About 50,000 people are expected to attend the 12-day climate summit in the Brazilian city of Belem.

The 30th annual United Nations climate change conference (COP30) has started in the Brazilian city of Belem, with leaders calling for countries to take a united approach against global warming.

“In this arena of COP30, your job here is not to fight one another – your job here is to fight this climate crisis, together,” the UN’s climate chief, Simon Stiell, told delegates on Monday.

Some 50,000 people from more than 190 countries are expected to attend the 12-day event, which is being held at the edge of the Brazilian Amazon rainforest.

Addressing the conference, Stiell said that previous climate talks had helped, but that there was “much more work to do”.

The UN climate boss noted that countries would have to move “much, much faster” in driving down greenhouse gas emissions. “Lamenting is not a strategy. We need solutions,” he said.

His comments came as a new UN analysis of countries’ climate plans found that the pledged reductions fall far short of the drop needed by 2035 to limit temperatures to 1.5C (2.7F) above pre-industrial temperatures.

If this threshold is breached, the world will experience far more severe impacts than it has so far, experts say.

“Climate change is no longer a threat of the future. It is a tragedy of the present,” Brazilian President Luiz Inacio Lula da Silva stressed at the start of COP30.

Brazil’s leader condemned those seeking to undermine efforts to combat the climate crisis.

“They attack institutions, they attack science and universities,” he said. “It’s time to inflict a new defeat on the deniers.”

The United States is not sending any delegates to COP30 in keeping with President Donald Trump’s anti-climate change stance.

“It’s a good thing that they are not sending anyone. It wasn’t going to be constructive if they did,” the US’s former special envoy for climate, Todd Stern, said of the Trump administration’s decision.

COP30 President Andre Correa do Lago said the US’s absence “has opened some space for the world to see what developing countries are doing”.

Pablo Inuma Flores, an Indigenous leader from Peru, urged world leaders to do more than simply give pledges at this year’s conference.

“We want to make sure that they don’t keep promising, that they will start protecting, because we as Indigenous people are the ones who suffer from these impacts of climate change,” he said.

In a letter to COP30 that was published on Monday, dozens of scientists expressed their fears about the melting of glaciers, ice sheets and other frozen parts of the planet.

“The cryosphere is destabilising at an alarming pace,” they wrote. “Geopolitical tensions or short-term national interests must not overshadow COP30. Climate change is the defining security and stability challenge of our time.”