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Climate change is undeniable and it is "absolutely essential" the world fights the problem together, United Nations Secretary-General Antonio Guterres argued on Tuesday.
His comments come as US President Donald Trump considers pulling out of the Paris climate deal.
Trump refused to endorse the landmark climate change accord at a summit of the G7 group of wealthy nations on Saturday, saying he needed more time to decide.
He then tweeted that he would make an announcement this week.
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"If any government doubts the global will and need for this accord, that is reason for all others to unite even stronger and stay the course," Mr Guterres said during an event at New York University.
“The message is simple: the sustainability train has left the station. Get on board or get left behind.”
“The world is in a mess,” Mr Guterres said. “It is absolutely essential that the world implements the Paris Agreement.”
Trump, who has previously called global warming a hoax, has come under concerted pressure from other world leaders to honour the 2015 Paris Agreement, the first to bind all nations to setting goals to curb carbon emissions.
“We believe that it will be important for the US not to leave the Paris agreement,” said Mr Guterres.
“But even if the US government decides to leave the Paris agreement, it’s very important for the US society as a whole - the cities, the states, the companies, the businesses - to remain engaged with the Paris agreement,” he added.
The United States is the world's biggest economy and the second largest emitter of greenhouse gases after China.
Mr Guterres said he intends to convene a climate summit in 2019 to review implementation of the global climate deal. He said that currently 147 parties representing more than 82 per cent of greenhouse gas emissions have ratified the Paris agreement.
“Climate change is undeniable. Climate action is unstoppable,” he said.
Big emitters led by China, the European Union and India have reaffirmed their commitment to the Paris deal, which seeks to phase out greenhouse gas emissions this century by shifting to clean energies.
By contrast, Trump wants to favour US coal.
A UN panel of climate scientists says it is at least 95 per cent probable that man-made greenhouse gas emissions, mainly from burning fossil fuels, are the main cause of climate change since 1950.
Global average temperatures have hit record highs in each of the past three years, and warming is projected to cause worsening droughts, sea level rises, floods, heat waves and extinctions of wildlife.
Reuters