In recent decades, better data and methods have become available for understanding the complex functioning of cities and their impacts on sustainability. This review synthesizes the recent developments in concepts and methods being used to measure the impacts of cities on environmental sustainability. It differentiates between a dominant trend in research literature that concentrates on the accounting and allocation of greenhouse gas emissions and energy use to cities and a reemergence of studies that focus on the direct and indirect material and resource flows in cities. The methodological approaches reviewed may consider cities as either producers or consumers, and all recognize that urban environmental impacts can be local, regional, or global. As well as giving an overview of the methodological debates, we examine the implications of the different approaches for policy and the challenges these approaches face in their application in the field.

Nigeria is Africa's largest economy and home to approximately 10% of the unelectrified population of Sub-Saharan Africa. In 2017, 77 million Nigerians or 40% of the population had no access to affordable, reliable and sustainable electricity. In practice, diesel-and petrol-fuelled back-up generators supply the vast majority of electricity in the country. In Nigeria's nationally-determined contribution (NDC) under the Paris Agreement, over 60% of the greenhouse gas emissions (GHG) reductions are foreseen in the power sector. The goal of this study is to identify and critically examine the pathways available to Nigeria to meet its 2030 electricity access, renewables and decarbonization goals in the power sector. Using published data and stakeholder interviews, we build three potential scenarios for electrification and growth in demand, generation and transmission capacity. The demand assumptions incorporate existing knowledge on pathways for electrification via grid extension, mini-grids and solar home systems (SHS). The supply assumptions are built upon an evaluation of the investment pipeline for generation and transmission capacity, and possible scale-up rates up to 2030. The results reveal that, in the most ambitious Green Transition scenario, Nigeria meets its electricity access goals, whereby those connected to the grid achieve a Tier 3 level of access, and those served by sustainable off-grid solutions (mini-grids and SHS) achieve Tier 2. Decarbonization pledges would be surpassed in all three scenarios but renewable energy goals would only be partly met. Fossil fuel-based back-up generation continues to play a substantial role in all scenarios. The implications and critical uncertainties of these findings are extensively discussed.Key policy insights . The 2030 electricity mix for Nigeria varies across the scenarios presented, with the most ambitious scenario achieving electricity access goals and partly meeting renewable energy goals. . All three scenarios surpass the decarbonization targets of Nigeria's NDC for the power sector. . The transformation of the power sector relies on a wide range of financial, policy and enabling environment-related conditions taking place in the near-term, some of which are in turn strongly influenced by larger political economy realities. . Fossil fuel-based back-up generation plays a substantial role in all scenarios. Data availability for this technology remains a significant source of uncertainty. ARTICLE HISTORY

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Abstract:The German federal state of North Rhine-Westphalia (NRW) is home to one of the most important industrial regions in Europe, and is the first German state to have adopted its own Climate Protection Law (CPL). This paper describes the long-term (up to 2050) mitigation scenarios for NRW's main energy-intensive industrial sub-sectors which served to support the implementation of the CPL. It also describes the process of scenario development, as these scenarios were developed through stakeholder participation. The scenarios considered three different pathways (best-available technologies, break-through technologies, and CO2 capture and storage). All pathways had optimistic assumptions on the rate of industrial growth and availability of low-carbon electricity. We find that a policy of "re-industrialisation" for NRW based on the current industrial structures (assumed here to represent an average growth of NRWs industrial gross value added (GVA) of 1.6% per year until 2030 and 0.6% per year from 2030 to 2050), would pose a significant challenge for the achievement of overall energy demand and German greenhouse gas (GHG) emission targets, in particular as remaining efficiency potentials in NRW are limited. In the best-available technology (BAT) scenario CO2 emission reductions of only 16% are achieved, whereas the OPEN ACCESSEnergies 2015, 8 11405 low carbon (LC) and the carbon capture and storage (CCS) scenario achieve 50% and 79% reduction respectively. Our results indicate the importance of successful development and implementation of a decarbonised electricity supply and breakthrough technologies in industry-such as electrification, hydrogen-based processes for steel, alternative cements or CCS-if significant growth is to be achieved in combination with climate mitigation. They, however, also show that technological solutions alone, together with unmitigated growth in consumption of material goods, could be insufficient to meet GHG reduction targets in industry.

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