This may be a conservative figure as well. The C40 is only counting cities that peaked at least five years ago (to rule out temporary blips), have committed to further emission reductions and have the data to prove that CO2 output has declined. A city that's inheren

There's no one effort that prompted the drop. Shifts to renewable energy and more efficient electricity use helped, but so did more efficient transportation (including shared transport and more walking), more recycling and reduced methane from landfills. Think of it as a combination of small initiatives that amounted to a lot.

You might not want to cheer too loudly. The declines were generally from known progressive cities in North America, Europe, Scandinavia and Australia. It would mean more if emissions declined in places where CO2 output is rampant and frequently dovetails with air pollution problems like smog. And of course, this only covers individual cities, not whole countries. Even a few major cities might not completely offset emissions across a larger country. Still, it's progress -- and it suggests that the Paris agreement's 2020 peak emissions target was somewhat realistic.