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U.N. Secretary General, Ban Ki-moon speaks during a press conference ahead of G20 Summit in Hangzhou, Zhejiang province, China, on Sept. 4, 2016. REUTERS

U.N. Secretary-General Ban Ki-moon said on Sunday that climate change scepticism is over, the day after the United States joined China to ratify the Paris agreement to curb climate-warming emissions.

Climate change skeptics dispute the widely held understanding that excessive levels of emissions in the atmosphere cause global warming and harm the environment, and have become increasingly side-lined at international summits.

"The debate over climate phenomenon is over scientifically and environmentally," said Ban, adding that the influence of climate change deniers or skeptics has waned.

"It is affecting our day-to-day life," Ban said, at a new conference ahead of a G20 summit in the eastern Chinese city of Hangzhou.

U.S. President Barack Obama and Chinese President Xi Jinping deposited the legal instruments to join the Paris agreement to curb climate-warming emissions to Ban on Saturday, Ban said.

Experts have said the temperature target is already in danger of being breached, with the U.N.'s weather agency saying 2016 is on course to be the warmest year since records began.