Read the foreword for the C40 Cities Annual Report 2025, written by the C40 Co-Chairs, Mayor of London Sadiq Khan and Mayor of Freetown Yvonne Aki-Sawyerr
As we write this, the terrifying consequences of the climate emergency are clearer than ever. In London, an unprecedented series of summer heatwaves – linked to thousands of deaths across the UK – has led to deepening drought. While across the world, we’ve seen similar extremes in recent months: disastrous wildfires in France, Spain and Greece, catastrophic storms in Chile, deadly floods in West Africa, and record-breaking heat in Japan and North Africa – where temperatures have reached as high as 49 degrees.
All the indicators for the world’s climate are flashing red. But, as we seek to show in this report, there’s hope. Because 2025 also marked a historic milestone: the 20th anniversary of C40. Over the last twenty years, C40 cities have met global inaction with bold local leadership and proven that our cities can step up and play a vital role in tackling the climate emergency.
City leaders have implemented almost 1,600 policies in the last decade – from reducing waste and decarbonising buildings to implementing clean air zones. This has meant 73 per cent of C40 cities for which we have data have peaked their emissions and are now on a downward trajectory. This collective action has also helped to spark a green jobs revolution that has already supported the creation of more than 21 million jobs.
In London, we’ve been determined to put our city at the forefront of the climate movement. We’ve planted more than 640,000 trees, rolled out more zero emission buses than any other city in Western Europe and expanded our cycle lanes fivefold, meaning they now stretch further than our entire Underground Tube network. In 2025, we also celebrated a defining moment for public health, as toxic nitrogen dioxide levels met legal limits almost two hundred years earlier than predicted, thanks to the expansion of our Ultra Low Emission Zone (ULEZ) – the largest urban clean air zone anywhere in the world. Now, we’re bringing that same ambition to our green and blue spaces, launching the Green Roots Fund to empower local communities to restore urban habitats, revive waterways and build a city that’s more resilient in the face of the climate emergency, as we strive to build a better, fairer and greener London for everyone.
In Freetown, we continue to champion a transition that’s not only green but just. Freetown is a regional hub for climate innovation, knowledge exchange and youth‑focused urban resilience. We are proud to have launched the Freetown Heat Action Plan, as the city’s roadmap to becoming heat resilient. We have installed shade and cooling infrastructure, reducing surface temperatures by up to 7°C in busy markets and introduced the Freetown Heat Pavilion, a first-of-its-kind public cooling model which has since been replicated by Paynesville City, Liberia. From protecting residents living in informal settlements through increased canopy cover, to overseeing the expansion of safe rainwater harvesting, our focus remains firmly on the communities most vulnerable to the impacts of climate breakdown. We have supported youth-led innovation via the Youth Climate Action Fund, awarding microgrants to 18 youth groups in 2025, building on the nine groups supported in 2024.
But the true strength of C40 lies not just in what our cities can achieve individually but also in what we can achieve together.
The 2025 C40 World Mayors Summit in Rio de Janeiro sent a defiant message – once again showing that while national governments dither and delay, it’s local leaders who get on and deliver. In the context of stalling global negotiations at COP30, mayors came together, not to make distant promises but to commit real resources to specific projects – from upgrading 170 schools in Barcelona with solar panels and advanced aerothermal systems, to expanding Bogotá’s electric bus fleet so that the city can cut a further 100,000 tonnes of CO₂ each year.
Throughout 2025, the resolve of our network never wavered. In fact, it grew, and we were proud to welcome Fortaleza and Casablanca as new members, further strengthening the leadership of the Global South at C40.
As we look ahead to 2026, our mission is clearer than ever: to deepen our partnerships, accelerate our action and maximise our impact. Together, we’ll keep showing the world what’s possible, continue to prove that cities are the primary source of global climate hope, and lead the way to a green, just and sustainable future for everyone.