There is no question that Joe Biden’s win will make a big difference to international efforts to deal with the climate emergency. A US president who recognises global heating as an “existential threat” will be a vital extra pillar propping up the teetering edifice of climate diplomacy. Four years of Donald Trump have done huge damage to the US’s reputation. But the world’s biggest economy, and second-biggest emitter of greenhouse gases (after China), remains vastly influential. With President Biden in charge, the prospects for next year’s Cop26 talks in Scotland, when drastic emissions cuts must be agreed if the world is to stand a chance of avoiding catastrophic heating, are already brighter.President Trump’s withdrawal of the US from the Paris agreement was a key plank of his nationalist “America first” agenda and an act of sabotage against both the UN climate process and the principle of a rules-based international order. It also gave cover to the world’s other climate vandals: Saudi Arabia, Brazil, Russia and Australia. With Mr Biden, that cover is gone, and ecocidal policies such as Amazon rainforest destruction and coal-power expansion should come under renewed and relentless pressure. It is striking that the president-elect put climate at the heart of his phone calls with foreign leaders.The path ahead is anything but smooth. A green stimulus package on the scale promised by Mr Biden’s campaign is unlikely to pass through Congress, with control of the Senate hinging on two undecided seats in Georgia. Conservative judges are a further roadblock. Legislation to limit emissions and punish polluters is certain to be challenged all the way to the supreme court. Fossil-fuel companies and other vested interests remain a formidable force. Nor can public support be taken for granted. Most voters are on board in principle, recognising the dangers of unchecked global heating. But the changes in lifestyle that will be needed to meet new targets, including reductions in meat-eating and flying, are challenging in the US as in other rich countries.Still, Mr Biden’s presence in the White House will be a huge opportunity, and one that the environmental movement and its supporters must seize with every hand they have. Global heating is a fact, not a hypothesis or ideology. It is not just the vast majority of Democrats who want their politicians to do more to tackle it, but also a sizable minority of Republicans. Younger people are the most anxious. Mr Biden will perform a valuable public service simply by doing the opposite of his predecessor, and telling the truth.Democrats have shown that climate can be a unifying force within their party. Mr Biden’s climate taskforce was chaired by Alexandria Ocasio-Cortez, the New York congresswoman and star of his party’s progressive wing. Now, if they are to create sufficient momentum, Democrats must look beyond the ranks of committed green supporters, as the writer Arlie Russell Hochschild did in her book Strangers in Their Own Land, about environmental politics in Tea Party-supporting Louisiana. Already, Mr Biden has signalled that the harm caused by pollution to poorer Americans will be among his priorities.In recent weeks China, South Korea and Japan have all announced net zero emissions targets. Rapid falls in the price of renewables have made the process of weaning away from fossil fuels far less painful than most experts predicted. Climate protesters have shown how effective they can be in mobilising support for strong action. Now that the election is over, they must keep pushing Mr Biden and other legislators as hard as they can.