Ahead of a meeting of the G20 group of leading nations in Hangzhou, China, 30 mayors from cities including London, Paris, Tokyo, Sydney, New York, Cape Town and Rio de Janeiro called on national leaders to work with them to "build a low carbon, climate safe world".
They welcomed government moves to secure the Paris Agreement, the world's first comprehensive global deal to tackle climate change, in December last year, and efforts to ratify it as soon as possible so it could come into force rapidly.
But they warned "this is only the first step along the road towards our low carbon, climate safe future".
In an open letter, the mayors from the C40 group of cities championing climate action said: "To limit the global temperature increase to 1.5C above pre-industrial levels, global greenhouse gas emissions need to peak by 2020.
"Achieving such a rapid shift is probably one of the greatest political, economic and practical challenges faced by every national leader, but you do have great allies in this task: we, the mayors of the megacities of the world."
They said they were already dealing with the consequences of climate change in their cities, battling floods to heatwaves.
But city leaders were also taking action such as banning the most polluting cars, rolling out fleets of electric buses and improving energy efficiency, which also had benefits for health, well-being and economic growth, they said.
"For the major cities of the world it is already clear that the faster we move to a low carbon economy, the greater will be the improvement in urban citizens' standards of living, and the stronger and more sustained will be the economic development that makes that possible."
The leaders have committed to setting out concrete plans for how they will deliver the greenhouse gas cuts in their cities needed to help meet the goals of the Paris Agreement to avoid dangerous climate change.
"We want our citizens, markets and other political leaders to know that we are serious about making the Paris Agreement a reality. We call on the heads of states from our respective nation states to do the same," they said.
