economic loss that led to the only case where the use of foreign tree populations wasprohibited by the European Union, which otherwise promotes the free exchange of seeds betweenecologically matched sites (EU Council Directive 1999/105/EC of 22 December 1999). An opinionpaper (Restoration Ecology 2013) and a reply letter (Bioscience 2013) are related to theseexperiences analyzing translocations in the past.A second step was devoted to propose new scenarios for translocations in the future based on climatedata and the fitness (measured as the volume/year as recorded in NFI). To do that, we producedscenarios (theoretical representation) of assisted migration actions for two important candidates forAM in Europe: Pinus pinaster and Pinus halepensis. We harmonized the Spanish and FrenchNational Forest Inventories for standing volume and mortality, which allowed us to run modelscovering Western Europe. Target zones were selected from provenances with high sensitivity andseed zones from provenances with low sensitivity to climate change; the latter can be considered“seed refugia” as the climate changes. Three plausible scenarios for translocation to the target zonewere developed on the basis of volume simulations calibrated with different planting Abies albastrategies: 1) seeds only from foreign provenances; 2) foreign provenances plus local seeds; and 3)only local seeds. Our results suggested that volume and mortality trends were not always correlatedwith seed sources and targets, that projected provenances mortality do not follow always asouthern-northern pattern and that seed refugia, if any, may be useful for compensating for the effectsof climate change only in a subset of provenances (this approximation has been published in thejournal New Forests 2015).To evaluate which native forests are likely candidates for AM we estimated the vulnerability of thenative species in Western Europe including the sensitivity, the exposure and the adaptive capacity ofthe major tree species in ecological models for creating scenarios for current climatic conditions andfuture ones. The results of these simulations are considering for publication in the journal EcologicalApplications. Our results suggest that management programs should be different for mountainconifers populations occurring at their rear-edge of their distribution that need to be sampled moreintensively to preserve their genetic resources compared to lowland widespread temperate specieswhere vulnerabilities seem lower and populations more homogeneous. Likewise, for rear-edgepopulations, ex-situ programs considering some compensation for climate change (assistedProject No.: 327778Period number: 1stRef: 327778_AMECO_Final_Report11_20151222_165135_CET.pdfPage - 3 of 11migration) may be essential to preserve the populations for which conditions become too harsh butthat may have a better chance in higher latitudes and altitudes.The adaptation of socio-ecological systems to climate change depends not only on the evolutionarypotential of natural populations but on the decisions taken by social actors that mediate naturalprocesses. To succeed in moving trees to compensate for climate change the development of adecision framework based on variations in tree fitness in relation with climate is a priority. In trees,provenance trials provide new insights to understand the future of populations under climate change.We use data from an European network of 33 provenance tests of Abies alba planted in 5 sites inFrance to predict spatial variation in tree height at 9 years old in the present and in the near future.We integrated the outputs of the height tree model in combination with the ones of habitat suitabilityfor each of the climate scenarios in a decision framework to guide assisted colonization tocompensate for climate change. We apply three decision rules to cope with the uncertain states of theworld represented by four climate change scenarios showing that there is no unique ‘best decision’and that the general recommendation of mixtures of populations, although probably not the mostproductive, can reduce uncertainty in the long term by allowing natural forests to evolve.

International audience; Looking back over three and a half Assessment Reports, we see that the Intergovernmental Panel on Climate Change (IPCC) has given increasing attention to the management and reporting of uncertainties, but coordination across working groups (WGs) has remained an issue. We argue that there are good reasons for working groups to use different methods to assess uncertainty, thus it is better that working groups agree to disagree rather than seek to bring everybody on one party line.

International audience; Learning – i.e. the acquisition of new information that leads to changes in our assessment of uncertainty – plays a prominent role in the international climate policy debate. For example, the view that we should postpone actions until we know more continues to be influential. The latest work on learning and climate change includes new theoretical models, better informed simulations of how learning affects the optimal timing of emissions reductions, analyses of how new information could affect the prospects for reaching and maintaining political agreements and for adapting to climate change, and explorations of how learning could lead us astray rather than closer to the truth. Despite the diversity of this new work, a clear consensus on a central point is that the prospect of learning does not support the postponement of emissions reductions today.

Learning—i.e. the acquisition of new information that leads to changes in our assessment of uncertainty—plays a prominent role in the international climate policy debate. For example, the view that we should postpone actions until we know more continues to be influential. The latest work on learning and climate change includes new theoretical models, better informed simulations of how learning affects the optimal timing of emissions reductions, analyses of how new information could affect the prospects for reaching and maintaining political agreements and for adapting to climate change, and explorations of how learning could lead us astray rather than closer to the truth. Despite the diversity of this new work, a clear consensus on a central point is that the prospect of learning does not support the postponement of emissions reductions today.

International audience; Problem: Should we use seeds from warmer locations to adapt forests to climate change ? We define assisted migration as the project to translocate populations to compensate for observed or future climate changes. It is justified since the pace of climate change is too rapid for most populations to track changing climates. The idea of assisted migration questions the current paradigm in forestry management, which is that local provenances are always the best first choice. The cases of Oak and Pine populations in France illustrate the timeliness but also the dangers of this idea.Method: A decision-making framework based on imprecises probabilities and multiple worldviews is used, which accounts for rationality and precaution. We represent the deep uncertainty on climate change by using worldviews on long term climate change based on simulations of 3 RCP scenarios and 3 climate models.Data: Experimental data from the British Columbia Ministry of Forests and Range's lodgepole pine provenance test, which consists of 140 populations tested at 62 sites, were used. An incomplete testing design tested 60 of the 140 populations at each site, so that each population was tested at 30–40 sites. Total height was measured at 43 of the 62 sites in autumn 2005, after 32 field growing seasons (35 growing seasons from seed).Conclusions: We find that assisted migration is already a policy issue. Our work illustrates a decision making criteria that is both rational and precautionary, by focusing on trades between two options instead of focusing on a choice considering each option in isolation . Canadian data was much easier to access and use than French. Early results points that assisted migration could be considered in higher latitute stations.Early results for assisted migration in higher latitude stations

The paper investigates a climate-economy model with an iso-elastic welfare function in which one parameter gamma measures relative risk-aversion and a distinct parameter rho measures resistance to intertemporal substitution. We show both theoretically and numerically that climate policy responds differently to variations in the two parameters. In particular, we show that higher gamma but lower rho leads to increase emissions control. We also argue that climate-economy models based on intertemporal expected utility maximization, i.e. models where gamma = rho, may misinterpret the sensitivity of the climate policy to risk-aversion., Ce papier examine un modèle économie-climat avec une fonction de bien être isoélastique dans laquelle un paramètre gamma mesure l'aversion relative au risque et un paramètre distinct rho mesure la résistance à la substitution intertemporelle. Nous montrons théoriquement et numériquement que la politique climatique optimale répond différement à ces deux paramètres. En particulier, nous montrons que c'est une augmentation de gamma ou une diminution de rho qui conduit à des réductions d'émissions plus strictes. Nous concluons que les modèles économie-climat basés sur la maximisation de la somme intertemporelle actualisée de l'utilité espérée, c'est à dire les modèles dans lesquels rho = gamma, sous-estiment la sensibilité de la politique climatique à l'aversion au risque.

This paper examines the consequences of various attitudes towards climate damages through a family of stochastic optimal control models (RESPONSE): cost-effectiveness for a given temperature ceiling; cost-benefit analysis with a “pure preference for current climate regime” and full cost-benefit analysis. The choice of a given proxy of climate change risks is actually more than a technical option. It is essentially motivated by the degree of distrust regarding the legitimacy of an assessment of climate damages and the possibility of providing in due time reliable and non controversial estimates. Our results demonstrate that (a) for early decades abatement, the difference between various decision-making frameworks appears to matter less than the difference between stochastic and non stochastic approach given the cascade of uncertainty from emissions to damages; (b) in a stochastic approach, the possibility of non-catastrophic singularities in the damage function is sufficient to significantly increase earlier optimal abatements; (c) a window of opportunity for action exists up to 2040: abatements further delayed may induce significant regret in case of bad news about climate response or singularities in damages.

International audience; The portfolio of approaches to respond to the challenges posed by anthropogenic climate change has broadened beyond mitigation and adaptation with the recent discussion of potential climate engineering options. How to define and categorize climate engineering options has been a recurring issue in both public and specialist discussions. We assert here that current definitions of mitigation, adaptation and climate engineering are ambiguous, overlap with each other and thus contribute to confusing the discourse on how to tackle anthropogenic climate change. We propose a new and more inclusive categorization into five different classes: anthropogenic emissions reductions (of short-lived climate agents and long-lived greenhouse gases, abbreviated AER, territorial or domestic removal of atmospheric CO2 and other greenhouse gases (D-GGR), trans-territorial or trans-boundary removal of atmospheric CO2 and other greenhouse gases (T-GGR), regional to planetary targeted climate and environmental modification (TCM), and climate change adaptation measures (including local targeted climate and environmental modification, abbreviated CCAM). Thus, we suggest that techniques for domestic greenhouse gas removal might better be thought of as forming a separate category alongside more traditional mitigation techniques that consist of emissions reductions. Local targeted climate modification can be seen as an adaptation measure as long as there are no detectable remote environmental effects. In both cases, the scale and intensity of action are essential attributes from the technological, climatic and political viewpoints. Whilst some of the boundaries in this revised classification depend on policy and judgement, it offers a foundation for debating on how to define and categorize climate engineering options and differentiate them from both mitigation and adaptation measures to climate change.

International audience; Recently, Pedlar et al. (2012) stated that assisted migration in forestry (forestry AM) differs from species-rescue-assisted migration (species rescue AM) because the risks of invasiveness, hybridization with local species, and spread of diseases are minimized in managed forests. The rationale behind this assertion for forestry AM is that it involves the translocation of populations within the existing geographic range of the species, whereas species rescue AM involves the introduction of exotic species. However, while we agree that forestry AM is less risky than species rescue AM for the recipient ecosystem, forestry AM can not only fail but can also incur enormous financial costs. The failure of efforts that involved planting maritime pine (Pinus pinaster Aït) trees in Southwest France (Aquitaine) with seeds from more southerly populations from Portugal for production purposes is a textbook case. The climate variability in Aquitaine includes periods of intense frost that are sufficiently rare (every 10 to 20 years) to be overlooked when establishing tree populations. The frost of the winter of 1985, the most intense frost event since records began with temperatures dropping as low as -22 °C (Boisseaux, 1986), affecting about 350 km2 of tree plantations in the region (Doré & Varoquaux, 2006). The highest mortality related to frost was observed in populations harvested from Leiria in Portugal, for which nearby records show that the absolute minimum temperature was only -7.8 °C in the last 60 years. Climate averages over the last 30 years differ only slightly between Leiria and Aquitaine, which would erroneously suggest that samples from Portugal would have survived in the Aquitaine region. Newly emerging climates (Williams et al. 2007) and the uncertainty related to climate change extreme events (Easterling, 2000) will make the search for southern locations with climatic conditions similar to those of northern populations of trees extremely difficult. Policies of forest adaptation to climate change should account for extreme cold events in the target populations even if climate change will likely decrease the number of extreme cold events (Easterling, 2000), that remain in our opinion, the hidden element behind the maladaptation of southern populations to northern locations.

This paper reviews evidence that energy technologies and systems adapt over time to accommodate external pressures: that technical innovation and systemic change in the energy sector is largely induced by need, and restrained by potentially large transitional costs. A simple integrated model of optimal greenhouse gas abatement over time is presented, in which the abatement cost depends on both fixed and transitional elements. It is shown that the optimal current response and long-run prospects differ radically between the classical economic case — in which the cost of a given cutback in emissions is fixed exogenously — and the adaptive case — in which the response is ultimately adaptive but heavily constrained by inertia (ie low fixed but high transitional cost). If energy systems are indeed to a large degree adaptive, the results demonstrate that as compared with the classical non-adaptive case: long-run stabilization of atmospheric CO2 may be optimal even with moderate damages from climate change; greater near-term abatement efforts are justified; and the cost of a given delay in response may be several times higher. Neglect of the issue of induced technical change and other adaptive responses may invalidate the policy implications drawn from most integrated assessment models developed to date.

This paper discusses aspects of risk and uncertainty relevant in an interdisciplinary assessment of climate change policy. It opposes not only the objective approach versus the subjective approach, but also situations when precise probabilities are well founded versus situations of broader forms of error such as Knightian or deep uncertainty, incompleteness, vagueness. Additional human and social dimensions of ignorance: strategic uncertainties, surprises, values diversity, and taboos, are discussed. We argue that the broader forms of error affect all sciences, including those studying Nature. For these aspects the IPCC guidance notes provides an interdisciplinary unified approach on risk and uncertainty. This is a significant advance from a simple multidisciplinary justaposition of approaches. However, these guidance notes are not universal, they mostly omit the human and social dimensions of ignorance., Ce papier discute les divers aspects du risque et de l'incertitude pertinents dans le cadre de l'évaluation interdisciplinaire des politiques climatiques. Il marque non seulement l'opposition entre l'approche objective (qui voit les probabilités comme des degrés de vérité) et l'approche bayésienne (qui les voit comme des degrés de certitude), mais encore l'opposition entre les situations de risque (quand on dispose de probabilités précises et bien fondées) et les situations d'incertitude (des formes d'ignorance plus générale, comme l'incertitude au sens de Knight, l'incomplétude ou le vague). L'évolution des directives IPCC sur le risque et l'incertitude entre le troisième et le quatrième rapport peuvent se lire comme un mouvement s'écartant de la position objectiviste et probabiliste, pour inclure des aspects plus complexes de l'incertitude. Cependant, il reste encore des dimensions humaines comme l'ignorance stratégique, les surprises, les aspects métaphysiques, les taboos et l'incertitude épistémique qui manquent dans les directives IPCC.

International audience; Dealing consistently with risk and uncertainty across the IPCC reports is a difficult challenge. Huge practical difficulties arise from the Panel's scale and interdisciplinary context, the complexity of the climate change issue and its political context. The key question of this paper is if the observed differences in the handling of uncertainties by the three IPCC Working Groups can be clarified. To address this question, the paper reviews a few key issues on the foundations of uncertainty analysis, and summarizes the history of the treatment of uncertainty by the IPCC. One of the key findings is that there is reason to agree to disagree: the fundamental differences between the issues covered by the IPCC's three interdisciplinary Working Groups, between the type of information available, and between the dominant paradigms of the practitioners, legitimately lead to different approaches. We argue that properly using the IPCC's Guidance Notes for Lead Authors for addressing uncertainty, adding a pedigree analysis for key findings, and particularly communicating the diverse nature of uncertainty to the users of the assessment would increase the quality of the assessment. This approach would provide information about the nature of the uncertainties in addition to their magnitude and the confidence assessors have in their findings.; Ce texte examine le traitement de l'incertitude dans les rapports du Groupe d'experts Intergouvernemental sur les Changements Climatiques (IPCC). La première partie rappelle les différences fondamentales entre probabilités objectives et subjectives, entre probabilités précises et imprécises, ainsi qu'entre les systèmes causaux et intentionels. La seconde partie met en évidence les divergences entre les trois groupes de travail persistantes au cours de l'histoire de l'IPCC. On montre en particulier que les incertitudes sur la notion de coût et de potentiel de réduction ne sont pas quantifiables en termes de probabilités. En conclusion, le texte recommande de reconnaitre la diversité des approches épistémiques dans les différentes disciplines, et d'adopter une caractérisation multidimensionnelle permettant de jauger la confiance des jugements (approche par pedigree).

We describe a method to represent the results of climate simulation models with analogues. An analogue to a city A is a city B whose climate today represents A's simulated future climate. Climates were characterized and compared non-parametrically, using the 30-years distribution of three indicators : Aridity Index, Heating Degree Days and Cooling Degree Days. Analogy was evaluated statistically with the two-samples Kolmogorov-Smirnov test, generalized to 3 dimensions. We looked at the climate of 12 European cities at the end of the century under an A2 climate change scenario. We used two datasets produced with high-resolution regional climate simulation models from the Hadley Center and Meteo France. Climate analogues were generally found southward of present locations, a clear warming trend even if much model and scenario uncertainty remains. Climate analogues provide an intuitive way to show the possible effects of climate change on urban areas, offering a holistic approach to think about how cities adapt to different climates. Evidence of its communication value comes from the reuse of our maps in teaching and in several European mass-media., Nous décrivons une méthode permettant de représenter les résultats de modèles de simulation climatique au moyen d'analogues. Un analogue d'une ville A est une ville B dont le climat actuel représente le climat futur simulé pour A. Pour définir les climats locaux, on a utilisé la distribution sur trente années de trois indicateurs : l'index d'aridité, le nombre de degrés-jours au dessus de 18°C, et le nombre de degrés-jours sous 18°C. L'analogie entre deux climats a été évaluée statistiquement par le test non-paramétrique de Kolmogorov-Smirnov sur deux échantillons, généralisé à trois dimensions. Nous avons examiné le climat de 12 villes européennes à la fin du siècle dans le scénario d'émission de gaz à effet de serre A2. Pour cela, nous avons utilisé les résultats de deux simulations par des modèles régionaux à haute résolution, en provenance du Hadley Center et de Météo-France respectivement. Les analogues climatiques trouvés varient beaucoup en fonction du modèle utilisé et du scénario d'émission, mais ils sont dans l'ensemble situés bien au sud des localisations actuelles, ce qui traduit une tendance manifeste au réchauffement. Les analogues climatiques procurent un moyen intuitif de parler des effets attendus du changement climatique sur les villes. Ils offrent une approche holistique pour réfléchir à l'adaptation urbaine aux différents climats, en montrant aussi l'étendue des incertitudes actuelles. La valeur pédagogique de ce concept est confirmée par la réutilisation des cartes décrites dans ce travail par plusieurs mass-media européens.

immobilize it in geological structures. Air capture differs from conventional mitigation in three key aspects. First, it removes emissions from any part of the economy with equal ease or difficulty, so its cost provides an absolute cap on the cost of mitigation. Second, it permits reduction in concentrations faster than the natural carbon cycle: the effects of irreversibility are thus partly alleviated. Third, because it is less coupled with existing energy infrastructure, air capture may offer stronger economies of scale and smaller adjustment costs than the more conventional mitigation technologies.We assess the ultimate physical limits on the amount of energy and land required for air capture and describe two systems that might achieve air capture at prices under 200 $/tC using current technology.Like geoengineering, air capture limits the cost of a worst-case climate scenario. In an optimal sequential decision framework with uncertainty, existence of air capture decreases the need for near-term precautionary abatement. The long-term abatement effect is the opposite, assuming that marginal cost of mitigation decreases with time, while marginal climate change damage increases. With air capture this implies an environmental Kuznets curve, returning towards preindustrial concentration levels of greenhouse gases., Ce texte porte sur la question de la réversibilité à long terme des émissions de CO2. Il donne un exemple de technologies de capture directe à partir de l'air, qui permet de borner le coût marginal de réduction du CO2 dans toute l'économie. Il exploré les aspects thermodynamiques de la capture du dioxyde de carbone à partir de l'air. Le modèle DIAM a été étendu pour prendre en compte ces options et examiner les conséquences sur les politiques climatiques optimales.

Mémoire d'Habilitation à Diriger les Recherches.; This thesis' ultimate goal is to improve the risk and uncertainty analysis of global environmental issues. It starts with a review of the various uncertainty analysis techniques used in integrated assessment models. Then it presents results of the Dynamics of Inertia and Adaptability Model (DIAM) about the optimal timing of global change policy. That model follows the classical approach of precaution as sequential decision-making, mathematically formalized using stochastic dynamic programming. But that classical approach requires precise subjective probabilities. The central claim of this thesis is that uncertainty about long-run issues such as climate change are better represented using (objective or subjective) imprecise probabilities. This claim is supported both theoretically and with applications. Chapters 5 to 7 are an introduction to the mathematical theory of imprecise probabilities. Applications form the last section of this work. Applications in chapters 8 to 10 deal with expert elicitation, information fusion and scenario-making. They use special kinds of imprecise probabilities, namely those of the transferable belief model for chapter 8 and 9, along with distributions of possibility in chapters 9 and 10.; Ce mémoire présente une recherche pour mieux traiter les problèmes d'environnement globaux en améliorant la prise en compte du risque et de l'incertitude dans les modèles numériques interdisciplinaires. La revue des approches de la décision ---utilisant ou non les probabilités subjectives--- montre que la première représentation de la précaution est le modèle de décision séquentielle. Dans ce cadre, ce texte présente les résultats du modèle intégré DIAM résolu en programmation dynamique stochastique concernant la question du tempo de l'action contre le changement climatique. La seconde partie de l'ouvrage propose une vision alternative de la précaution, basée sur le rejet de l'axiome de complétude des préférences de Savage. Cela conduit à définir un critère de décision rationnel en situation d'information imprécise. Ce critère est discuté avec la théorie des probabilités imprécises, en visant à unifier les différentes approches proposées dans la littérature (Shackle et Zadeh, Dempster et Shafer, Smets, Gilboa et Schmeidler, Walley). Les applications présentées en troisième partie concernent la causalité en épidémiologie, l'élicitation et la fusion des opinions d'experts ainsi que les règles de la prospective.

This paper studies uncertainty about the non-linearity of climate change impact. The DIAM 2.3 model is used to compute the sensitivity of optimal CO2 emissions paths with respect to damage function parameters. This builds upon results of the EMF–14 uncertainty subgroup study by explicitly allowing for the possibility of threshold effects and hockey stick damage functions. It also extends to the cost-benefit framework previous studies about inertia of energy systems. Results show that the existence of a threshold in the damage function is critical to precautionary action. Optimal path are much less sensitive to uncertainty on the scale of the damages than on the threshold values.

Fossil fuels can be used with minimal atmospheric emissions of carbon dioxide by capturing and storing the CO2 away in geologic structures. However, stored CO2 can leak back to the atmosphere reducing the utility of this technology. To explore the trade-offs between discounting, leakage, the cost of sequestration and the energy penalty (the energy necessary to capture, transport and inject carbon underground), we derive analytic expressions for the value of leaky CO2 storage compared to perfect storage when storage is a marginal component of the energy system. If the annual leak rate is 1% and the discount rate is 4%, for example, then CO2 mitigation using leaky storage is worth 80% of mitigation with perfect storage. Using an integrated assessment numerical model (DIAM) to explore the role of leakage when CO2 storage is non-marginal, we find that a leakage rate of 0.1% is nearly the same as perfect storage while a leakage rate of 0.5% renders storage unattractive. The possibility of capturing CO2 from the air, not only from flue gases, makes storage with higher leakage rates interesting. Finally, we speculate about the role of imperfect carbon storage in carbon accounting and trading., Les combustibles fossiles peuvent être utilisés avec peu d'émissions en capturant le dioxyde de carbone et en le stockant dans des structures géologiques. Cependant, une fraction du carbone stocké pourrait fuir vers l'atmosphère, ce qui limiterait l'utilité de cette technologie. Afin d'explorer les compromis entre actualisation, étanchéité du réservoir, coût de séquestration et pénalité énergétique (l'énergie nécessaire pour capturer, transporter et injecter le carbone en sous-sol), nous développons un modèle analytique marginal simple de la valeur d'un stockage imparfait comparativement à stockage idéal. Si le taux de fuite annuel est de 1 pour cent et le taux d'actualisation de 4 pour cent, par exemple, alors le stockage temporaire) de la tonne de CO2 se compare à l'élimination (permanente) de 8 quintaux de ce même gaz.

Perspectives on climate change policy The past decade achieved significant progress in how humanity understand and cope with global climate change. It is increasingly known that the central hypothesis for the next century is that global average temperature will increase by about 2 °C in 2100, with carbon dioxyde concentration raising over 550 PPM. A catastrophic change of pattern in climate dynamics remains possible, which may be a risk more serious than gradual and anticipated warming. It's more and more understood that burning the world reserves of oil, gas and coal is technically very feasible, but environmentally very catastrophic. International negociations are led within the framework of the Climate Convention. Initially, they followed the model of the ozone layer treaty negociations. But climate change is a much wider issue, comparable in scope to law-of-the-sea or to free-trade negociations, with the additional complexity of scientific ignorance and intergenerational aspects. Negociation must be expected to go on for several decades. Industrialized countries agreed in Kyoto, in 1997, to reduce the level of their greenhouse gases emissions as soon as 2008–2012, about 5% below their 1990 level. This objective is ambitious considering the inertia of energy systems, since the current global trend is more like a 1 % annual growth. But this is not enought to insure a stable long-term rentability for industrial investments in less emitting technologies.

International audience; This paper examines the economic rationale behind both the quantitative targets and the flexibility mechanisms adopted in the Kyoto Protocol. It synthesises some theoretical dimensions of the debate about the so-called "when flexibility" of climate policies, explaining the importance of the interplay between uncertainty and technico-economic inertia. Numerical results shows that the aggregate Kyoto abatement target is consistent with a stochastic dynamic optimum in which a 450 ppm concentration ceiling is seriously considered. Turning to the EU-US debate about the interpretation of the "supplemental to" condition in Article 3 of the Kyoto Protocol regarding the articulation between international trading systems and domestic policies and measures, this text illuminates the risk of dynamic inconsistencies due to the heterogeneity of capital stocks in the economy, if price signals do not emerge in due time from GHGs trading systems because of the ''hot air'' in some countries and the discovery of low costs abatement potentials in Annex B countries. Numerical simulations show that a delay of action on sectors with large inertia of capital stocks and of the consumption styles may under such circumstances ultimately undermine the economic viability of climate policies beyond 2012. Some lessons are derived for the future of climate policies and negotiations about the implementation of the Kyoto Protocol.

International audience; In the climate change issue, the environmental irreversibility (risk of an acceleration of mitigation policies if the worse happen) has to be balanced with the investment irreversibility (risk of over-cautious policies). To explore this balance, we define an option value for a precautionary climate policy. Using the simplest decision-making model, we expose how option value relates to the expected value of future information. Using quantitative data from an integrated as sessment model, we find that most of the times the environmental irreversibility dominates the investment irreversibility. For all cases explored here, the order of magnitude of the option value was significant, about 50% of the opportunity cost.

International audience; Current debates on climate mitigation emphasize the role of the inertia of the economic system. Our aim in this paper is to study more in depth how sectorally differentiated inertia impacts on optimal C02-emission abatement policies. Using the STARTS model, we show that optimal abatement levels and costs differ sensibly among sectors. Differential inertia is the critical determinant of this trade-off, especially in case of a 20-year delay in the action, or in an underestimation of the growth of the transportation sector. In particular, the burden of any additional abatement effort falls on the most flexible sector, i.e. the industry.Debates on mitigation emphasize the role of inertia of the economic system. This paper aims at studying more in depth how sectorally differentiated inertia should influence optimal CO2 emission abatement policies. Using a two-sector version of STARTS, we show that under perfect expectations, optimal abatement profiles and associated costs differ sensibly between a flexible and a rigid sector (transportation). In a second step, we scrutinize the role of the uncertainty by testing the case of a 20-year delay of action and an underestimated growth of the transportation sector. We do this for three concentration ceilings and we point out the magnitude of the burden which falls on the flexible sector. We derive some policy implications for the ranking of public policies and for incentive instruments to be set up at international level.

Climate change is one of the biggest challenges facing humankind for the next century. We expose how we build a Model about the Dynamics of Inertia and Adaptability in energy systems (DIAM) to study the question \"Shall we wait another decade before taking costly measures to curb greenhouse gases emissions\". Using primarily Mathematica, we had to use also the GAMS language, not only to be understood by other researchers in our field, but also to gain access to a powerful non linear constrained optimisation solver. It seems urgent to bridge the gap between these kind of solvers and Mathematica because when one builds models of complex systems, intertemporal optimisation is often preferred to recursive simulation., Ce texte expose la conception du modèle intégré DIAM - Dynamics of Inertia and Adaptability Model. Nous expliquons pourquoi nous avons utilisé, à côté de Mathematica, le langage GAMS pour utiliser des solveurs dóptimisation non linéaire.

This presentation has two parts. The first part is more theoretical. We will examine the definition of Integrated Assessment Models of Climate Change and identify their main aims. We will also have look at the differences between climate and socioeconomic models. The second part will be applied. We present the Dynamics of Inertia and Adaptability Model (DIAM). That model will be used to study some of the fundamentals economic parameters of IAMs: the discount rate, the technical progress rate, the flexibility of energy systems, uncertainty and the final concentration target. We won\'t try to be exhaustive, see Weyant1 leading author of the corresponding IPCC chapter for that. Our goal is simply to convince the reader that these non realistic models can profitably be used to analyse real issues., Dans la première partie, ce texte, examine ce que sont les modèles d\'évaluation intégrée du changement climatique et discute leurs buts principaux. La seccnde partie, plus appliquée, présente le modèle DIAM - Dynamics of Inertia and Adaptability Model. Il est utilisé pour discuter les paramètres socioéconomiques fondamentaux du problème: taux dáctualisation, taux de progrès technique, flexibilité des systèmes énergétiques, incertitude et cible ultime de concentration.

International audience; The challenges of climate change and the COP21: ambitionand aims. In Climate change forum: Favouring a green economy andsustainable urban development, Colombo, Sri Lanka, June 10 2015. Invitedkeynote opening talk. Also given at the University of Moratuwa on June9th.

We find that approximately a quarter of the world’s productive capital could be sensitive to climate; therefore, this capital faces the risk of accelerated obsolescence in a world warming by an average of 0.2 °C per decade. We examine the question of optimal adaptation to climate change in a vintage capital growth model without uncertainty. Along the optimal pathway, adaptation is proactive with an anticipation period of approximately twenty years. While there is additional investment in this scenario compared with a no-climate-change baseline, the overall cost to adapt is low relative to the potential losses from maladaptation. Over-investment in protection capital allows the economy to be consistently well-adapted to climate; thus, such a policy prevents transient maladaptation costs. Sensitivity analysis with an integrated assessment model suggests that costs could be ten times larger if adaptation only begins after vulnerable sectors are impacted.