The world's cities must invest around $375 billion in climate action and low carbon infrastructure over the next four years in order to avoid catastrophic global climate change, the C40 Cities Climate Leadership Group has warned.

Published this week, 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. 

The Deadline 2020 report argues world cities must be at the forefront of avoiding that scenario, highlighting the need for megacities to reduce their average emissions from more than five tonnes of carbon per capita today to around 2.9 tonnes over the next decade.

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.

Doing so is "achievable but extremely ambitious," the report conceded, with C40 cities taking just an estimated 11,000 climate actions between 2005 and 2016. Cities need to ramp up their climate actions by 125 percent in less than half the time.

If we are going to avoid the worst impacts of climate change, cities are going to have to play an even bigger role in the months and years ahead.

Yet, if C40 cities and their partners take on the recommended actions, the report estimates they can deliver 51 percent of the carbon reductions necessary to ensure cities are on course with Paris Agreement objectives. The remaining 49 percent of emission reductions, meanwhile, then would need to come from external structural changes, such as de-carbonizing national energy supplies.

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.

"Cities, along with businesses and citizens, are taking action to address climate change because they know it is in their best interests, and if we are going to avoid the worst impacts of climate change, cities are going to have to play an even bigger role in the months and years ahead," he said.

Actions are set out in the report in terms of priorities up to 2100, with a broad focus on work towards greener urban planning and climate adaptation, cleaner public and private transport, clean and efficient heat and power sources and use, greener buildings and better waste prevention and recycling.

Some joint city actions are already taking place in these areas, such as the Coalition for Urban Transitions, which launched earlier this year, and the formation of the new Global Covenant of Mayors for Climate and Energy.

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.

The report was launched today at the start of the C40 Mayors Summit in Mexico, at which the mayor of Paris, Anne Hidalgo, who was welcomed as the group's new chair, said the investment would "support new jobs, support the emergence of new industries, improve quality of life and health outcomes and benefit the global climate."

"We know that cities have the capacity to significantly cut global emissions, but we acknowledge that we cannot meet the challenge alone," Hidalgo added, calling on governments and private backers to support the wider transition to a low carbon economy.

Depending on the region, the report estimates around $10 billion to $30 billion may be required on average by 2050 by individual cities. African and South and West Asian cities are expected to need the most low-carbon investment.

And in total world cities are likely to need more than $1 trillion in green investment up to 2050, the report contends, with the money largely leveraged from a combination of international institutions, national governments and private investors.

If all cities with populations of 100,000 people or more act on the recommendations in the report, the world would achieve 40 percent of the emissions reductions necessary by 2100 to avoid catastrophic climate change.

But according to Bloomberg, 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.

While recent reports suggest more cities are reporting on their actions towards tackling climate change, the report points out that if all cities with populations of 100,000 people or more act on the recommendations in the report, the world would achieve 40 percent of the emissions reductions necessary by 2100 to avoid catastrophic climate change. 

"Cities are where people are, it's where the pollution is and the damage to our climate, and that's where the solutions are," said Bloomberg. "Mayors represent the people that elect them, and while they should lead from the front, they have to rally the troops, if you will, to cooperate in closing coal-fired power plants, using less energy and all of the things you would do to stop this warning that, every time a new piece of data comes out, gets more and more worrisome."

In order to help fund some of the actions needed, therefore, Bloomberg — 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.

Addressing the recent U.S. elections and the potential impact of a Trump presidency on climate action, though, Bloomberg emphasized the great power of city mayors to take action on climate change regardless of who is in the White House.

"Mayors, as we know, have never waited for Washington to act here in the U.S., and have never waited for international treaties to protect their citizens and improve public health. And no matter what happens, mayors will continue leading by example," he said.

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. Both the green economy and city dwellers hope the money can be found quickly.