File Photo: Travelers are stuck in a traffic jam as people hit the road before the busy Thanksgiving Day weekend in Chicago, Illinois, U.S., Nov. 21, 2017. (Xinhua/REUTERS)

SAN FRANCISCO, Sept. 13 (Xinhua) -- An international climate leadership group said Thursday that 27 of the biggest cities in the world have reached their peak of greenhouse gas emissions while their populations and economies continue to grow.

C40 Cities, an emissions reduction advocacy group connecting more than 90 of the world's greatest cities, said the 27 cities have seen emissions fall over a five-year period and are now at least 10 percent lower than their peak.

These cities represent over 650 million people and one quarter of the global economy, and their emissions continued to drop by an average of 2 percent every year since their peak, while their populations grew by 1.4 percent per year and their economies by 3 percent per year on average, C40 Cities said during the on-going 2018 Global Climate Action Summit (GCAS) that opened earlier Thursday in San Francisco.

The cities include Spain's Madrid and Barcelona, London in Britain, Sydney and Melbourne in Australia, as well as Chicago, New York City, San Francisco and Washington D.C. in the United States.

A C40 research revealed in 2015 said that the world greatest cities need maximize their efforts to deliver on the goal of the Paris Agreement -- to limit the global temperature increase to only 1.5 degrees Celsius.

"To prevent the worst impacts of climate change, we have to cut greenhouse gas emissions even as the population grows," said Michael Bloomberg, president of the C40 Board, who is also UN Special Envoy for Climate Action.

San Francisco Mayor London Breed also asserted the city's pledge to lead in the fight against climate change, though U.S. President Donald Trump announced last year that he was withdrawing from the landmark 2015 Paris accord.

California Governor Jerry Brown Thursday took a stab at Trump for retreating from global warming, saying that it "borders on criminality."

He said the Trump administration's attempt to weaken regulation of the heat-trapping gas methane is "a major assault on the well-being of the people of California in America and the world."

C40 said the world's leading scientists have calculated that global greenhouse gas emissions should peak at the latest by 2020 and subsequently sharply drop.

The 2018 GCAS, hosted by the U.S. state of California, attracted more than 4,500 representatives from states, regions, cities, businesses as well as investors for discussions on policy and measures to bring down emissions.