Boris Johnson’s decision to press ahead with a new coalmine in Cumbria shows “contemptuous disregard for the future of young people” and will lead to “ignominy and humiliation”, one of the world’s foremost climate voices has warned, in a stark message to the prime minister ahead of the UN Cop26 climate summit later this year in Glasgow. James Hansen, the former Nasa scientist who has been called the “godfather of climate change”, has written to the UK prime minister calling on him to stop all support for fossil fuels and “earn a special place in history” for tackling the climate crisis. He wrote: “In leading the UK, as host to the Cop, you have a chance to change the course of our climate trajectory – or you can stick with business-almost-as-usual and be vilified in the streets of Glasgow, London and around the world.” Pointing to the recent decision to allow the UK’s first new deep coalmine for three decades, Hansen went on: “It would be easy to achieve this latter ignominy and humiliation. Just continue with the plan to open a new coalmine in Cumbria and continue to invest funds of the British public in fossil fuel projects overseas, in contemptuous disregard of the future of young people and nature.” Hansen is one of the most respected figures in global climate science. As an expert at Nasa, he testified to the US Congress in 1988 – including then-senators Joe Biden and John Kerry – of the dangers of climate change. That landmark warning helped pave the way to the 1992 UN Framework Convention on Climate Change, parent treaty to the 2015 Paris accord. Hansen’s letter to Johnson is copied to Kerry, now special climate envoy to Biden, who has made climate action a top priority for his presidency, and who will hold a global climate summit on 22 April as a precursor to Cop26, scheduled for November. Tim Crosland, director of the climate litigation charity Plan B, said: “Since the US election, there’s a new dynamic in play. Boris Johnson can no longer get away with his climate hypocrisy, talking the talk of climate leadership while opening a new coalmine and permitting ongoing financial support for fossil fuel interests in the UK and around the world. If he does, it’s clear there will be no way back for him with the new US administration. We must trust he has the political pragmatism to act on Dr Hansen’s warning.” Hansen is concerned that going ahead with the Cumbrian mine will be seized on by supporters of fossil fuels around the world, as the UK prepares to host the Cop26 summit in Glasgow this November. “It shows they [the government] are really not serious,” Hansen told the Guardian in an interview. “We don’t need new mines. This shows that leaders will say the right words but in fact fossil fuels are still winning.” Young people in particular would feel betrayed, he warned. “That’s why I’m trying to encourage Johnson to think this through – he is going to really get hammered. But he could be heroic, if he would just point the way to a path that would work.” Companies profiting from fossil fuels must be faced down, according to Hansen. “The great obstacle you must overcome – where others have failed – is that posed by the special financial interests that have bribed our governments and trashed our planet,” he wrote, in the letter sent to Johnson on Wednesday and seen by the Guardian. He said that developing countries would see the UK’s actions in opening a new coalmine as an encouragement to continue using coal. “Developed countries have to work with emerging economies to help them – otherwise, they are going to use coal,” he said. The Committee on Climate Change, the government’s statutory advisers on achieving the UK’s goal of net zero emissions by 2050, has also warned ministers on the Cumbrian mine. In a letter published last Friday, the chairman of the committee, Lord Deben, urged the planning secretary Robert Jenrick to consider the impacts, as the mine – permission for which has been granted up to 2049, the year before the government must achieve net zero emissions – would increase global greenhouse gas emissions. Deben wrote: “it is also important to note that this decision gives a negative impression of the UK’s climate priorities in the year of Cop26.” Kwasi Kwarteng, business secretary, told MPs who questioned the Cumbrian decision that the mine should be allowed because it would provide coking coal, which was needed for industrial processes, such as making steel, rather than coal for power generation. Hansen rejected this argument. “We have to replace the old ways of doing things – there are alternatives,” he said. “This is possible and we have to do it, because science tells us we can’t continue business as usual.” Johnson pledged late last year to halt UK government investments in fossil fuel overseas. However, campaigners are concerned at loopholes in these plans. A No 10 spokesperson said: “The UK continues to lead the fight against climate change, cutting emissions by more than any major economy so far – at the fastest rate – and putting the prime minister’s bold 10 point plan for a green Industrial Revolution by 2030 into action. “We have already committed to ending the use of coal for electricity by 2025 and ending direct government support for the fossil fuel energy sector overseas. “As we go further and faster to build back greener this year, we will continue to urge countries to drive clean economic recoveries too, as hosts of Cop26 and president of the G7. The UK co-hosted Climate Ambition Summit helped see 75 leaders set out new commitments to climate action.”