Celebrating two decades of city climate leadership with C40

C40 Cities

A global network of mayors working to confront the climate crisis through city-led action, collaboration and leadership. Cities, climate governance and international policy.

Sector and focus area

Cities, climate governance and international policy

Context and challenge

In 2025, C40 Cities marked its twentieth anniversary, coinciding with a pivotal moment where cities are increasingly recognised as critical actors in delivering practical solutions to the climate crisis. The organisation sought a way to celebrate their milestone while reinforcing its role as a bold, collaborative force within a complex and fast-moving multilateral landscape.

The challenge was to create a clear, cohesive narrative that could hold together C40’s history, ambition and impact across a year-long campaign. This narrative needed to resonate with mayors, policymakers and international stakeholders, while strengthening C40’s presence at high-profile moments, including major global summits.

Approach and strategy

Greenhouse worked with C40 to develop a unifying brand and communications platform that aligned storytelling across multiple channels and moments. Central to the approach was the creation of a distinct C40 at 20 sub-brand, designed to celebrate city leadership while reinforcing credibility and collective purpose.

The strategy focused on delivering high-quality, flagship content that could be deployed at key moments, particularly around the World Mayors Summit and COP30. Consistency, clarity and coherence were prioritised to ensure that C40’s story was told with confidence and authority across diverse audiences and formats.

Work delivered

• • Development of a unifying brand and communications platform, including the C40 at 20 sub-brand
• • Production of a 200-page anniversary publication capturing C40’s history, impact and city leadership. The book featured case studies and key learnings from current and previous C40 Chairs, all prominent internationally recognised city mayors like Mike Bloomberg, Anne Hidalgo and Sidiq Khan and was translated into four additional languages.
• • Creation of a hero campaign film and supporting creative assets
• • Design of a wearable lanyard booklet to increase visibility at COP30 when the organisation had no official pavilion presence
• • Development of a comprehensive messaging house for use across three flagship campaign moments throughout the year.

Outcomes and impact

The campaign strengthened C40’s presence at a critical moment for multilateral climate action and helped articulate a clear, compelling narrative around city leadership.

Reported outcomes include:

Stakeholders praised the campaign for its creativity, clarity and cohesion, reinforcing C40’s reputation as a collaborative and credible global climate force.

Why this work matters

As cities take on an increasingly central role in responding to the climate crisis, the ability to communicate leadership, legitimacy and collective impact is essential. This work supported C40 in marking a significant milestone while strengthening its positioning within the global climate architecture.

By bringing coherence to a complex network story and aligning communications across moments and audiences, the campaign helped reinforce the influence of city-led climate action at a time when it matters most.

What our clients say

“Working with Greenhouse is always a tremendous experience. The team has a wealth of knowledge, expertise and understanding that helps turn our ideas into high-impact deliverables. They are incredible at interpreting a brief and bringing exceptional project management and creative flair to complex projects. Reliable, can-doers. They always feel like an essential part of the team from day one”

Luke Upchurch, C40 Communications Director