A warning from the Climate Leadership Group said that cities around the world must invest around $375 billion just in climate action and low carbon infrastructure over the next four years in  order to avoid catastrophic global change.

A report by consultancy Arup on behalf of the C40 group argues that without serious action before 2020, the world will have "locked-in" future emissions that will see global temperatures surpass the 2 degrees Celsius mark set out as the upper "safe" limit in the Paris Agreement. 

Established 11 years ago, the C40 Cities Climate Leadership Group alone includes more than 85 world cities, representing more than 650 million people and one-quarter of the global economy.

The report sets out a roadmap for C40 cities to take 14,000 climate actions over the next four years in areas such as transportation, efficiency, energy production and waste management in order to reduce the world's urban emissions.

Michael Bloomberg, president of the C40 board of directors and U.N. special envoy for cities and climate, said despite the challenges he was optimistic cities would be able to take the climate actions necessary within the next four years.

He said that cities, along with businesses and citizens are taking actions to address climate change because they know that it is in their best interest.

In order to fund further necessary actions, around $375 billion of investment in low carbon infrastructure is needed in cities over the next four years, which the report argues will transform economies and boost infrastructure - although some of that investment already may be covered in existing city budgets.

Action from the C40 cities on climate change is also about setting standards and city mayors can lead the way in spreading success towards driving down emissions in more cities around the world.

A former mayor of New York City, also announced a joint $40 million commitment over the next four years from charitable organizations Bloomberg Philanthropies, the Children's Investment Fund Foundation and Realdania.

Four years is certainly a short deadline to find $375 billion investment in the low carbon economy, but it also highlights the scale of the climate crisis, which studies have shown is likely to hit urban areas the hardest.