Welcome to the front lines of climate change

THE BIG STORY | November 24, 2025

Welcome to the front lines of climate change

Just before the United Nations’ annual climate conference kicked off in Brazil November 10, three hundred mayors gathered in Rio de Janeiro. C40, a coalition of big-city leaders that has long pushed to be included in decision-making about climate action, hosted the mayors for talks about dealing with the extreme heat, turbocharged storms, and other results of a changed climate. As the summit opened, the conference’s president André Corrêa do Lago stressed “the need to place cities at the center of climate negotiations.”

Is the mayors’ moment here?

By Peter Engelke

Imagine standing on any street corner in any city center in the world: New York, Los Angeles, Tokyo, Shanghai, Delhi, Berlin, Dubai, Lagos, São Paulo, or thousands of others. You would see a bustling hive of activity, people conversing, working, selling, buying, playing, demonstrating, shipping, transporting, exercising, building, demolishing, loading, unloading, and a million other things besides. These scenes are both ancient and new: ancient in that they repeat countless similar activities on countless street corners over thousands of years; new in that the massive amounts of energy required by such activities in cities today are transforming the very planet we live upon.

Cities are where most of the world’s population lives, where the vast majority of its buildings, factories, companies, homes, and vehicles are located. They are also a central driver of anthropogenic climate change, accounting for 75 percent of global energy use and 70 percent of global carbon dioxide (CO₂) emissions.¹ But they are also where climate solutions will be found. The trick will be in minimizing the first and maximizing the second. As United Nations (UN) Secretary-General António Guterres has said, “Cities are where the climate battle will largely be won or lost.”²

If we are to decarbonize the global economy in time to ward off the worst climate impacts, then we are going to have to transform how thousands of cities around the world function, and how billions of people live, work, travel, and entertain themselves in cities.

Are cities up to the challenge of shaping a livable future for billions of people? Will the world’s nation-states enlist these powerful assets in the fight against climate change, helping cities find and scale up workable solutions?

Why cities matter

Cities can be a partner for—or a counterweight to—national governments.

The debate over how to reduce carbon emissions—decarbonization—typically focuses on nations: how China’s emissions stack up against India’s, for instance. Nations have the power as sovereigns to raise funds, direct investment, and drive policy. Yet the world’s national governments have failed to place binding limits on carbon emissions, with the nonbinding nationally determined contributions under the 2015 Paris Agreement being the most successful outcome of the United Nations Framework Convention on Climate Change process thus far. Unfortunately (and predictably), there has been little progress implementing those emissions caps: National governments’ current policies are insufficient to reach the Paris Agreement’s target of limiting warming to 1.5 degrees Celsius above preindustrial levels.³ (The 1.5-degree goal is swiftly disappearing as a realistic target. In 2024, the world’s hottest year on record, the global average temperature was 1.6 degrees Celsius higher than preindustrial levels.)⁴ Nor is progress guaranteed, as states’ decarbonization policies can change—including in the wrong direction—with electoral outcomes and shifts in priorities.⁵ The United States, to give a prominent example, scrapped its Paris Agreement commitments to cut emissions when President Donald Trump again withdrew the United States from the Paris Agreement on the first day of his second term.

Cities create wealth and power.

There is every reason for national governments to work with city governments on climate, for cities are among the greatest assets that countries have. Cities are an important element of national power: They are, the economist Edward Glaeser argued in his landmark 2011 book, Triumph of the City, gigantic value-adding machines. Why? People live close together in cities. This physical proximity leads to exchange, invention, cooperation, planning, and execution, enabling the formation of institutions—governments, stock exchanges, start-ups, factories, corporations, universities, philanthropies, hospitals, and much more.⁶ Proximity thereby creates wealth, through invention, innovation, collaboration, manufacturing, servicing, and a great many other valuable things besides. The data bear this out: Globally and on balance, the most urbanized countries are the wealthiest, the least urbanized the poorest.⁷

Cities are where most people live.

Although cities are among humanity’s oldest complex creations (the world’s first cities emerged at least five thousand years ago), it was not until recently in historical terms that a truly urbanized world emerged.⁸ Rising fossil fuel use was at the center of this shift. The Industrial Revolution, which captured energy from coal, was the turning point. Starting in northern Europe and expanding outward, the Industrial Revolution enabled mass urbanization, as steam-powered factories pulled workers off the land and into cities.⁹ The city’s appeal today is the same as it was when the revolutionary Boulton & Watt steam engines appeared in Birmingham, England, in the 1770s: Cities, per Glaeser, are where economic opportunities lie.¹⁰

The world is still going through this demographic realignment that began with the Industrial Revolution. In 2022, cities were home to 4.5 billion people, about 57 percent of the global population.¹¹ Those figures will rise for decades to come, by 2050 to nearly 70 percent of the world’s population (6.7 billion city dwellers), with most growth occurring in Asia and Africa.¹²

The five sectors to green up

Cities will need to swiftly cut carbon emissions if global warming is to be limited to 1.5 degrees Celsius above preindustrial levels, the goal set under the 2015 Paris Agreement, or even 2 degrees given that the 1.5-degree goal in 2025 is already nearly moot.¹³ There are numerous ways in which cities can cut their emissions, including removing fossil fuels from electricity generation and power grids, making buildings more efficient (buildings account for roughly 40 percent of all greenhouse gas emissions), electrifying urban transport, shifting to nonmotorized modes such as the bicycle, reducing waste streams, decarbonizing urban infrastructure, and expanding local carbon sinks such as urban tree cover.¹⁴ This diversity of urban decarbonization can be grouped into five main categories—electricity, buildings, transportation, waste, and carbon sinks and offsets—from which most local emissions are generated.

Examples of urban creativity across these five priority sectors can be found nearly everywhere in the world.

Tokyo in 2010 became the first city in the world to implement a mandatory CO₂ cap-and-trade program, focused on the metropolitan area’s largest CO₂ emitters (factories, large buildings, and so on).¹⁵

Paris is implementing Plan Vélo to substantially increase the share of residents traveling by the greenest mode of travel—the bicycle. The plan will dramatically expand the city’s network of bicycle lanes, prioritize cyclists on streets and at intersections (via traffic lights and other traffic rules), expand bicycle parking throughout the city, and otherwise encourage cycling at the expense of driving.¹⁶

Cape Town is piloting waste-to-energy plants to reduce methane releases from the city’s landfills, with an eventual goal of generating 7-9 megawatts of electricity. For decades, Cape Town has been a sustainability pioneer, having experimented with numerous approaches to reducing the city’s carbon emissions through renewable energy projects.¹⁷

Melbourne is piloting a neighborhood battery system that will store renewable energy generated during the daytime and then release it to communities—residents and small businesses—in the evenings when it is most needed.¹⁸

Medellín has planted thousands of trees and hundreds of thousands of shrubs along “green corridors”—roads and waterways—to cool temperatures in the city during heatwaves.¹⁹

A neighborhood battery in Melbourne, Australia. The local government installed three large batteries around the city in 2024 to store renewable energy for city residents to use. Photo courtesy energy.vic.gov.au

The scale of urban decarbonization at the global level is enormous, given the thousands of cities and billions of structures, vehicles, and people that will be involved if the world’s cities are to be successfully decarbonized. In the building sector alone, new urban construction will have to be much less carbon intensive—projections show that some 40 percent of all buildings that will exist in 2050 are not yet built—while the stock of existing buildings will have to be renovated.²⁰ Global energy demand in buildings continues to rise, owing (mostly) to rapid urbanization in a few world regions—Africa, the Middle East, the Asia-Pacific, and Eurasia. Most of the projected growth will occur in emerging economies that lack advanced energy-efficient building codes. However, there is enormous opportunity here as well: The adoption of efficiency codes for new buildings can help all economies, advanced and emerging; make buildings more energy efficient; spur demand for renewable sources of energy; and boost climate resilience by, for example, enabling buildings to better withstand heatwaves.²¹

An important piece of good news here is that local leaders tend to see decarbonization as a positive for their cities, rather than a negative. Decarbonization strategies in cities, if envisioned and implemented smartly, can enhance urban livability, address inequities for historically marginalized communities, reduce climate impacts, improve the local environment, and boost local economic competitiveness all at once. This positivity springs in large part from the pragmatism that abounds in cities. Cities are in constant flux as economic, demographic, environmental, technological, cultural, social, political, and other driving forces change in real time. To stay in office, mayors and city councils must competently anticipate and manage these forces.²²Increasingly, they seem to understand that the creation of a livable, healthy, safe, and sustainable environment buttresses their cities’ economic fortunes. They understand that the global competition for talent requires that their cities offer, all at once, a high quality of life, a clean environment, economic opportunity, and inclusive and competent governance.

In cities, these things are always linked because cities—as Glaeser has written—are where everything happens in one physical place. There is simply no way for local officials to avoid having to manage all these items at the same time. Hence cities are laboratories of policy experimentation, where bold initiatives can be trialed and then adopted, refined, or discarded.

The policymaking behind bus lanes and better building codes

Policy experimentation is commonplace in the world’s cities. In Valencia, Spain, the city government has built a decarbonization agenda around four pillars—energy efficiency, renewable energy, green infrastructure, and sustainable mobility. In each of these areas, Valencia’s efforts have focused on reducing carbon emissions while increasing the city’s livability and economic viability, a trifecta that city leaders insist is the right way to look at the decarbonization problem.²³ While Valencia has been working to reduce emissions, in October 2024 it fell victim to climate-driven disaster. Extreme rainfall generated unprecedented flooding in the city, demolishing parts of it and costing hundreds of lives. The event speaks to the need to build greater urban disaster resilience including through nature-based solutions.²⁴ In Rotterdam, Europe’s busiest port city, energy and shipping companies, local politicians, academics, architects and city planners, and civil society leaders generated the 2019 Rotterdam Climate Agreement, which sought to reduce greenhouse gas emissions by nearly 50 percent by 2030 through a three-pronged strategy: energy efficiency, low-carbon energy generation, and sustainable transportation.²⁵ Six years on, the latest data available from the Port of Rotterdam and surrounding industrial area showed a faster-than-projected drop in greenhouse gas emissions.

In January 2024, New York City began implementing a long-awaited building performance standard, Local Law 97, which requires owners to retrofit their buildings to reduce carbon emissions (buildings generate about two-thirds of New York City’s greenhouse gas emissions).²⁶New York’s regulation, viewed as one of the earliest and most important in the world, was well ahead of the federal government’s policymaking curve.²⁷ This phenomenon is not unusual. A 2022 survey conducted by the Organisation for Economic Co-operation and Development found that 88 percent of cities and regions required higher energy standards in buildings than did their national governments.²⁸

Cities are also banding together. For example, mayors and city councils in Mexico City, Medellín, Rio de Janeiro, Curitiba, San Salvador, São Paulo, and elsewhere in Latin America have been experimenting with a variety of efforts aimed at decarbonization and environmental protection, in some cases for many years, and have been working together in city-based coalitions to engage on these issues.²⁹

A few of these Latin American cities long have been acknowledged as world leaders—Curitiba, Brazil, for instance, is regarded as one of the world’s most sustainable cities. Starting under the visionary guidance of legendary mayor Jaime Lerner, Curitiba in the 1970s began innovative and highly successful programs focused on green space, environmental education, recycling, and transportation. (Lerner once enlisted hundreds of children to sit upon and paint a newly opened pedestrian street, thereby preventing a motorists’ protest that was planned for the same time on the same street; his creative intervention allowed the people-friendly innovation to survive.)³⁰ Curitiba’s most famous innovation, bus rapid transit, privileged mass transit over cars on the city’s streets and became a model for city planners around the world.³¹

For cities to maximize their individual and collective decarbonization efforts, local and national climate policies should match in a mutually reinforcing cycle. National governments have a critical, even foundational, role to play, through setting performance floors and providing much-needed funding to support local investments of all kinds, including but not limited to infrastructural investments. It is critical that national governments also encourage state and local governments to experiment with their own solutions, as with New York’s Local Law 97.

Synergies between local and national authority are not a given, however. California, for example, long has been empowered under federal Clean Air Act requirements to set its own more stringent automobile emissions standards. Owing to California’s massive consumer market, historically the state’s adoption of stricter emissions standards has driven automobile manufacturers to comply with these tighter standards—an example of a subnational standard leading to national-level improvements the national government might not have been able to achieve on its own.³²Yet the so-called California waiver under the Clean Air Act might be coming to an end. The Trump administration is suing the state of California to end its implementation of more stringent emissions requirements under the waiver.³³

Decarbonization policies are often controversial when they are first proposed and implemented, as New York’s building performance standard has been. This political fact requires city leaders to design inclusive processes that incorporate the views of residents. Attempts to do otherwise will fail on political grounds. Cleveland, for example, has spent years building the groundwork for a broad-based decarbonization strategy that is inclusive of historically marginalized perspectives—the city residents whose neighborhoods were divided by highway construction or lost jobs when factories closed or who live too far from bike lanes or bus stops to benefit from those investments. In conversation, Cleveland’s stakeholders stress that such efforts are difficult and time consuming, yet ultimately rewarding.³⁴When citizens and key stakeholders are systematically consulted, as they have been in Cleveland, Valencia, Rotterdam, and many other places, cities can enjoy sustained progress around a coherent decarbonization agenda.

*Children take part in a 2022 workshop in Paris to teach children to bike safely and independently. The workshops are part of the city’s Plan Vélo* launched in 2015 with a goal of making the entire city bikeable by 2026. Bicycle use in Paris rose by 54 percent in a single year during the plan’s first phase. Xose Bouzas / Hans Lucas via Reuters Connect

As nations stall, cities band together

Cities and intercity networks form a parallel global governance architecture, a counterweight to an unreliable international system. Cities therefore constitute a steady presence on the global landscape, a cooperative constant within an international system that is becoming less so. National governments, international institutions, and foreign and security policy communities alike should regard cities and the people who run them—city governments and local stakeholders—as allies in the fight against all manner of international challenges, climate change first among them. Cities bring unique strengths, including creativity, innovativeness, global outlook, and flexibility, to the table in finding positive-sum solutions to concrete problems.

Cities are far from parochial actors focused only on their own issues: Transnational cooperation is a hallmark of urban governance, especially for the world’s largest cities such as New York or Tokyo (but also for many of its smaller ones).³⁵ Cities do not exist in a state of geopolitical competition as do nation-states. China and the United States both have an interest in slowing climate change, but their on-again, off-again bilateral climate diplomacy has been hampered by geopolitical suspicion.³⁶ City governments are freed from existential worry about geopolitical power, allowing them to devote their efforts to pragmatic and cooperative problem-solving across national borders. (Geopolitics does have a hand here, if indirectly. China’s rapid growth to global power starting in the early 1980s was centered on its cities. Now China, in a bid to augment its power via the Belt and Road Initiative, is pouring huge sums into transforming cities from Asia to Europe to Africa.)³⁷

Over the past several decades, local governments have formed numerous city associations dedicated to tackling climate change and other problems at the global level. These include C40 Cities, United Cities and Local Governments, ICLEI–Local Governments for Sustainability, Metropolis, the Global Covenant of Mayors for Climate and Energy, and the Resilient Cities Network.³⁸ These networked associations share best practices, build interurban solidarity, and augment cities’ collective impact within global climate negotiations. For example, two organizations, the Global Taskforce of Local and Regional Governments and the Local Governments and Municipal Authorities Constituency, formally represent subnational governments within the UN system, including at the UN’s annual climate summits.³⁹

The upshot is that cities and their associations are not vassals but agents in world affairs. As Simon Curtis, a scholar of intercity diplomacy, has written, “Nation-states need quickly to realize the potential of global cities and take steps to empower them to meet the global challenges of the twenty-first century … [by allowing them] more fiscal autonomy and [giving] them a louder, more influential voice in the deliberations of international organizations.”⁴⁰ And individual cities and city networks (along with supportive institutions such as philanthropies) should continue to deepen and expand their cooperative decarbonization efforts.

Channeling Guterres, the climate problem is an urban one. Cities are at the heart of the global economy and therefore at the center of both the problem of climate change as well as its solution. If the world’s cities can be decarbonized, they will deliver what they are supposed to deliver—prosperity and a high quality of life to billions of people—at low ecological cost.

Cities around the world have proven their willingness to innovate in pursuit of these goals. Whether national governments will support their efforts remains an open question—and one on which the planet’s future may hinge.

About the author

Peter Engelke is a senior fellow at the Atlantic Council’s Scowcroft Center for Strategy and Security and its Global Energy Center. He is on the adjunct faculty at Georgetown University’s School of Continuing Studies and is a frequent lecturer to the US Department of State’s Foreign Service Institute. He was previously a member of the World Economic Forum’s Global Future Council on Complex Risks, an executive-in-residence at the Geneva Centre for Security Policy, a Bosch fellow with the Robert Bosch Foundation, and a visiting fellow at the Stimson Center.

Articulated buses pull into a terminal in Curitiba, Brazil. Curitiba pioneered bus rapid transit in the 1970s with dedicated bus lanes and protected bus shelters—creating a mass-transit system that reduces car use for much less money than building a new subway or light rail would cost. Video by bydronevideos/Storyblocks

THE BIG STORY | November 24, 2025