^{An artificial ski slope shows that powering cities no longer needs to be a dirty business}

Mayors from 94 of the world's major cities have gathered in the Danish capital Copenhagen for the C40 Summit.

The annual meeting is aimed at building a broad coalition of politicians, business people and civil society groups to take action on climate change.

The event opened with a joint call for a new global "green deal" – essentially a plan to redirect the world's economic resources towards a more sustainable future.

Cities are an unavoidable part of a discussion on curbing global warming. Fifty five percent of the world's population live in them and it is where 44 percent of global emissions originate.

Among the list of attendees lending their support is UN Secretary General Antonio Gutueres.

With the world watching, the host city has been keen to show it is leading by example. It has a plan to become the first carbon-neutral capital by 2025.

A very visible sign of this commitment is a ski slope built on the roof of what is said to be the most efficient waste-to-energy plant in the world. 

Municipal authorities here call Copenhagen's target "radical," a word heard often in the conference halls and meeting rooms at this year's C40.

No one at the summit disputes that time is short, or that bold action is needed now.