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.”
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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.
Some 60 leaders, including the UK’s Boris Johnson and France’s Emmanuel Macron are expected to take the podium to deliver their pledges, but the sensitive issue of coal reportedly means that Japan and Australia – whose leaders are not attending – will not be given a platform.
The US, Brazil and Saudi Arabia will also not speak.
“People can only speak if they come with positive steps. That is kind of a ticket,” Mr Guterres said. “For bad news, don’t come.”
Japan’s Environment Minister, Shinjiro Koizumi, denied Japan had been barred from speaking, saying it had received an invitation, but Prime Minister Shinzo Abe could not attend.
However, China and India have been given a podium place despite leading the world in developing new coal capacity. The majority of India’s electricity still comes from coal burning, and Chinese institutions are the largest finances of coal plants across the globe, according to data quoted by the CarbonBrief website.
Mr Trump is also in New York and will be at the UN headquarters, but will attend an event on religious freedom instead. The US will be represented at the climate summit by a senior official for the environment.
Prime Minister Narendra Modi will represent India, but Chinese President Xi Jinping will not attend. Beijing will instead be represented by Foreign Minister Wang Yi.
The 2015 Paris Agreement pledged to keep global temperatures “well below” 2C (3.6F) above pre-industrial times and “endeavour to limit” them even more, to 1.5C.
But scientists warn current pledges will mean the world will fall well short of that goal.
The WMO report recognised that global temperatures had risen by 1.1C since 1850, and gone up by 0.2C between 2011 and 2015.
Scientists plead with politicians
By Roger Harrabin, BBC environment analyst
As the dangers of climate heating become ever more apparent, so does the absence of collective will to tackle the issue.
In 2015 in Paris, all the world’s leaders sounded their determination to curb the emissions that were heating the climate.
Today will see a host of initiatives from businesses and small- and medium-sized nations.
But President Trump is encouraging fossil fuel in every way he can.
And China – in spite of its trend-setting commitment to solar and wind power – is still building new coal-fired power stations.
Even the UK, a global leader in climate policy-making, is veering away from its own medium-term targets to cut emissions.
The government is still aiming to expand Heathrow airport and increase the road network in a way that will increase emissions in those sectors.
Politicians appear to believe climate change can be challenged with a version of economic business as usual.
Their scientists are telling them with increasingly desperation that we humans are facing an unprecedented threat in need of an unprecedented response.
Follow Roger on Twitter @rharrabin
UN climate summit: World leaders gather to present plans