A special climate summit organised by UN Secretary General António Guterres is set to open in New York, but some key world leaders, including Donald Trump, will not take part.
Mr Guterres expects leaders to come with new and ambitious plans to reduce carbon dioxide emissions.
Youth activists, including Greta Thunberg, will address delegates.
Ahead of the one-day summit, scientists warned the signs and impacts of global warming were speeding up.
The World Meteorological Organization (WMO) said the amount of carbon dioxide going into the atmosphere between 2015 and 2019 had grown by 20% compared to the previous five years.
“We should listen to the loud cry coming from the schoolchildren,” said Prof Brian Hoskins, chair of the Grantham Institute, Imperial College London, and professor of meteorology at the University of Reading.
“There is an emergency – one for action in both rapidly reducing our greenhouse gas emissions towards zero and adapting to the inevitable changes in climate.”
Mr Guterres has said he expects “a number of meaningful plans on dramatically reducing emissions during the next decade, and on reaching carbon neutrality by 2050” will be unveiled at Monday’s summit.
He wants the world to move away from fossil fuels and to stop building new coal-fired power stations after 2020.
The special meeting comes ahead of the next international climate negotiations next year.