In a video statement from the White House, Mr Trump told the G20 summit hosted by Audi Arabia: “To protect American workers, I withdrew the United States from the unfair and one-sided Paris climate accord, a very unfair act for the United States.”
He said the international accord was “not designed to save the environment. It was designed to kill the American economy”.
But US president-elect Joe Biden, who will take office in January, has said he will rejoin the pact at the earliest opportunity.
The US was instrumental in the creation of the agreement, and Barack Obama described it as a “turning point” in the global response to the climate crisis.
As president, Mr Trump has worked to undo most of President Barack Obama’s efforts to fight climate change, rolling back numerous environmental protections and pulling the US out of the Paris climate agreement.
On Sunday, during a discussion about safeguarding the Earth from the climate crisis, Mr Trump said his government had “made environmental stewardship a sacred obligation”.
He also claimed the US has “among the cleanest air and cleanest water on the planet”, and said since he pulled the US out of the agreement, the country has reduced carbon emissions more than any nation.
That is true, but not particularly remarkable. Energy emissions have been falling in the US since their peak in 2007.
With its giant economy, the US has far more raw emissions of climate-damaging carbon dioxide to cut than any other country except China.
A more telling measure of progress in various countries is to look at what percentage of emissions a county has cut. Since 2005, the United States hasn’t been even in the top 10 in percentage of greenhouse gas emission reductions.
More than 180 nations have ratified the Paris climate agreement, which aims to keep the increase in average temperatures worldwide “well below” 2C and ideally no more than 1.5C, compared with pre-industrial levels.
Scientists have warned any rise above 2C could have a devastating impact on large parts of the world, raising sea levels, stoking tropical storms and worsening droughts and floods.
The US formally exited the Paris pact on 4 November 2020, and on Saturday, the US formally left the Open Skies Treaty, which permits more than 30 nations to conduct unarmed, observation flights over each other’s territory. These overflights were set up decades ago to promote international trust and avert conflict.
The Trump administration said it wanted to exit the treaty because Russia was violating the pact, and imagery collected during the flights can be obtained quickly at less cost from US or commercial satellites.
During the discussions at the climate session, President Xi Jinping of China, the world’s largest emitter, said the G-20 should continue to take the lead in tackling climate change and push for the full implementation of the Paris accord.
“Not long ago, I announced China’s initiative to scale up its nationally determined contributions and strive to peak carbon dioxide emissions by 2030 and achieve carbon neutrality by 2060,” he said. “China will honor its commitment and see the implementation through.”
India’s prime minister, Narendra Modi, said “climate change must be fought not in silos, but in an integrated, comprehensive and holistic way”.
Additional reporting by AP.