In a month, this year’s UN climate summit, Cop30, begins in Belém, Brazil – preceded by a key leaders’ meeting. It is a crucial moment. The UN’s scientists have yet to publish calculations based on the latest round of nationally determined contributions (NDCs) – as countries’ emissions pledges are known. But Brazil’s president, Luiz Inácio Lula da Silva, has called leaders together because it is already clear that the current emissions pledges are nowhere near enough. The Paris 1.5C threshold was crossed last year. Without stronger action, that temporary breach will become irreversible – with devastating consequences for people and planet.

Lula’s diplomatic outreach to Donald Trump, who calls the climate crisis a “con job”, suggests he is trying to bring key players into the fold ahead of Belém. Having heavyweights in the room can make all the difference. Ten years ago in Paris, world leaders’ presence proved crucial to securing an ambitious deal. That’s why it is important that Sir Keir Starmer attends. He may not be the most powerful world leader, but his presence is a moral and diplomatic imperative. If King Charles is able to, he ought to go too. Soft power can help to rebuild the spirit of cooperation to keep the hopes in the Paris agreement alive.

The UK is one of the world’s biggest polluters historically, due to its early industrialisation and its empire. But the climate policies of the last two decades have been ambitious compared with other wealthy countries, and the UK’s NDC is one of the few that climate analysts regard as helping to keep the 1.5C goal in view. All of which makes it even more important that Sir Keir should be in Brazil, in person, standing up for evidence-based policy in multilateral climate talks.

Cop30 comes at a fraught moment for climate politics in Britain. The Tories’ promise to repeal the Climate Change Act was populist posturing, aimed at pandering to an anti-science, Trumpian right. Labour must not flinch. It has been reported that Sir Keir’s aides are advising against going to Belém, fearing backlash from Reform UK. Retreat would be cowardly and wrong. Sir Keir should instead reaffirm his commitment to national targets and the Cop process – confident that shifting to renewable energy is both right and popular.

Sir Keir is, of course, not the only leader who should be making travel plans. The leaders of Australia and Canada both made climate policies part of their pitches to voters in recent elections. They too should be heading for Belém, as should the leaders of the EU, China, India and Japan. As the former UN secretary general Ban Ki-moon noted, attendance is not ceremonial but a test of leadership. He warned that the world was watching – and said that history would remember who showed up.

Mr Trump’s election win was a dangerous setback to the global green energy transition. But 33 years after the Brazilian Earth Summit that set the process going, he is also a reason for other governments to step up. Perhaps Lula could even convince Mr Trump to change his mind? If there was ever a time to show that the climate is a diplomatic priority, this is it. Sir Keir must not leave the question of his attendance hanging any longer. He must decide – and go.