Patrice Dumas

Optimal control models and elicitation of attitudes towards climate damages

This paper examines the consequences of various attitudes towards climate damages through a family of stochastic optimal control models (RESPONSE): cost-efficiency 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.

Adaptation to an uncertain climate change: cost benefit analysis and robust decision making for dam dimensioning

Climate models project large changes in rainfall, but disagree on their magnitude and sign. The consequences of this uncertainty on optimal dam dimensioning is assessed for a small mountainous catchment in Greece. Optimal dam design is estimated using a Cost-Benefit Analysis (CBA) based on trends in seasonal temperature and precipitations from 19 IPCC-AR4 climate models driven by the the SRES A2 emission scenario. Optimal reservoir volumes are modified by climate change, leading to up to 34% differences between optimal volumes. Contrary to widely-used target-based approaches, the CBA suggests that reduced rainfall should lead to smaller water reservoirs. The resulting change in the Net Present Value (NPV) of water supply is also substantial, ranging from no change to a large 25% loss, depending on the climate model, even assuming optimal adaptation and perfect foresight. In addition, climate change uncertainty can lead to design errors, with a cost ranging from 0.3 to 2.8% of the NPV, depending on site characteristics. This paper proposes to complement the CBA with a robust decision-making approach that focuses on reducing design-error costs. It also suggests that climate change impacts in the water sector may reveal large, that water reservoirs do not always provide a cost-efficient adaptation strategy, and that alternative adaptation strategies based on water conservation and non-conventional water production need to be considered.

The Impact of Globalization on Food and Agriculture: The Case of the Diet Convergence

Globalization drives a process of diet convergence among developing and developed countries that challenges the predictions about future patterns of food consumption. To address this issue, the objective of this article is to map the range of the possible future diet changes and to explore their impact on agriculture using the Nexus Land-Use model. This model computes agricultural intensification in the crop and livestock sectors at the global scale, based on an architecture accounting for the different types of food calories. By considering four scenarios built on distinct assumptions regarding diet convergence, this article sheds light on the pivotal role of diet changes as drivers of tensions on agriculture and land use and shows the uncertainty associated with processes of diet convergence for foresight exercises on food and agriculture. Finally, the interactions between food production and the other land-use patterns are explored by testing the sensitivity of our results to assumptions regarding biofuel production, deforestation, potential crop yields, and nutrient-use efficiency.

Impacts of Climate Change on Freshwater Bodies: Quantitative Aspects

In this chapter we present the results of the impact assessment on freshwater bodies in the Mediterranean region. Starting from the characterization of the general features of Mediterranean hydrology, main focus is given on large river basins discharging into the Mediterranean sea as well as to small and medium scale catchments representing almost half of the entire discharging basin. Groundwater representing a fundamental water resource for Mediterranean countries was also considered. Climate change impacts on the hydrological behavior of large river basins is investigated through the IRIS computational tool which was proved to be a versatile instrument for both climate studies and the assessment of model ability to simulate the hydrological cycle at catchment scale, taking advantage of the available observed discharge series to evaluate the reliability of future discharge projections. The results regarding some representative Mediterranean rivers using multiple climate models developed inside Circe have highlighted an open spread among twenty-first century projections. The problem of the effective information content of climate model simulations with respect to small scale impact studies is developed at the scale of medium and small catchments. Particularly at the space-time scales needed to describe the terrestrial water cycle in Mediterranean environments this is recognized among the most difficult problems facing both science and society. Therefore downscaling and bias-correction requirements have been treated in this chapter through specific methodologies which integrate dynamical downscaling with statistical downscaling always adopting ground based observation of climate variables as a powerful means to obtain more robust climate forcing for hydrological models. The assessment of climate change impacts on small and medium size catchments is developed through some representative case studies in which downscaling methodologies have been applied thanks to the availability of dense climate measurement networks. The impact assessment of water resources in the Apulia region (southern Italy) revealed a marked increase in the variability of hydrologic regimes as consequence of the increased rainfall variability predicted for the twenty-first century. Conversely only slight decreasing trends were detected in the annual water balance components. Similar results were found on a carbonate aquifer in Southern Italy in which a large Apennine spring have been selected as a significant hydrogeological systems with minimal anthropogenic pressures in the recharge areas. Finally a specific session is dedicated to the role of artificial dams in reducing the possible impacts of climate change. In particular, methodologies for the assessment of optimal dam dimensioning under climate change are presented as well as a reliability assessment based on water supply and demand imbalances.

Assessing Climate Change Impacts, Sea Level Rise and Storm Surge Risk in Port Cities

This study illustrates a methodology to assess economic impacts of climate change at city scale, focusing on sea level rise and storm surge. It is based on a statistical analysis of past storm surges in the studied city, matched to a geographical-information analysis of the population and asset exposure in the city, for various sea levels and storm surge characteristics. An assessment of direct losses in case of storm surge (i.e. of the damages to buildings and building content) can then be computed and the corresponding indirect losses – in the form of production and job losses, reconstruction duration, amongst other loses – deduced, allowing a risk analysis of the effectiveness of coastal flood protections, including risk changes due to climate change and sea level rise. This methodology is applied in the city of Copenhagen, capital of Denmark, which is potentially vulnerable to the effects of variability in sea level, as a low lying city....

Damage Assessment from Storm Surge to Coastal Cities: Lessons from the Miami Area

Coastal cities are growing at a very rapid pace, both in population and in terms of assets; therefore, flood risk is likely to increase substantially in these areas in the absence of specific protections. In addition, great uncertainty surrounds the future evolution of hurricane intensity and sea level rise. The area of Miami represents a clear hotspot of human and economic coastal flood exposure: there are more than 5 million inhabitants in the Miami metropolitan area and the population is growing. It is also a low-lying city with most of the population living below an elevation of 10m and is located in a region where tropical cyclones hit frequently. The present study is focused on the two contiguous counties of Miami, Dade and Broward. In this analysis, we consider the impact of dif- ferent storm surges predicted by the computerized model SLOSH 1 and in- vestigate flood risks with current sea level, considering different hurricane parameters (storm category and direction, wind speed, and tide level). For each impact, we apply a damage function and determine if the considered storm surges potentially lead to asset loss, considering both properties and their contents. The results show that, in absence of protections, losses will be very high for large storm surges reaching up to tens of billions USD. In the second part of the analysis, we demonstrate how the economic impact

Optimal timing of CO2 mitigation policies for a cost-effectiveness model

This paper deals with the regulation of greenhouse gases emissions related to climate change. We consider a stylized climate-economy sequential model and use a cost-effectiveness approach. The analytical study is based on a dynamic programming method. It provides both a tolerable ceiling of concentration and, under simple conditions involving the marginal abatement cost and emission functions, optimal and effective abatement rates. In particular, we prove how the cost effective abatement rate increases with time. Through the optimal time to act function, we examine in detail the role played by greenhouse gases absorption, growth and discount rates. We also analyze the paths from an intergenerational equity perspective. Numerical examples illustrate the general statements.

Why are climate policies of the present decade so crucial for keeping the 2 °C target credible?

Decision-makers have confirmed the long term objective of preventing a temperature increase greater than 2 °C. This paper aims at appraising by means of a cost-benefit analysis whether decision makers’ commitment to meet the 2 °C objective is credible or not. Within the framework of a cost-benefit type integrated assessment model, we consider that the economy faces climate damages with a threshold at 2 °C. We run the model for a broad set of scenarios accounting for the diversity of “worldviews” in the climate debate. For a significant share of scenarios we observe that it is considered optimal to exceed the threshold. Among those “non-compliers” we discriminate ”involuntary non-compliers” who cannot avoid the exceedance due to physical constraint from ”deliberate compliers” for whom the exceedance results from a deliberate costs-benefit analysis. A second result is that the later mitigation efforts begin, the more difficult it becomes to prevent the exceedance. In particular, the number of ”deliberate non-compliers” dramatically increases if mitigation efforts do not start by 2020, and the influx of involuntary non-compliers become overwhelming f efforts are delayed to 2040. In light of these results we argue that the window of opportunity for reaching the 2 °C objective with a credible chance of success is rapidly closing during the present decade. Further delay in finding a climate agreement critically undermines the credibility of the objective.

Disentangling the Stern/Nordhaus Controversy: Beyond the Discounting Clash

The Stern/Nordhaus controversy has polarized the widely disparate beliefs about what to do in order to tackle the climate challenge. To explain differences in results and policy recommendations, comments following the publication of the Stern Review have mainly focused on the role played by the discount rate. A closer look at the actual drivers of the controversy reveals however that Stern and Nordhaus also disagree on two other parameters: technical progress on abatement costs and the climate sensitivity. This paper aims at appraising the relative impacts of such key drivers of the controversy on the social cost of carbon and climate policy recommendations. To this end, we use the flexible integrated assessment model RESPONSE which allows us to compare very diverse worldviews, including Stern and Nordhaus’ ones within the same modelling framework and map the relative impacts of beliefs on the three key drivers of the controversy. Furthermore we appraise quantitatively, by means of a linear statistical model, the impacts on results of an extended set of core parameters of RESPONSE. We show that beliefs on long term economic growth, technical progress, the form of the climate damage function and the climate sensitivity have an impact as important as beliefs on pure time preference. Hence, we can qualify the role played by the discount rate in the Stern/Nordhaus controversy and more broadly in the definition of climate policies.

Feedback characteristics of nonlinear dynamical systems

We propose a method to extend the concept of feedback gain to nonlinear models. The method is designed to dynamically characterise a feedback mechanism along the system natural trajectory. The numerical efficiency of the method is proved using the Lorenz (1963) classical model. Finally, a simple climate model of water vapour feedback shows how nonlinearity impacts feedback intensity along the seasonal cycle.