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AbstractAssessment of climate change policies requires aggregation of costs and benefits over time and across generations, a process ordinarily done through discounting. Choosing the correct discount rate has proved to be controversial and highly consequential. To clarify past analysis and guide future work, we decompose discounting along two dimensions. First, we distinguish discounting by individuals, an empirical matter that determines their behavior in models, and discounting by an outside evaluator, an ethical matter involving the choice of a social welfare function. Second, for each type of discounting, we distinguish it due to pure time preference from that attributable to curvature of the pertinent function: utility functions (of consumption) for individuals and the social welfare function (of utilities) for the evaluator. We apply our analysis to leading integrated assessment models used to evaluate climate policies. We find that past work often confounds different sources of discounting, and we offer suggestions for avoiding these difficulties. Finally, we relate the standard intergenerational framework that combines considerations of efficiency and distribution to more familiar modes of analysis that assess most policies in terms of efficiency, leaving distributive concerns to the tax and transfer system.

NO2 concentrations at the street level are a major concern for urban air quality in Europe and have been regulated under the EU Thematic Strategy on Air Pollution. Despite the legal requirements, limit values are exceeded at many monitoring stations with little or no improvement in recent years. In order to assess the effects of future emission control regulations on roadside NO2 concentrations, a downscaling module has been implemented in the GAINS integrated assessment model. The module follows a hybrid approach based on atmospheric dispersion calculations and observations from the AirBase European air quality database that are used to estimate site-specific parameters. Pollutant concentrations at every monitoring site with sufficient data coverage are disaggregated into contributions from regional background, urban increment, and local roadside increment. The future evolution of each contribution is assessed with a model of the appropriate scale: 28 x 28 km grid based on the EMEP Model for the regional background, 7 x 7 km urban increment based on the CHIMERE Chemistry Transport Model, and a chemical box model for the roadside increment. Thus, different emission scenarios and control options for long-range transport as well as regional and local emissions can be analysed. Observed concentrations and historical trends are well captured, in particular the differing NO2 and total NOx = NO + NO2 trends. Altogether, more than 1950 air quality monitoring stations in the EU are covered by the model, including more than 400 traffic stations and 70% of the critical stations. Together with its well-established bottom-up emission and dispersion calculation scheme, GAINS is thus able to bridge the scales from European-wide policies to impacts in street canyons. As an application of the model, we assess the evolution of attainment of NO2 limit values under current legislation until 2030. Strong improvements are expected with the introduction of the Euro 6 emission standard for light duty vehicles; however, for some major ...

NO 2 concentrations at the street level are a major concern for urban air quality in Europe and have been regulated under the EU Thematic Strategy on Air Pollution. Despite the legal requirements, limit values are exceeded at many monitoring stations with little or no improvement in recent years. In order to assess the effects of future emission control regulations on roadside NO 2 concentrations, a downscaling module has been implemented in the GAINS integrated assessment model. The module follows a hybrid approach based on atmospheric dispersion calculations and observations from the AirBase European air quality database that are used to estimate site-specific parameters. Pollutant concentrations at every monitoring site with sufficient data coverage are disaggregated into contributions from regional background, urban increment, and local roadside increment. The future evolution of each contribution is assessed with a model of the appropriate scale: 28 × 28 km grid based on the EMEP Model for the regional background, 7 × 7 km urban increment based on the CHIMERE Chemistry Transport Model, and a chemical box model for the roadside increment. Thus, different emission scenarios and control options for long-range transport as well as regional and local emissions can be analysed. Observed concentrations and historical trends are well captured, in particular the differing NO 2 and total NO x = NO + NO 2 trends. Altogether, more than 1950 air quality monitoring stations in the EU are covered by the model, including more than 400 traffic stations and 70% of the critical stations. Together with its well-established bottom-up emission and dispersion calculation scheme, GAINS is thus able to bridge the scales from European-wide policies to impacts in street canyons. As an application of the model, we assess the evolution of attainment of NO 2 limit values under current legislation until 2030. Strong improvements are expected with the introduction of the Euro 6 emission standard for light duty vehicles; however, for some major European cities, further measures may be required, in particular if aiming to achieve compliance at an earlier time.

NO2 concentrations at the street level are a major concern for urban air quality in Europe and have been regulated under the EU Thematic Strategy on Air Pollution. Despite the legal requirements, limit values are exceeded at many monitoring stations with little or no improvement in recent years. In order to assess the effects of future emission control regulations on roadside NO2 concentrations, a downscaling module has been implemented in the GAINS integrated assessment model. The module follows a hybrid approach based on atmospheric dispersion calculations and observations from the AirBase European air quality database that are used to estimate site-specific parameters. Pollutant concentrations at every monitoring site with sufficient data coverage are disaggregated into contributions from regional background, urban increment, and local roadside increment. The future evolution of each contribution is assessed with a model of the appropriate scale: 28 × 28 km grid based on the EMEP Model for the regional background, 7 × 7 km urban increment based on the CHIMERE Chemistry Transport Model, and a chemical box model for the roadside increment. Thus, different emission scenarios and control options for long-range transport as well as regional and local emissions can be analysed. Observed concentrations and historical trends are well captured, in particular the differing NO2 and total NOx = NO + NO2 trends. Altogether, more than 1950 air quality monitoring stations in the EU are covered by the model, including more than 400 traffic stations and 70% of the critical stations. Together with its well-established bottom-up emission and dispersion calculation scheme, GAINS is thus able to bridge the scales from European-wide policies to impacts in street canyons. As an application of the model, we assess the evolution of attainment of NO2 limit values under current legislation until 2030. Strong improvements are expected with the introduction of the Euro 6 emission standard for light duty vehicles; however, for some major European cities, further measures may be required, in particular if aiming to achieve compliance at an earlier time.

NO2 concentrations at the street level are a major concern for urban air quality in Europe and have been regulated under the EU Thematic Strategy on Air Pollution. Despite the legal requirements, limit values are exceeded at many monitoring stations with little or no improvement in recent years. In order to assess the effects of future emission control regulations on roadside NO2 concentrations, a downscaling module has been implemented in the GAINS integrated assessment model. The module follows a hybrid approach based on atmospheric dispersion calculations and observations from the AirBase European air quality database that are used to estimate site-specific parameters. Pollutant concentrations at every monitoring site with sufficient data coverage are disaggregated into contributions from regional background, urban increment, and local roadside increment. The future evolution of each contribution is assessed with a model of the appropriate scale: 28 x 28 km grid based on the EMEP Model for the regional background, 7 x 7 km urban increment based on the CHIMERE Chemistry Transport Model, and a chemical box model for the roadside increment. Thus, different emission scenarios and control options for long-range transport as well as regional and local emissions can be analysed. Observed concentrations and historical trends are well captured, in particular the differing NO2 and total NOx = NO + NO2 trends. Altogether, more than 1950 air quality monitoring stations in the EU are covered by the model, including more than 400 traffic stations and 70% of the critical stations. Together with its well-established bottom-up emission and dispersion calculation scheme, GAINS is thus able to bridge the scales from European-wide policies to impacts in street canyons. As an application of the model, we assess the evolution of attainment of NO2 limit values under current legislation until 2030. Strong improvements are expected with the introduction of the Euro 6 emission standard for light duty vehicles; however, for some major European cities, further measures may be required, in particular if aiming to achieve compliance at an earlier time.

Integrated assessment models (IAM) and resulting scenarios have become increasingly institutionalised and relevant in the science-policy interface of climate policy. Despite their analytical strengths to conceive low-carbon futures, their co-evolution with the transnational science-policy interface of climate politics has also led to a focus on a specific set of techno-economic futures that are typically based on a relatively narrow set of assumptions. This deviates attention from alternatives that are hardly studied by IAMs, but might be more desirable from a societal perspective. We argue that research-based models and scenarios should support rather than narrow down deliberations on possible and desirable futures and provide an impetus to enact socially desirable change. Accordingly, we propose three future directions regarding the development and use of IAMs: 1) incorporate a plurality of perspectives on plausibility and desirability through iterative participatory engagement and worldview-based scenario exploration, 2) seek collaboration with the arts and humanities to expand the range of imagined futures beyond the status quo and 3) make projected futures more tangible and experiential so that diverse societal actor groups can understand and genuinely engage with them. By deploying the indisputable analytical strengths of IAMs optimally within these suggestions, we believe they can facilitate broader societal debates about transformative pathways to low-carbon futures.

Integrated assessment models (IAM) and resulting scenarios have become increasingly institutionalised and relevant in the science-policy interface of climate policy. Despite their analytical strengths to conceive low-carbon futures, their co-evolution with the transnational science-policy interface of climate politics has also led to a focus on a specific set of techno-economic futures that are typically based on a relatively narrow set of assumptions. This deviates attention from alternatives that are hardly studied by IAMs, but might be more desirable from a societal perspective. We argue that research-based models and scenarios should support rather than narrow down deliberations on possible and desirable futures and provide an impetus to enact socially desirable change. Accordingly, we propose three future directions regarding the development and use of IAMs: 1) incorporate a plurality of perspectives on plausibility and desirability through iterative participatory engagement and worldview-based scenario exploration, 2) seek collaboration with the arts and humanities to expand the range of imagined futures beyond the status quo and 3) make projected futures more tangible and experiential so that diverse societal actor groups can understand and genuinely engage with them. By deploying the indisputable analytical strengths of IAMs optimally within these suggestions, we believe they can facilitate broader societal debates about transformative pathways to low-carbon futures.

World leaders signed the Paris Agreement in 2015 to combat climate change. They agreed to limit global warming to 'well below' 2 °C relative to pre-industrial levels and strive to limit it further to 1.5 °C. The Agreement has a bottom-up architecture: it is up to the 192 Parties (191 countries plus the European Union) to formulate their targets and policies. At the moment, the sum of these individual targets for 2030 is not enough to reach the global emissions needed to meet the Paris climate goals. This difference is called the ambition gap. Moreover, current policies implemented by the countries are not yet sufficient to meet their pledged ambitions, which is called the implementation gap. This thesis addresses three main questions: 1) How large are the ambition and implementation gaps?; 2) When can countries achieve net-zero greenhouse gas emissions?; and 3) How can the gaps be bridged? Integrated Assessment Models were used at both the global level and the level of individual countries to answer these questions. We found that the global ambition and implementation gaps are roughly similar in size, so both need closing. To close the ambition gap, net-zero emission targets could be, if fully implemented, an important step in the right direction. However, they will need to be aligned with ambitious, short-term targets and backed up by climate policies to avoid widening of the implementation gap. To that end, replicating policies that have been successful in some countries (good practice policies) could be a stepping stone.