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The current nationally determined contributions, pledged by the countries under the Paris Agreement, are far from limiting climate change to below 2 ∘C temperature increase by the end of the century. The necessary ratcheting up of climate policy is projected to come with a wide array of additional benefits, in particular a reduction of today's 4.5 million annual premature deaths due to poor air quality. This paper therefore addresses the question how climate policy and air pollution–related health impacts interplay until 2050 by developing a comprehensive global modeling framework along the cause and effect chain of air pollution–induced social costs. We find that ratcheting up climate policy to a 2 ∘ compliant pathway results in welfare benefits through reduced air pollution that are larger than mitigation costs, even with avoided climate change damages neglected. The regional analysis demonstrates that the 2 ∘C pathway is therefore, from a social cost perspective, a "no-regret option" in the global aggregate, but in particular for China and India due to high air quality benefits, and also for developed regions due to net negative mitigation costs. Energy and resource exporting regions, on the other hand, face higher mitigation cost than benefits. Our analysis further shows that the result of higher health benefits than mitigation costs is robust across various air pollution control scenarios. However, although climate mitigation results in substantial air pollution emission reductions overall, we find significant remaining emissions in the transport and industry sectors even in a 2 ∘C world. We therefore call for further research in how to optimally exploit climate policy and air pollution control, deriving climate change mitigation pathways that maximize co-benefits. [ABSTRACT FROM AUTHOR]

The most stringent emission scenarios published by the Intergovernmental Panel on Climate Change (IPCC) would result in the stabilization of atmospheric carbon dioxide (CO2) at concentrations of approximately 550 ppm which would produce a global temperature increase of at least 2°C by 2100. Given the large uncertainties regarding the potential risks associated with this degree of global warming, it would be more prudent to stabilize atmospheric CO2 concentrations at or below current levels which, in turn, would require more than 20-fold reduction (i.e.,≥95%) in per capita carbon emissions in industrialized nations within the next 50–100 years. Using the Kaya equation as a conceptual framework, this paper examines whether CO2 mitigation approaches such as energy efficiency improvements, carbon sequestration, and the development of carbon-free energy sources would be sufficient to bring about the required reduction in per capita carbon emissions without creating unforeseen negative impacts elsewhere. In terms of energy efficiency, large improvements (≥5-fold) are in principle possible through aggressive investments in R&D and the removal of market imperfections such as corporate subsidies. However, energy efficiency improvements per se will not result in a reduction in carbon emissions if, as predicted by the IPCC, the size of the global economy expands 12-26-fold by 2100. Terrestrial carbon sequestration via reforestation and improved agricultural soil management has many environmental advantages, but has only limited CO2 mitigation potential because the global terrestrial carbon sink (ca. 200 Gt C) is small relative to the size of fossil fuel deposits (≥4000 Gt C). By contrast, very large amounts of CO2 can potentially be removed from the atmosphere via sequestration in geologic formations and oceans, but carbon storage is not permanent and is likely to create many unpredictable environmental consequences. Renewable energy can in theory provide large amounts of carbon-free power. However, biomass and hydroelectric energy can only be marginally expanded, and large-scale solar energy installations (i.e., wind, photovoltaics, and direct thermal) are likely to have significant negative environmental impacts. Expansion of nuclear energy is highly unlikely due to concerns over reactor safety, radioactive waste management, weapons proliferation, and cost. In view of the serious limitations and liabilities of many proposed CO2 mitigation approaches, it appears that there remain only few no-regrets options such as drastic energy efficiency improvements, extensive terrestrial carbon sequestration, and cautious expansion of renewable energy generation. These promising CO2 mitigation technologies have the potential to bring about the required 20-fold reduction in per capita carbon emission only if population and economic growth are halted without delay. Therefore, addressing the problem of global warming requires not only technological research and development but also a reexamination of core values that equate material consumption and economic growth with happiness and wellbeing. [ABSTRACT FROM AUTHOR]

Climate change has considerably dominated science-policy dialogue, public debate, and subsequently environmental policies since the three "Rio Conventions" were born. This has led to practically independent courses of action of climate change mitigation and biodiversity conservation actions, neglecting potential conflicts among outcomes and with missed opportunities for synergistic measures. Transformative governance principles have been proposed to overcome these limitations. Using a transformative governance lens, we use the case of the Norwegian "Climate Cure 2030" for the Land Use, Land-Use Change and Forestry (LULUCF) sector to, first, illustrate the mechanisms that have led to the choice of climate mitigation measures; second, to analyze the potential consequences of these measures on biodiversity and greenhouse gas (GHG) emissions; and, third, to evaluate alternative measures with potential positive outcomes for biodiversity and GHG emissions/removals. We point to some mechanisms that could support the implementation of these positive actions. [ABSTRACT FROM AUTHOR]

Land-use emissions of greenhouse gases make up over one-third of current total anthropogenic emissions of greenhouse gases and about three-quarters of the total anthropogenic emissions of CH4 and N2O. Considering their contribution to global emissions, it is important to understand their future trends in order to anticipate and mitigate climate change. This paper reviews published scenarios of major categories of these emissions with the aim to provide background information for the development of new scenarios. These categories include CO2 from deforestation, CH4 from rice cultivation, CH4 from enteric fermentation of cattle, and N2O from fertilizer application. Base year estimates of all these categories varied greatly from reference to reference, and hence emissions of all scenarios were normalized relative to their 1990 value before being compared to one another. The range of published scenarios of CO2 emissions from deforestation is widest around the middle of the 21st century and then all scenarios converge to low values towards 2100. By contrast, the different scenarios of CH4 and N2O diverge with time, showing their widest range in 2100. Global emissions of CH4 from rice cultivation vary by a factor of three in 2100 and N2O from fertilized soils by a factor of 2.3. Emissions of CH4 from enteric fermentation of animals have the smallest range (factor of 2.0). The typical long-range trends of land-use emission scenarios vary greatly from region to region - they stabilize in industrialized regions after a few decades, but tend to stabilize later in developing regions or continue to grow throughout the 21st century. To improve the realism of the estimates of future trends of land-use emissions, it is especially important to improve the estimation of the future extent of agricultural land and the rate of deforestation, while taking into account significant driving forces such as the demand for agricultural commodities and crop yields. [ABSTRACT FROM AUTHOR]

We consider alternative history scenarios in which explicit climate mitigation begins before the present day, estimating the total costs to date of delayed action. Considering a 2(1.5) degree Celsius stabilization target, peak costs are greater and reached sooner with a later start to mitigation, reaching 15(17)% of global GDP in 2085(2070) for a 1990 start and 18(35)% in 2080(2035) for a 2020 start. Further mitigation delay costs a best estimate of an additional 0.5(5) trillion dollars per year. Additional simulations show how optimal mitigation pathways evolve without imposing a warming limit, finding that median abatement levels and costs are not strongly dependent on start date. However, whereas 18(5) percent of optimal solutions starting in 1980 meet the 2(or 1.5) degree target, 5(or 0)% of 2020 simulations meet the goals. Discounted damages due to delayed mitigation action rise by 0.6 trillion US dollars per year in 2020. [ABSTRACT FROM AUTHOR]

In light of the persistent failure to reduce emissions decisively, facilitate long-term resilience against climate change and account for the connectedness of climate change with other social, environmental and economic concerns, we present a conceptual framework of capacities for transformative climate governance. Transformative climate governance enables climate mitigation and adaptation while purposefully steering societies towards low-carbon, resilient and sustainable objectives. The framework provides a systematic analytical tool for understanding and supporting the already ongoing changes of the climate governance landscape towards more experimental approaches that include multi-scale, cross-sectoral and public-private collaborations. It distinguishes between different types of capacities needed to address transformation dynamics, including responding to disturbances (stewarding capacity), phasing-out drivers of path dependency (unlocking capacity), creating and embedding novelties (transformative capacity) and coordinating multi-actor processes (orchestrating capacity). Our case study of climate governance in Rotterdam, the Netherlands, demonstrates how the framework helps to map the activities by which multiple actors create new types of conditions for transformative climate governance, assess the effectiveness of the capacities and identify capacity gaps. Transformative and orchestrating capacities in Rotterdam emerged through the creation of space and informal networks for strategic and operational innovation, which also propelled new types of governance arrangements and structures. Both capacities support stewarding and unlocking by integrating and mainstreaming different goals, connecting actors to each other for the development of solutions and mediating interests. Key challenges across capacities remain because of limited mainstreaming of long-term and integrated thinking into institutional and regulatory frameworks. As the ongoing changes in climate governance open up multiple questions about actor roles, effective governance processes, legitimacy and how effective climate governance in the context of transformations can be supported, we invite future research to apply the capacities framework to explore these questions. [ABSTRACT FROM AUTHOR]

Social scientists and science communicators are concerned about the apparent discrepancy between the scientific consensus on climate change (Anderegg et al. Proc Natl Acad Sci 107:12107-12109, 2010; Doran and Zimmerman EOS Trans Am Geophys Union 90:22-3, 2009) and the general public’s views (Knight Environ Sociol 2:101-113, 2016; Lee et al. Nat Clim Chang 5:1014-1020, 2015). It is reasoned that increased public awareness and perceived threat of climate change may pressure governments to enact policy to counteract climate change (e.g. setting stringent carbon emissions targets). Despite a logical link between public awareness and government-set emissions targets, this relationship remains untested. We examined the relationship between public awareness about and perceived threat of climate change and governmental emissions targets across 71 countries and 1 region. We found a positive association between the proportions of a country’s population that are aware of climate change and the unconditional emissions reduction targets set by that country in the Paris Agreement (Rogelj et al. Nature 534:631-639, 2016). However, the proportion of people in a country who perceive climate change as a personal threat was not associated with higher emissions reduction targets. Our results suggest that public awareness may be an important part of garnering the public support required for policies designed to mitigate climate change to succeed. [ABSTRACT FROM AUTHOR]

In this essay I argue that the research on emission scenarios over the past few years has focused on low emission scenarios while producing few medium level emission scenarios. As a consequence, there is a gap in the literature on emission scenarios which prohibits answering several basic questions. This relative lack of medium emission scenarios is problematic for three reasons. First, scientific integrity, as the research community should produce scenarios along the full range of future options. Second, there is currently a very limited number of scenarios that describe real-world conditions of imperfect climate policy combined with technology restrictions. Finally, I observe a growing mismatch between the mitigation research focus on low emission scenarios and research to impacts and adaptation on medium and high emission scenarios. If this trend continues, it will be difficult to draw overarching conclusions from mitigation and impacts research in IPCCs 6th Assessment Report. Therefore, this essay ends with several recommendations for the mitigation research community to stimulate production and publication of medium level emission scenarios. [ABSTRACT FROM AUTHOR]

The Global Warming Potential (GWP) index is currently used to create CO-equivalent emission totals for multi-gas greenhouse targets. While many alternatives have been proposed, it is not possible to uniquely define a metric that captures the different impacts of emissions of substances with widely disparate atmospheric lifetimes, which leads to a wide range of possible index values. We examine the sensitivity of emissions and climate outcomes to the value of the index used to aggregate methane emissions using a technologically detailed integrated assessment model. The methane index is varied between 4 and 70, with a central value of 21, which is the 100-year GWP value currently used in policy contexts. We find that the sensitivity to index value is, at most, 10-18 % in terms of methane emissions but only 2-3 % in terms of the maximum total radiative forcing change, with larger regional emissions differences in some cases. The choice of index also affects estimates of the cost of meeting a given end of century forcing target, with total two-gas mitigation cost increasing by 7-9 % if the index is increased, and increasing in most scenarios from 4 to 23 % if the index is lowered, with a slight (1 %) decrease in total cost in one case. We find that much of the methane abatement occurs as the induced effect of CO abatement rather than explicit abatement, which is one reason why climate outcomes are relatively insensitive to the index value. We also find that the near-term climate benefit of increasing the methane index is small. [ABSTRACT FROM AUTHOR]

The majority of climate change impacts assessments account for climate change uncertainty by adopting the scenario-based approach. This typically involves assessing the impacts for a small number of emissions scenarios but neglecting the role of climate model physics uncertainty. Perturbed physics ensemble (PPE) climate simulations offer a unique opportunity to explore this uncertainty. Furthermore, PPEs mean it is now possible to make risk-based impacts estimates because they allow for a range of estimates to be presented to decision-makers, which spans the range of climate model physics uncertainty inherent from a given climate model and emissions scenario, due to uncertainty associated with the understanding of physical processes in the climate model. This is generally not possible with the scenario-based approach. Here, we present the first application of a PPE to estimate the impact of climate change on heat-related mortality. By using the estimated impacts of climate change on heat-related mortality in six cities, we demonstrate the benefits of quantifying climate model physics uncertainty in climate change impacts assessment over the more common scenario-based approach. We also show that the impacts are more sensitive to climate model physics uncertainty than they are to emissions scenario uncertainty, and least sensitive to whether the climate change projections are from a global climate model or a regional climate model. The results demonstrate the importance of presenting model uncertainties in climate change impacts assessments if the impacts are to be placed within a climate risk management framework. [ABSTRACT FROM AUTHOR]