The world is facing a “pivotal moment” in the climate crisis, Kamala Harris has told the Cop28 summit after the US vowed to phase out coal plants and slash methane emissions, but also came under attack for meagre assistance to developing countries and for its own booming oil and gas extraction industry. The US vice-president, deputising for Joe Biden – who skipped the UN climate talks – said Biden’s administration had made the “largest climate investment in the history of our country, and some have said the world” via the Inflation Reduction Act. The world is badly off track in efforts to slow the climate crisis, however, and Harris warned of the dangers posed by opponents of climate action, which could be seen as a reference to Donald Trump, who is likely to be Biden and Harris’s electoral opponent next year. “This is a pivotal moment – our action collectively, or worse our inaction, will impact billions of people for decades to come,” Harris told delegates, including dozens of world leaders, at the summit in the United Arab Emirates. “Around the world there are those who seek to slow or stop our progress. Leaders who deny climate science, delay climate action and spread misinformation.” Harris, who also used the trip to Dubai to discuss the war in Gaza, has been the highest-profile US attender of the talks, which have drawn representatives from nearly 200 countries to the two-week summit. Biden, who oversaw the lighting of the White House Christmas tree on Thursday, decided to not make the trip, citing various world crises including the Israel-Hamas conflict. The American delegation has unveiled some major actions at Cop28, including sweeping new regulations to cut emissions of methane from its oil and gas sector and a decision to join a pledge not to build any new coal plants. Harris also announced a $3bn (£2.4bn) infusion to the Green Climate Fund, which aims to boost renewable energy and climate resiliency around the world, although this money would have to be agreed to by a US Congress, which is partly controlled by Republicans who oppose most measures to deal with the climate crisis. John Kerry, Biden’s climate envoy, has talked up US and global progress on climate while in Dubai. “I feel optimistic, I really do,” he said on Saturday. “I am in Dubai with a sense that something different is really happening.” Kerry said the US under Biden had taken a global leadership role in pushing countries to cut emissions and curb deforestation. However, the US has come under fire for the symbolic absence of Biden, as well as the revelation that it has only pledged $17.5m (£13.8m), far less than several less wealthy countries, to the loss and damage fund, a structure set up to assist developing countries most at risk from climate impacts. Climate campaigners called the contribution from the world’s richest economic power “embarrassing”. There is also unease among climate campaigners about the US’s ongoing expansion of its own fossil fuel drilling. Already the world’s leading producer of oil and gas, the US is expected to break records for extraction this year, with Biden handing out leases for drilling at a rate comparable to Trump, for projects such as the controversial Willow oil complex in Alaska, as well as assenting to a huge build out of gas export infrastructure along the Gulf of Mexico coast. The former US vice-president turned climate advocate Al Gore said the US’s progress was somewhat mixed. “The methane announcement is a welcome and helpful one, and of course the Inflation Reduction Act remains the most impressive and most efficacious climate legislation any nation has ever passed,” Gore told the Guardian. “But of course the continued leasing of oil and gas drilling rights on public lands is a concern. In that sense you have to balance out all of the good things president Biden and his team have done with an appropriate expression of concern about all this leasing.” The opening of the US pavilion at Cop28 reflected some of this unease, with a group of climate activists braving restrictions on freedom of expression at the summit by chanting for an end to fossil fuels and posing with stickers calling for a phase out. “You can’t talk about reductions when you are talking about more exploration and leasing and drilling,” said John Beard, one of the protesters. Beard, a former Exxon engineer who now campaigns against the industry’s impact upon communities along the Gulf coast, added: “You can’t talk about decarbonising when you’re re-carbonising, it doesn’t make sense. Talk is cheap and sending your big dog officials here isn’t going to do it, it’s action that will do it.”