The world has embarked on a “vast, frightening experiment” on the natural world, King Charles has told world leaders, which risks triggering “feedback loops” in the climate system that will cause irreversible disaster.

Noting that 2023 was the hottest year on record, the king told the Cop28 UN climate summit on Friday: “Records are now being broken so often that we are perhaps becoming immune to what they are really telling us. We need to pause to process what this actually means: we are taking the natural world outside balanced norms and limits, and into dangerous, uncharted territory.”

In an opening speech calling on leaders to make Cop28 “a critical turning point”, he warned: “We are carrying out a vast, frightening experiment of changing every ecological condition, all at once, at a pace that far outstrips nature’s ability to cope … Our choice is now a starker, and darker, one: how dangerous are we actually prepared to make our world?”

More than 130 heads of state and government have gathered in Dubai for the opening days of a two-week summit at which nations are aiming to chart a path for the world to avoid breaching the global heating limit of 1.5C (2.7F) above pre-industrialised temperatures.

António Guterres, the secretary general of the UN, said the world was “miles” from fulfilling the Paris agreement, and only “minutes to midnight” when it came to the 1.5C goal. He insisted that leaders could still make a difference if they exercised “political will”.

“It is not too late,” he told them. “You can prevent planetary crash and burn. We have the technologies to avoid the worst of climate chaos – if we act now.”

More countries came forward on Friday with contributions to a fund for loss and damage, announced on the first day, which will help rescue and rehabilitate vulnerable countries stricken by climate disaster. Some developing countries told the Guardian the $420m of finance so far agreed was not all it seemed, with some funding appearing to be repurposed from existing aid and some taking the form of loans. Saudi Arabia was also rumoured to be considering contributing to the fund, which would be a first for the oil-producing state.

World leaders signed a declaration on transforming food systems – the first Cop resolution that directly tackles the symbiotic relationship between what people eat and the changing climate. The resolution recognises that “unprecedented adverse climate impacts are increasingly threatening the resilience of agriculture and food systems as well as the ability of many, especially the most vulnerable, to produce and access food in the face of mounting hunger, malnutrition and economic stresses”.

More than 100 countries signed the declaration and committed to including food and land use in their climate plans by 2025. The declaration was widely welcomed by small-scale and indigenous farmers – who produce a third of the world’s food – as well as food rights campaigners, consumer associations and small business groups.

A UN report warned that droughts supercharged by global heating were “an unprecedented emergency on a planetary scale”, leading to food shortages and famine.

While other climate impacts, such as heatwaves, wildfires and floods, often hit the headlines, droughts were often silent disasters, the report said, and “the massive impacts of human-induced droughts are only starting to unfold”.

The report said those who had done the least to cause the climate crisis were the most exposed: 85% of those affected by droughts live in low- or middle-income countries.

“Several countries already experience climate change-induced famine,” said the report by the UN Convention to Combat Desertification. “Forced migration surges globally; violent water conflicts are on the rise; the ecological base that enables all life on Earth is eroding more quickly than at any time in known human history.”

Luiz Inácio Lula da Silva, Brazil’s president, called for more support from developed countries. “The planet is fed up with unfulfilled climate agreements. Governments cannot escape their responsibilities,” he told the conference. “No country will solve its problems alone. We are all obliged to act together beyond our borders.”

He told delegates that the trillions of dollars spent on weapons should be used instead against hunger, inequality and climate change: “The world has naturalised unacceptable disparities in income, gender and race, and it is not possible to face climate change without combating inequality.”

He spoke of climate suffering in the Amazon, which is experiencing one of the “most tragic droughts in its history”, while cyclones in the south of Brazil had left a trail of “destruction and death”.

Brazil’s energy minister announced on Thursday that the country would align more closely with the world’s biggest oil cartel, Opec, but Lula said it was necessary to “work for an economy less dependent on fossil fuels”.

William Ruto, the president of Kenya, said his region was also facing the horrific effects of climate breakdown. “In eastern Africa, catastrophic flooding has followed the most severe drought the region has seen in over 40 years,” he said, adding that the extreme weather this year had “seized lives and destroyed communities” as well as infrastructure and supply chains.

He said the world needed to invest in green energy and other infrastructure in Africa. “A tendency to ignore Africa’s developmental and industrial needs … is no longer a tenable position,” he said. “Turning Africa into a green powerhouse is not just essential for the continent, it is also vital for global industrialisation and decarbonisation.”

Rishi Sunak, the UK prime minister, is at the conference, along with European heads of state including France’s president, Emmanuel Macron, and the EU Commission president, Ursula von der Leyen, and leaders of developing countries including India’s prime minister, Narendra Modi.

Sunak told the conference “the mounting science and evidence of climate-related disasters prove we are not moving fast enough” and said that “everyone can do more”. But he also admitted that he had rolled back the UK’s decarbonisation policies, and claimed this was to reduce costs for households.

One Cop summit veteran from the UK said delegates were focused more on Sunak’s actions, particularly in pushing new North Sea oil and gas licences, than on his words.

“It doesn’t matter what Sunak says today,” they said. “We used to be a climate leader. Now we are going backwards.”

World leaders will continue their meetings into the weekend, then when they depart, their negotiators will thrash out the final details of a deal over 10 days. Major differences remain to be bridged, including over a potential resolution to phase out or phase down fossil fuels, and how to ensure that the world’s biggest emitters return to the negotiating table with far more stringent cuts to greenhouse gas emissions.