Cities have massive carbon footprints, and how they develop is crucial for addressing climate change. The Paris climate summit allowed cities to unify - and post-Paris, cities' collective action is breeding green hopes.
Cities are major sources of greenhouse gases, and a new report estimates that around 500 cities will be responsible for half of energy-related greenhouse gas emissions growth between now and 2030.
"A third of the global carbon policies will be impacted by urban decision-makers," says Mark Watts, executive director of C40 - a network of more than 80 of the largest cities of the world committed to combating climate change.
Cities are gaining ever more influence on the international climate stage. In 2014, a global compact of mayors was launched, billed as the world's largest effort for cities to fight climate change.
The compact aims to enable cities around the world to publicly commit to deep greenhouse gas emission reductions and to low-carbon development by, among other things, exploring synergies among mayors.
Cities driving climate-friendly development
Coinciding with COP21 in Paris in early December 2015, Paris Mayor Anne Hidalgo - along with former New York City Mayor Michael Bloomberg - co-hosted the Climate Summit for Local Leaders. This was the largest global convention to date of mayors, governors and local leaders focused on climate change.
Emerging from the Climate Summit for Local Leaders was an ambition to reduce urban greenhouse gas emissions by up to 3.7 gigatons annually by 2030.
"Cities are drivers of progress and innovation, and by taking this step, they can help nations set new, necessary and aggressive greenhouse gas targets over the next year," Bloomberg said.
From words to action
Experts point out that climate action needs coherence, and to be coordinated around the globe. Malini Mehra, chief executive of GLOBE International, said "This [COP 21] is a generational opportunity to set a universal agenda."
Amidst the rhetoric that echoes across the halls of massive events such as the UN Conference of Parties, it is easy to lose sight of what is already being achieved, participants at the Local Leaders' Climate Summit pointed out.
"There are two kinds of outcomes: success and real success," said Daniel Esty, an environmental law and policy professor at Yale University. Over the past two decades, Esty thinks, climate negotiations saw more rhetoric than action to curb emissions.
The Paris Agreement, for all the praise and criticism around it, does send a signal. Progressive local leaders see that now is the time to take up that signal.
"It is critical that Paris becomes an inflexion point moving into real success territory," Esty said. And he believes that cities can help.
Leveraging private money
They key outcome is not what happened in Paris, "but implementation of the commitments and tracking of mitigation efforts after Paris," Esty said.
One of the core elements for real success is a commitment to broader engagement, Esty thinks. "Many government officials don't have day-to-day knowledge on the carbon footprint of their cities," he said.
Megacities like Tokyo will account for around more than 10 percent of emissions growth between now and 2030
And according to Esty, the number-one factor here is money. "There will never be enough funding for the transformation that is needed," he said.
"They key is to use the limited public finance to leverage private funds to get to the scale we need."
Cities around the world are already making great strides in this regard. Mpho Parks Tau, Johannesburg's mayor, said "We have been looking at financing internally and finding innovative ways of financing."
In 2014, the city of Johannesburg designed the first green bond, floating it on the stock exchange to mobilize private investment.
'Carry the spirit of Paris to your city'
In Esty's words, the spirit of Paris - that is, of collaboration - is already transforming the global approach to climate change, to being fueled by urgency and optimism for reaching a common goal of safeguarding the planet's future.
Esty hopes this will spread from the bottom up - he urged conference participants: "In your regions, carry the spirit of Paris and bring on-the-ground transformation in your city."
How cities develop over coming decades will play a major role in determining the success of climate change mitigation efforts - and the degree to which climate change affects those at risk.
With regard to climate action, history may be written by local governments.