I began my journey on city-scale carbon accounting back in 2005, when I first started to work with the City and County of Denver on developing its greenhouse gas inventory, and associated climate action plan. Recognizing that cities and urban areas generate more than 80% of the world’s GDP and contribute to over 70% of greenhouse gases (GHGs), several US mayors committed to GHG mitigation through the Climate Protection agreement. However, back in 2005, there were no protocols for cities to measure GHG emissions. This stimulated a long period of research developed in partnership with Denver, several other cities, as well as networks of cities like ICLEI-USA.
Considering essential requirements for both homes and businesses, seven key provisioning systems that provide food, energy, mobility-connectivity, water, shelter/construction materials, waste management, and green infrastructure emerge as critical for community-wide functioning.
Back in 2005, I quickly realized that measuring GHG emissions associated with cities is complicated and different from both how nations and businesses were tracking their emissions. What makes it so challenging is that cities are smaller-scaled open systems embedded within larger-scaled infrastructure and trade networks. That means a lot of materials, energy, water, and other goods and services flow in and out of cities, which really complicates carbon accounting. Considering essential requirements for both homes and businesses, seven key provisioning systems that provide food, energy, mobility-connectivity, water, shelter/construction materials, waste management, and green infrastructure emerge as critical for community-wide functioning. Many of these sectors rely on transboundary supply chains, i.e., food, energy and construction materials used in cities are often produced in farms, powerplants and factories that maybe be located in proximate hinterlands or across the globe. This transboundary feature of cities has stimulated a new field of study wherein cities are studied not purely within their administrative boundaries but from a local-to-global urban system perspective.
Among practitioners as well, urban carbon (GHG) accounting has evolved from an initial focus on direct territorial in-boundary emissions (called Scope 1 emissions) to recognizing the importance of supply chain emissions serving cities. Back in 2005, cities intuitively recognized accounting for emissions associated with imported electricity (called Scope 2 emissions). But there was little standardization of other supply chains that cities should address. When I started work with Denver, I made the case for focusing on the transboundary supply chain associated with the seven key sectors noted above. The city found the argument compelling, and we co-developed the concept of transboundary carbon footprinting, focusing on the seven key sectors, which we called community-wide infrastructure supply chain carbon footprinting. This was also one of my first papers wherein science co-produced with practitioners led the way for researchers to conceptualize cities as transboundary systems. Practitioners as well started to think about Scope 3 emissions, i.e., supply chain emissions associated with producing goods and services that are imported to cities.
However, the puzzle didn’t end there. A major question that arose was how many more Scope 3 emissions should be counted in cities. The approach that Denver and I had pioneered looked at the seven key provisioning systems. At the same time, others were testing out a consumption-based approach, such as the CoolClimate calculator, that accounted for emissions embodied in all goods and services serving households in cities. This approach, however, does not address emissions from businesses and industries that serve visitors and produce exports elsewhere. For example, the consumption approach in a ski town like Aspen, USA, would ignore all emissions associated with ski resorts that cities have a lot of influence over, but that serve visitors. For New York City (NYC), a consumption approach would not count energy-related emissions from all financial services that serve customers outside of NYC.
In 2011, I was part of a multi-stakeholder technical advisory council convened by ICLEI-USA to understand the similarities and differences in the two approaches. Deliberations in the committee became the catalyst for me to identify mathematical relationships that clarify the various flows in and out of cities, mapping them to different GHG accounting approaches. Based on our deliberations and the new science that emerged, the ICLEI-USA Community Protocol recommended both a community-wide Scope 1+2+3 approach and a separate consumption-based account.
However, fast forward 10 years, cities and researchers in 2021 have still not settled on this question of how and what to account for in terms of GHG emissions. For one thing, new technologies, including satellite-based remote sensing, are now providing unprecedented capacity to measure place-based GHG emissions, renewing interest in Scope 1 territorial accounts. Transboundary approaches have also evolved to a total supply chain approach that theoretically can track all carbon embodied in goods and services flowing in and out of the cities, although data to do this for most world cities are lacking. With the emerging science, more and more cities are feeling pressure to count for more and more emissions, yet recognizing the complexity of doing so. Indeed, at a meeting of the Global Carbon Project in 2019, it became apparent that there was little clarity on the (now four) approaches that had emerged for carbon accounting. Both practitioner and researcher communities needed a new way to think about urban emissions considering a net-zero carbon future. That became the catalyst for our paper, now co-produced with several researchers from all over the world, along with representatives of organizations that help develop GHG protocols for cities (ICLEI-USA & WRI).
We make the case for defining a net-zero carbon city as one with net-zero carbon transboundary infrastructure & food provisioning systems.
Our paper contributes in three ways. First, we argue that counting more emissions does not, in fact, enhance decarbonization policy per se. We show that each of the four accounting approaches - territorial, community-wide infrastructure, consumption-based, and total supply chain footprinting – inform different aspects of decarbonization policy, namely source-based tracking of GHGs, urban infrastructure transition planning, informing household actions, and, informing carbon embodied in trade (beyond the infrastructure sectors). Second, we make the case for defining a net-zero carbon city as one with net-zero carbon transboundary infrastructure & food provisioning systems (see figure below). We highlight the focus on these seven sectors can be particularly strategic, because these sectors contribute to about 90% of global GHG emissions and they also shape additional UN-SDGs (e.g., inequality, health and wellbeing, land, water, energy, climate, and cities). Furthermore, by focusing on these key sectors, carbon mitigation can be linked with other agendas, e.g., energy transitions, mobility transitions, smart cities, and nature-based solutions. Indeed, by focusing on these seven sectors systemically in ways that connect urban demand with production, we can align urban actions with national net-zero goals for energy, mobility, construction materials, agriculture, and land-use. So, we make a strong case to focus community-wide Scope 1+2+3 GHG accounting on the seven key provisioning sectors. The last and most important insight is that, in a net-zero future driven by net-zero energy, mobility and agricultural systems, carbon emissions embodied in trade of other goods/services will automatically become near zero. Rather than complex accounting of emissions embodied in all trade flows across cities, we argue that cities can strategically focus on seven provisioning systems that will be critical to achieve a net-zero carbon and equitable world. In tandem, a new urban data science (e.g., see Stokes & Seto 2018) is needed to characterize all seven sectors across urban areas worldwide.
Figure: A net-zero carbon city = net-zero carbon transboundary infrastructure and food systems across seven sectors
I hope our paper stimulates dialogue among and across researcher and practitioner communities on what to count in community-wide Scope 1+2+3 emissions for a net-zero future, finally setting this 20-year debate to rest. I also hope that this arc of knowledge co-production inspires further collaboration to address the carbon-inequality nexus, carbon-health nexus, and the carbon-resource nexus to advance multiple UN-SDGs across all urban areas worldwide.
Read the paper here: Carbon analytics for net-zero emissions sustainable cities