Rich countries must come forward with detailed plans on how they hope to meet their climate targets, and Boris Johnson must forge much closer relationships with developing countries to bring about the breakthrough needed on the climate crisis this year, one of the architects of the Paris agreement has said. The G7 summit, which ended on Sunday in Cornwall, achieved much less than campaigners had hoped, with no significant new cash forthcoming for the world’s poorest and most vulnerable, on the frontlines of climate breakdown. There were promises by the world’s richest economies to halt funding for coal, but they fell short of the pledge to end all new fossil fuel development that experts have said will be needed. The UK now has just 20 weeks to forge a global consensus on the climate crisis before UN climate talks called Cop26 to be held in Glasgow this November. Cop26 will be the most important climate meeting since the Paris agreement was signed in 2015, and one of the last chances to put the world on track to meet its goal of limiting temperature rises to 1.5C above pre-industrial levels. Laurence Tubiana, the chief executive of the European Climate Foundation, who was the top French diplomat in charge of the 2015 Paris summit, said the key was for rich countries to build trust among other countries by drawing up detailed plans on how to meet their own targets on greenhouse gas emissions, and come up with improved offers of finance for the most vulnerable. “Cop26 will be a moment of truth,” she told the Guardian. “Countries have to display their precise plans and accountability mechanisms [to ensure that emissions cuts take place]. There is a lot of work to do to deliver this. We are at the end of the era of coal, oil and gas burning, and that [shift] has to start in earnest.” She added: “Leaders will need to use the next four months before they meet again at the Cop26 summit in Glasgow to crack on with fleshing out the details of this global Marshall plan for green recovery and getting everyone everywhere vaccinated – and crucially, that means putting up the investment to make all this possible.” The G7 meeting left Johnson looking exposed on the world stage to some observers. Pledges by the G7 club of the richest democracies – the UK, US, Japan, Canada, Germany, France, Italy and the EU – on vaccines were disappointing to many, and the summit was overshadowed by tensions over Brexit and the Northern Ireland protocol. The UK row about overseas aid spending – which Johnson is cutting from 0.7% of GDP to 0.5%, with the loss of £4bn to the developing world – also put him in a poor position to browbeat his fellow leaders into stumping up more cash for climate finance. Poor nations were promised $100bn a year from the rich world to help them cut greenhouse gas emissions and cope with the impacts of climate breakdown, but that longstanding pledge has fallen short by about $20bn. Mohamed Adow, the director of the Power Shift Africa thinktank, said: “It’s clear the G7 was a huge disappointment and it has undermined Johnson’s credibility. It’s hard not to think that the host of Cop26 cutting their aid budget is going to undermine efforts to raise climate finance from other countries. Overpromising and underdelivering is a bad look for the man overseeing the biggest climate talks since the Paris agreement.” Saleemul Huq, the director of the International Centre for Climate Change and Development in Bangladesh, said: “The UK having reneged on their own parliamentary act [on overseas aid] made the situation of Johnson as chair of the G7 non-credible for the other leaders. All of this does not bode well for Cop26.” Tubiana said Johnson must do much more to reassure the developing world. At Cop26, every one of the 196 signatory countries to the Paris agreement has – in theory, at least – an equal say, and a deal can only be forged by consensus, so developing countries can make or break any compromise. “You have to deliver a signal to developing countries that they are taken seriously,” she said. “Overseas aid is not just about aid, it is about political outreach and it shows developing countries that they have a seat at the table, alongside the US and China and so on.” Johnson can take comfort from the fact that the road to the Paris agreement was far from smooth. Tubiana recalls that a key meeting of the leaders of the G20 nations – the G7 and 13 other less democratic or smaller economies, including China, India, Russia, Brazil, Australia, Saudi Arabia, South Africa and Turkey – just weeks before the climactic summit in Paris in December 2015 went very badly. “Six months before Paris it was super-pessimistic, people thought there would not be an ambitious agreement,” she said. “And the G20 just before Paris was a disaster. After the G20, anyone would have said it is not worth even going to Paris.” Yet with painstaking diplomacy the French repaired the tensions and managed to achieve a global consensus in the final weeks. Fortunately, the previous two years had been spent carefully listening to developing country concerns and building reserves of trust that meant the French were seen as honest brokers for a fair deal. Given the outcome of the G7, Johnson must hope he has as much credit to draw upon.