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Leaders or laggards in climate action? Assessing GHG trends and mitigation targets of global megacities [1]

['Mahendra Sethi', 'Chair Of Sustainability Economics In Human Settlements', 'Technical University Berlin', 'Berlin', 'Indian Society For Applied Research', 'Development', 'New Delhi', 'Felix Creutzig', 'Mercator Research Institute On Global Commons', 'Climate Change']

Date: 2023-01

Urban areas account for between 71% and 76% of CO 2 emissions from global final energy use and between 67–76% of global energy use. The highest emitting 100 urban areas (defined as contiguous population clusters) account for 18% of the global greenhouse gas (GHG) emissions. To date there is no comprehensive study of megacities (10 million+ population) analysing their historic population, economic and emission patterns and contributions to global GHGs. A key challenge is that a majority of these megacities (33 out of 41) are located in developing countries, making it challenging to track their rapidly mounting emissions. In this research, we capitalize on recently released open-access datasets—the Global Human Settlements Database (R2019A) and the World Urbanization Prospects (2018) for analyzing megacity development and GHG trends, vis-à-vis the mitigation targets outlined in their climate action plans. We find that as leading political and economic centres in their nations, though most megacities have initiated climate action plans, the aggregate impact of megacities on global emissions is limited. Based on this evidence, we explore how rapidly growing megacities can hedgehop to effectively reduce their GHG emissions while urbanizing and developing economically.

Funding: MS acknowledges the Alexander von Humboldt Foundation for the generous research fellowship hosted at the Technical University Berlin, Germany during the formative phase of this research. Further, MS’s research pertaining to the Asia-Pacific megacities is supported by grants from Asia-Pacific Network for Global Change Research under their Collaborative Regional Research Programme (CRRP) Project No. CRRP2020-04MY-Sethi, titled “Integrated climate action planning (ICLAP) 2050 tool in Asia-Pacific cities”. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

1. Introduction

As recent climate research undoubtedly indicates, global warming of 1.5°C and 2°C will be exceeded during the 21st century unless deep reductions in carbon dioxide (CO 2 ) and other greenhouse gas (GHG) emissions occur in the coming decades [1, 2]. This challenge mandates rapid and far-reaching transitions in energy, land, urban and infrastructure (including transport and buildings), and industrial systems [3]. Urban areas account for between 71% and 76% of CO 2 emissions from global final energy use and between 67–76% of global energy use [4]. Many international frameworks like the Sustainable Development Goals [5], the Sendai Framework for Disaster Risk Reduction [6], Paris Climate Agreements [7], and the New Urban Agenda [8] require human settlements information to feed indicators to form the basis for an empirical-informed policy for global climate action.

Studies on urban climate mitigation particularly offer robust regional insights into climate planning [9] that necessitate up-scaling for systematic review of global urban climate action plans (CAPs). In the last two-decades, a fair amount of groundwork has been set by urban emission studies, intensifying both in scale and scope [10]. These have largely focussed on preparing GHG accounts and mitigation policies for cities–individually and collectively. Some cities, like Toronto, New York, London, Barcelona, Tokyo, and Beijing, are disproportionally considered in studies of urban emissions and climate action [11]. In parallel, there are growing comparative studies between cities at the national or continental scale. For example, Brookings Institution [12] assessed the carbon footprint of the 100 largest metropolitan areas in the US considering highway transportation and energy consumption in residential buildings. In Europe, researchers evaluate and compare GHG emissions and climate plans of assorted European cities, samples ranging from 18 to 200 cities/city-regions [13–15]. Similarly, studies from South-Asia [16, 17] analyse GHG composition of prominent cities across India, Sri Lanka, Bangladesh & Nepal to demonstrate how certain urban spatial parameters like geo-physical location, settlement structure/organization and industrial landuse are strongly correlated with their GHGs.

A high-resolution gridded model demonstrates that 35% of the global GHGs are concentrated in 200 high-income cities and suburbs [18], 41 of these are located in countries with relatively low emissions [19]. This necessitates an in-depth analysis of global GHGs from megacities, i.e. cities with population over 10 million. Despite megacities hosting 626 million people in 2015 [20], there is no special assessment of their emission structures and progression over the years vis-à-vis the development trajectory. While some colleagues studied energy and material flows of 27 megacities [21], they put less attention into their GHG composition and trends. Meanwhile, few others concentrated on energy metabolism of megacities mainly in emerging economies [22]. One study investigated 274 cities worldwide and found 8 types of cities, sorted according to their GHG emission patterns, also exploring the prospective relevance of fuel pricing and urban planning for reducing GHG levels for 2050 [23]. Recently, a research tracked historical emission changes for 42 key world cities, though over two data points only [24]. In addition to the apparent deviation in urban focus (mainly targeting energy and transport sectors), there is deviation amongst different agencies in acknowledging which cities are considered megacities (elaborated in Section 3: Data and Methods) and so it is important that we consider all these definitional issues.

Moreover, the research scope of urban emissions studies is evolving, from mere GHG assessment and comparison to evaluating climate policies and actions. For instance, there has been a survey of climate change experiments in 100 world cities across all inhabited continents [25], as well as an investigation into how 885 EU cities [26] are combating climate change through local climate plans. In addition, there is plenty of grey literature from multilaterals, city networks, private companies and NGOs like the Covenant of Mayors, World Bank, UN-Habitat, ATKINS, IBM, ICLEI, Rockefeller Foundation, etc. documenting and disseminating best practices. As more and more cities perform GHG assessments, there are large variabilities and inconsistencies in methodologies inhibiting a transparent and comparative analysis [27], essentially because of input data, methods adopted and desired outputs [17]. A key methodological variation is on account of production-based or territorial emissions of a city as against consumption or footprint based assessment that extends beyond the city boundaries. In either case, any comparison of megacities should source reliable and consistent GHG data to draw historic trends and correlations with development indices.

Given the size, wealth and wherewithal, there are immense climate mitigation opportunities in developing megacities, yet vast fragmented literature of growing case studies, particularly in developing Asia and Africa largely remains underutilized for an exhaustive and mainstream policy application [18, 28, 29]. While global comparisons of city cases are sparse, often restricted by data unavailability, incomparability and sector-oriented inferences; open-access quantitative datasets released recently–in particular the World Urbanization Prospects [30] and Global Human Settlements (GHS) Database [20] that allow for systematic coding of urban development, energy and GHGs from 1970–2012, remain vastly unexplored to address certain crucial scientific and policy enquiries. For instance, some pointed queries include: How are megacity emissions evolving—are these expanding or shrinking? What are the specific drivers (population, affluence or technology), functions/ sectors significantly contributing (emission hotspots) in megacities and are these representative of their national circumstances? Secondly, emission data has not been utilized to interpret the qualitative data on mitigation strategies pursued under their urban climate policy. Here, we aim to systematically evaluate global megacity development and GHG trends, vis-à-vis the mitigation targets outlined in individual CAPs so as to reduce their GHG emissions while developing economically. In order to pursue this methodically, we first acknowledge the state-of-the-art literature on factors that influence or associate with urban GHGs (Section 2). We then adopt mixed methods (explained in Section 3) to analyze megacity development trends, emission compositions and their mutual correlations along with interpreting CAPs (section 4). We finally infer key findings and recommendations imperative to generate a more focused policy response for urban climate action and sustainable urbanization.

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[1] Url: https://journals.plos.org/climate/article?id=10.1371/journal.pclm.0000113

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