Twenty-seven major cities around the world may already have seen their greenhouse gas emissions peak, according to a new study.

San Francisco, Los Angeles, Chicago, London and Washington, D.C., are among the cities whose emissions have fallen more than 10 percent from their historic peaks, according to an analysis released yesterday by the C40 Cities Climate Leadership Group, a city-focused climate advocacy organization founded by former London Mayor Ken Livingstone and championed by former New York City Mayor Michael Bloomberg.

The group examined its 96 member cities’ emissions inventories and found 27 cities that have reduced emissions at least 10 percent from their peak levels.

In order to qualify, the cities had to have peaked at least six years ago, and the peak had to be at least 10 percent higher than the most recent year of emissions data. Those thresholds are thought to be high enough to exclude natural variations due to weather or other short-term shifts.

“We feel fairly confident in the methodology,” said Michael Doust, C40’s program director for measurement and planning. “We think the 10 percent figure and the five-year delay means it takes it out of the range of natural variability or any other changes that could affect emissions.”

San Francisco, for example, had peak emissions of about 8 million metric tons in 2000. Since then, its inventory shows steadily declining emissions, mostly due to changes in the electricity mix. In 2016, the city’s emissions were roughly 6 million metric tons.

Sydney, which reported a 20 percent emissions cut between 2007 and 2017, credits energy efficiency retrofits in the building sector with saving 1,000 megawatt-hours of electricity per year in 2017 compared with 2006 levels.

The full list of cities is: Barcelona, Spain; Basel, Switzerland; Berlin; Boston; Chicago; Copenhagen, Denmark; Heidelberg, Germany; London; Los Angeles; Madrid; Melbourne, Australia; Milan; Montreal; New Orleans; New York City; Oslo, Norway; Paris; Philadelphia; Portland, Ore.; Rome; San Francisco; Stockholm; Sydney; Toronto; Vancouver, British Columbia; Warsaw, Poland; and Washington, D.C.

The group found that 20 other cities may have also peaked but didn’t meet both thresholds of peaking by 2012 and declining more than 10 percent from the peak. An additional 66 cities are also on track to peak by 2020, according to the group.

Tokyo didn’t make the cut because it has been replacing its nuclear generation with natural gas since the 2011 Tohoku earthquake and subsequent tsunami damaged Japan’s Fukushima Daiichi nuclear plant. Although Tokyo’s energy consumption in 2016 was 21 percent below the 2000 peak, its emissions were only 7 percent below their 2003 peak.

“They’ve peaked their energy consumption,” Doust said, “but emissions have not gone down as drastically because of changes in the national grid, which is increasingly gas rather than nuclear.”

Reprinted from Climatewire with permission from E&E News. E&E provides daily coverage of essential energy and environmental news at www.eenews.net.