Paris (AFP) - Spurred on by growing social movements, cities across the world are getting serious with their climate action, vowing to slash pollution and implement greener laws.
But until now efforts to make some of the largest urban areas on the planet compliant with the Paris climate goals have been hampered by an inability to measure their effect.
The benchmark of a city's "carbon footprint" currently tracks estimated emissions of greenhouse gases based on voluntary disclosures from industry, traffic authorities, energy firms, among others.
Cities produce around 70 percent of all manmade greenhouse gases, yet estimates of each urban area's carbon footprint contain a huge margin for error -- as much as 30 percent in some cases.
"It requires lots of information to be assessed and generally that creates a lag of 2-3 years," said Thomas Lauvaux, a researcher at the Environment and Climate Sciences Laboratory (LSCE), told AFP.
Take for example Paris, home of the landmark 2015 agreement that saw nations commit to limiting global temperature rises to "well below" two degrees Celsius (3.6 Farenheit).
The latest official figures were published in 2016 but actually date from 2014: 25.6 million tonnes of CO2 equivalent that year.
While that's down 9.2 percent compared with 2004, it is hardly the most accurate way of comparing carbon cuts.
"Carbon footprints every five years isn't good enough, we need to have more reactive data," said Celia Blauel, Paris' deputy in charge of ecological transition.
Tuesday saw the unveiling of a pilot project aimed at changing how cities monitor their climate action: a network of 20 carbon dioxide monitors on rooftops across the greater Paris area providing constantly updated CO2 level data.
"No one at the moment has the ability to know what emissions levels are in real time," said Fouzi Benkhelifa, head of Origin.earth, a start up piloting the monitoring project, which received funding from energy giant Suez and the EU.
"There's a huge gap between the need to act against global warming and the tools we have at our disposal.
"Cities need to act on a daily basis but they only have a five-year-old snapshot (of emissions levels), it's really a problem," said Benkhelifa.
- 'More dynamic' -
The monitoring stations, currently used only for research purposes, will eventually allow the city and surrounding region to act more rapidly to fluctuating emissions levels.
Voters will also be better able to judge the climate action taken by the office of mayor Anne Hidalgo, and see whether it's living up to its green pledges ahead of elections next year.
Greater transparency brings greater scrutiny for policymakers, but Blauel insists Paris lawmakers aren't worried.
"This will allow us to be more dynamic," she said.
The mayor's office isn't anticipating any huge surprises in emissions levels since it has already targeted several areas of reduction -- such as retrofitting buildings, reducing waste and greening transport links -- leading up to carbon neutrality by 2050.
"But to measure our action in a more detailed manner... that will allow us to refine those policies," said Blauel.
The constant feedback will also allow Paris' 2.5 million inhabitants to track their own contribution to the climate fight, according to Lauvaux.
Thanks to an index to be published monthly next year, citizens "will become aware of the natural cycle of emissions, in summer and winter, and see the impact of them putting on the heating has for example," he said.
While several cities in North America, including Los Angeles and Mexico City, have launched projects to measure CO2 levels, Lauvaux said Paris was the first to use the data for constant monitoring.
"No one can lie about (the data)," he said.