In 2015, 192 nations and the EU agreed to limit global warming to 2°C before 2050, preferably 1.5°C, compared to pre-industrial levels. At present, the 1°C threshold has been crossed . Annual global greenhouse gas emissions are estimated at 50 gigatonnes (Gt) (50 billion metric tonnes) CO2-eq per year, with around 70% of those related to energy generation. This includes electricity, combustion for transportation and heat generation.
Cities currently host 56% of the world’s population, a proportion that is expected to increase over time. They currently account for 71–80% of the global emissions, and are therefore the place to implement solutions to mitigate climate change. To design solutions for climate mitigation, the following conditions need to be clear: (a) the current levels of greenhouse gases in the geographical area need to be understood; (b) the sources of those emissions need to be identified; (c) targets for future emission levels need to be set; and (d) the different actors in the city, including municipal governments, industry and private citizens, as well as other actors such as financiers, need to design, implement, monitor and evaluate strategies to reduce the emissions. This coordinated effort is especially important if we consider that all actors and not only the municipal government are responsible for part of the emissions.
Emissions can be divided into three scopes (Figure 1). Scope 1 emissions are direct greenhouse gas emissions produced in a geographical area, a city in this case, such as transportation and energy generation. In scope 2, indirect emissions from energy use within the city are incorporated. The different energy sources that are included in this scope are grid-supplied electricity, heat, steam and/or cooling. Scope 3 is the most encompassing, using the point of consumption and not production as its focus. It considers all other indirect emissions, belonging to the value chain of goods and services that are purchased from or sold to other markets. A review of nine Swedish cities’ climate plans found that most focus on scope 1 and 2 emissions, while some have started discussing scope 3 emissions.
Cities behave like a system, consisting of stocks interconnected by flows that replenish or empty them. Urban stocks are composed of any element that can be counted or measured, such as people, currency, energy or plants. The flows are the processes that bring elements in and out of each stock, replenishing or depleting them. Within this system, measures taken in one part of the system will often affect stocks and flows in other parts of the system. This urban characteristic requires that every strategy aimed at supporting the transition to net zero needs to be assessed in terms of emissions reduction or carbon capture potential and in terms of secondary effects in the rest of the system, where enabling effects of different solutions can have a multiplying effect on the outcomes.
To reduce emissions, mitigation strategies can be classified into three categories: (a) options that result in reduced consumption, such as avoiding and shifting consumption; (b) options that improve operational and manufacturing efficiency, mostly focusing on physical assets; and (c) options that allow the capture and storage of carbon, taking out some of the emissions currently in the atmosphere. With this report, we aim to provide insight into the technologies that can be deployed at city level to reach climate neutrality, namely technologies to improve efficiency and technologies that capture and store carbon. We exclude electricity and energy generation, and industry solutions. Large-scale electricity generation can be beyond the municipal scope and industry encompasses a wide variety of typologies; therefore, general technological strategies would be hard to define. Thus, these two areas are not addressed in this document.
Our report is structured as follows: in section 2 we define the methodology used for the data gathering and the management of the data sourced; in section 3 we recreate the emissions baseline by identifying the key emitting sectors in the current global market, namely, transportation and the built environment, and their subsectors. In section 4 we identify the technologies that can reduce emissions in those sectors, including transversal strategies such as digital infrastructure and circular economy. In section 5, we focus on negative emission technologies. Section 6 ranks all the potential strategies and technologies as means to provide orientation for future practice, ending with concluding remarks on the analysis.