President Trump, who is not attending the meeting in the Amazonian city of Belém, was called a liar by the leaders of Colombia and Chile for his rejection of climate science.
UK Prime Minister Sir Keir Starmer acknowledged the waning political support on climate change. He said it had been a unity issue internationally and in the UK but "today sadly that consensus is gone".
Over the next two weeks countries will try and negotiate a new deal on climate change, with a particular focus on channelling more money to forest protection.
Many leaders from the world's largest nations – India, Russia, US and China - are notably absent from this year's summit.
And while Trump isn't attending this meeting in Belém, his views on climate change are certainly on the minds of many of the other leaders present.
Speaking at the UN in September, the US president said that climate change was "the greatest con job ever perpetrated on the world".
He said: "The entire globalist concept, asking successful industrialised nations to inflict pain on themselves and radically disrupt their entire societies, must be rejected completely and totally."
Without naming the US leader, President Luiz Inácio Lula da Silva of Brazil warned on Thursday of "extremist forces that fabricate fake news and are condemning future generations to life on a planet altered forever by global warming".
The leaders of Chile and Colombia went further, calling the US president a liar, and asking other countries to ignore US efforts to move away from climate action.
Maisa Rojas, Chile's environment minister, told the BBC: "The science is very clear. It is very important not to falsify the truth."
But while Trump-bashing went down well with the audience, getting agreement on new steps to tackle warming is proving much harder.
Only a few dozen leaders have turned up here in Belém, and a majority of countries have failed to submit new plans to cut carbon emissions, the root cause of rising temperatures.
Despite Starmer acknowledging that global political support for the climate movement is waning, he told the gathering of those that were present: "My message is that the UK is all-in."
However, on Wednesday night, in a blow to the Brazilian hosts, the UK chose to opt out of its flagship $125bn (£95bn) fund to support the world's rainforests.
President Lula hoped that $25bn could be raised for the Tropical Forests Forever Facility from public sources – mainly from developed countries like the UK – to support governments and communities protecting the world's rainforests like the Amazon and the Congo Basin.
The protection of these ecosystems is crucial for tackling climate change - they cover just 6% of the world's land, yet store billions of tonnes of planet-warming gases and host half of the planet's species.
The move by the UK has come as a surprise as it had been heavily involved in the fund's design, and launched a global commitment for countries to halt deforestation by 2030 when it hosted the COP summit in Glasgow in 2021.
Lord Zach Goldsmith, who worked on the issue when he was former environment minister, told the BBC's PM programme: "The assumption was that the UK would be a leading participant and at the last minute the UK has walked away. It has caused real frustration to put it mildly here in Brazil... the Brazilian government behind the scenes is furious."
The decision also seems at odds with the stance of the Prince of Wales. Also addressing leaders on Thursday he declared the fund "a visionary step toward valuing nature's role in climate stability" and shortlisted it for his £1m Earthshot Prize.
