The C40 Cities Climate Leadership Group brings together nearly 100 major cities around a shared approach to urban climate action. Founded in 2005, the network has evolved from a forum for mayors into a more structured system for setting targets, developing climate plans, reporting emissions, and sharing policy tools. Its importance lies in the framework it provides for turning a global climate objective into decisions made at municipal level.
That framework starts with the Climate Action Planning Framework (CAPF), which asks cities to align their climate strategies with the Paris Agreement’s 1.5°C ambition. Cities are expected to establish emissions-reduction targets toward net zero by 2050, set interim goals for 2030, assess climate risks, address social equity, and integrate climate action into municipal planning and decision-making.
C40’s Deadline 2020 framework then attempted to translate those ambitions into city-level carbon trajectories, while its Climate Action Pathways work has assessed hundreds of possible interventions across buildings, energy, transportation, waste, and planning. The network’s Leadership Standards further formalize expectations around climate plans, delivery, mainstreaming, innovation, and collaboration.
This has turned climate action into something that reaches well beyond a city’s environmental department, shaping building regulations, transportation, land-use planning, public procurement, energy infrastructure, and municipal budgets. But C40’s policy framework only tells part of the story. The more useful measure is what happened on the ground, and, more specifically, which cities have actually reached peak emissions and continued to bring them down.
image © LI Xuejin
The cities that moved from targets to reductions
C40 has identified a group of cities that have already passed their emissions peak and are cutting greenhouse gas emissions. An earlier C40 analysis identified 27 cities that had peaked their emissions before 2012 and subsequently achieved sustained reductions, including Barcelona, Basel, Berlin, Boston, Chicago, Copenhagen, Heidelberg, London, Los Angeles, Madrid, Melbourne, Milan, Montréal, New Orleans, New York City, Oslo, Paris, Philadelphia, Portland, Rome, San Francisco, Stockholm, Sydney, Toronto, Vancouver, Warsaw and Washington, DC.
C40’s analysis found that the 27-city cohort reduced direct emissions by an average of around 2 percent per year after reaching peak emissions, while population and economic output continued to grow.
The trajectories are not identical, and neither are the policies behind them, though several patterns emerge.
Energy systems are one. Cities such as Copenhagen and Stockholm have reduced emissions through the decarbonization of district heating and electricity systems. Buildings are another, with increasingly stringent performance standards, benchmarking requirements and retrofit policies targeting the existing building stock. Transportation provides a third, with public transit, cycling infrastructure, vehicle regulation and electrification having become central tools for reducing urban emissions.
Copenhagen: large reductions, difficult final steps
Copenhagen is a good example of how far city-led climate action can go, and where it starts to run into harder limits. The Danish capital cut its territorial greenhouse gas emissions by around 80 percent between 2009 and 2022, with much of that progress coming from changes to its district heating system, renewable energy and long-term investment in cycling. These are not climate measures sitting apart from the city itself; they have reshaped how Copenhagen is heated, powered and navigated. But the last part has proved much harder. Copenhagen had set itself the goal of becoming carbon neutral by 2025, a target it ultimately did not meet. One of the sticking points was carbon capture at the Amager Bakke waste-to-energy facility. After major reductions in energy and transport emissions, technologies such as CCS became important for tackling the harder-to-remove remainder, but delivering them depended on financing and regulations beyond the city’s control. Copenhagen’s experience points to a familiar problem in urban decarbonization: the biggest reductions can come from changes a city can make itself, while the final ones may depend on infrastructure, money and decisions at another level of government.
image by Nick Karvounis
Oslo: when carbon becomes part of the city budget
Oslo has taken a different route, bringing climate targets into the city’s everyday budgeting process. Its Klimabudsjett, or Climate Budget, sets out emissions targets and measures alongside the city’s ordinary financial budget, giving municipal departments responsibility for specific reductions and tracking their progress over time.
The approach has been paired with practical changes across the city. Oslo has cut direct territorial emissions since 2009 and used its purchasing power to push fossil-free and zero-emission machinery on municipal construction projects. Electric vehicles and charging infrastructure have also become part of the city’s wider transition. But the harder reductions are proving more difficult. Oslo’s longer-term target depends in part on carbon capture at the Klemetsrud waste-to-energy plant, tying the city’s climate goals to a large and technically demanding infrastructure project, as well as the financing and construction time needed to deliver it. Oslo shows both the strength and the limits of putting climate action into the machinery of city government. A city can set the budget, change its procurement rules and push new infrastructure, but some of the biggest remaining emissions depend on projects and decisions beyond City Hall.
image by Andriy Oliynyk
Tokyo demonstrates another model of municipal climate action.
Tokyo offers a different approach, using regulation to tackle emissions from the city’s large buildings. In 2010, the Tokyo Metropolitan Government introduced its Tokyo Cap-and-Trade System, requiring large commercial and industrial facilities to meet mandatory emissions-reduction targets. Instead of relying mainly on voluntary action, the program sets facility-level baselines and requires emissions to be monitored and verified. What makes the system interesting is its treatment of buildings as part of a much larger energy system.
Tokyo uses regulation to push changes across a large part of the commercial building stock. That matters as cities turn their attention to existing buildings, where much of the energy demand is already locked in. Retrofitting heating, cooling and energy systems can affect thousands of buildings at once, making the accumulated impact far greater than that of individual new projects. Tokyo’s approach shows how regulation can work alongside design and construction, setting the conditions that push an entire sector in the same direction.
image via C40
The infrastructure behind the numbers
Across the C40 cities that have sustained emissions reductions, the changes are visible in the way cities are built and run, through cleaner electricity and district heating, tighter building standards, energy retrofits, expanded public transportation, cycling networks, low-emission zones and electric vehicles. Climate targets can sound abstract on paper, net zero by a certain year, or a percentage reduction from a baseline. In practice, they come down to much more familiar parts of urban life. Building codes determine what gets built. Transit networks shape how people move around the city. Low-emission zones change which vehicles can use its streets. Municipal procurement can create a market for electric construction equipment, while a climate budget can make individual departments responsible for delivering specific reductions. Looking across the C40 cities that have made measurable progress, one thing stands out: the targets tend to work when they are tied to the everyday decisions that shape the city.
C40 cities generally use the Global Protocol for Community-Scale Greenhouse Gas Emission Inventories (GPC) to track emissions produced within their boundaries. The system gives cities a common way to measure and compare emissions, but it leaves out some of the carbon associated with what those cities consume. A city can cut the emissions produced within its borders while still relying on materials, products and energy-intensive supply chains elsewhere. This can be particularly significant in cities with economies dominated by services, finance and other relatively low-emission activities.
C40’s own research found that the combined consumption footprint of 79 cities was substantially larger than their territorial emissions, with much of the difference coming from goods and supply chains outside the city. Construction is a good example. The emissions from producing cement, steel and other building materials are generated before those materials ever reach the site, so they do not necessarily appear in a city’s territorial inventory. A building can therefore look efficient once it is in operation while carrying a much larger carbon footprint from the materials used to construct it. There is a similar challenge when city inventories are compared with independent atmospheric measurements. Studies have found significant differences between self-reported inventories and atmospheric estimates in some cities, underlining how difficult it can be to pin emissions neatly to an administrative boundary.
image by Timo Volz
From ambition to evidence
C40 has given cities a shared framework for turning climate targets into policies, regulations and investment, but the cities that stand out are not necessarily those with the most ambitious targets. They are the ones where those targets have translated into changes on the ground and where emissions data shows that those changes are having an effect.
Copenhagen, Oslo and Tokyo offer three different approaches: changing the energy and transport systems, putting emissions into the municipal budget, and regulating the building stock. Each uses powers that already exist within city government to tackle a different part of the emissions problem.
The wider group of early-peaking C40 cities shows that emissions can fall while cities continue to grow. It also points to a harder phase ahead. As the easier reductions are made, cities have to deal with waste, industrial emissions, embodied carbon, national energy systems and supply chains that extend well beyond their boundaries. That makes the next test fairly straightforward: can cities keep cutting emissions as the remaining sources become harder to reach, and do the numbers still hold when their carbon footprint is measured beyond the city itself?
map of the C40 cities
This article is part of designboom’s MODES OF HABITATION chapter, exploring how the spaces we inhabit can reshape the ways we live together. Discover more stories rethinking domesticity, shared living, and the many forms a home can take here.
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