Here is a summary from their own documentation regarding what they do (from their report from Leeds).

C40 is a global network of nearly 100 mayors of the world’s leading cities that are united in action to confront the climate crisis. [1]

Remember, it's not just climate change. It's a climate crisis and must be addressed now or the planet is doomed...doomed. They are kind enough to break down for us their overarching goal into bite sized bullets.

• Raising climate ambition through 1.5°C climate action plan support, high-impact accelerators and fostering innovation
• Building equitable and thriving communities via global and regional programmes (sic)
• Building a global movement through robust international advocacy and diplomacy
• Scaling up climate action and sharing best practices across high-impact sectors
• Facilitating access to finance for investment in green jobs and projects that improve resilience in cities

Lest we think that this is only representative of mayors and leaders in Europe, please think again. In their same report, they have a listing of all places throughout the world where mayors and leaders are involved in helping to mitigate against climate change. It fascinates me that under the heading "North America," there is only one mayor in Canada and it's the mayor of Montreal. All the other listings are mayors over major cities in the USA. Take a look at them and you'll quickly notice that all of them are run by Leftists or "blue" mayors and the cities themselves are considered blue or Democrat.

So what do these illustrious folks do? They agree with the principles set forth in the Leeds/C40 cities document that promises to reduce consumption of goods, services, food and automobiles by a huge percentage by 2030. For instance, under Food Consumption Interventions, please note they have a progressive target and an ambitious target that is to be completed by the aforesaid 2030.