The US has rejoined the High Ambition Coalition at the UN climate talks, the group of developed and developing countries that ensured the 1.5C goal was a key plank of the Paris agreement.

The decision by the world’s biggest economy and second biggest emitter, after China, to return to the High Ambition Coalition group of countries marks a significant boost to attempts to focus the Cop26 summit on limiting temperature rises to 1.5C, the tougher of the two goals of the Paris agreement.

The coalition, which numbered scores of countries at the 2015 Paris talks, will on Tuesday call on governments to step up their efforts on greenhouse gas emissions and phasing out coal, consistent with a 1.5C limit, and urge rich nations to double the amount of climate finance they make available for poor countries to adapt to the impacts of the climate crisis. They also want to bring an end to subsidies for fossil fuels.

A senior US official said: “The High Ambition Coalition was instrumental in Paris in making sure that high ambition was written into the Paris agreement and will be instrumental in Glasgow in making sure it’s delivered.”

Tina Stege, the climate envoy for the Marshall Islands, said: “The High Ambition Coalition has set the bar for what needs to happen at this Cop: getting on track to limiting temperature rise to 1.5C with enhanced [nationally determined contributions] and with real, actual actions, like phasing out coal; a sea-change on adaptation, with at least a doubling of current levels of adaptation financing; and making sure that we all have the resources we need to face this crisis, including the loss and damage we’re already experiencing today.

“These heads of state have given their marching orders for ambition.”

One negotiator said fears that the 1.5C target was in danger of slipping out of reach had prompted the group’s resurgence. “We are extremely concerned about 1.5C,” they said. “That’s why we are calling for a way to keep 1.5C as a viable option.”

The High Ambition Coalition was formed in the run-up to Paris by the chief negotiator for the Marshall Islands, Tony de Brum. Though the tiny Pacific island state, made up of 29 atolls, has a population of only 60,000, the charismatic De Brum had a major influence at climate Cops.

The goal of holding temperature rises to 1.5C rather than 2C is much harder to achieve, as it requires emissions cuts of at least 45% by 2030, compared with 2010 levels. But science shows it is much safer – beyond 1.5C, many of the impacts of climate breakdown, such as melting ice sheets, become irreversible, and many small islands would face inundation from rising sea levels and storm surges.

De Brum spent months taking soundings from developed and developing countries, and in the closing days, when it appeared that the 1.5C could be in danger, the coalition was announced.

John Kerry, the US climate envoy, said earlier this year that the Paris goal of “pursuing efforts” to 1.5C was “based on hard work by the High Ambition Coalition and the small island developing states. They felt it that it was imperative – and thank heavens they did. Science has now caught up to that fact, the IPCC [Intergovernmental Panel on Climate Change] and IEA [International Energy Agency] and others have been pretty clear, that this is what we need to try to achieve.”

De Brum died in 2017. Stege is his niece.