“Climate change is an issue determining our destiny as mankind – it will determine the wellbeing of all of us,” the German chancellor, Angela Merkel, has told the world’s nations gathered at a climate summit.

The delegates heard a series of strong political messages on Wednesday, urging them to use the final two days of the summit to complete important work on putting the landmark 2015 Paris deal into action. Without this, the world faces a devastating 3C or more of global warming.

The UN secretary general, António Guterres, told the conference in Bonn of his visit to the Caribbean after this year’s hurricanes.

“The catastrophic damage of climate change is upon us and when the frontline is devastated, the whole army is lost” he said, railing against the $825bn invested in fossil fuels in 2016. “We must stop making bets on an unsustainable future.”

The French president, Emmanuel Macron, who has been among the most vocal critics of Donald Trump’s decision to pull the US out of the Paris deal, got the loudest applause when he committed France and European partners to filling the funding gap for the UN’s climate science panel, left by the US withdrawal. “They will not miss a single euro,” he said.

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Macron also emphasised the global importance of tackling global warming, saying: “The fight against climate change is by far the most significant struggle of our times.” He also addressed the issue that underpins most disputes at the global negotiations: the responsibility of rich nations that caused climate change to pay for the solution and compensate for the damage: “Climate change adds further injustice to an already unfair world.”

President Ali Bongo Ondimba of Gabon, representing African nations, said the need for faster action was urgent: “The fire is right under our feet. That is why I am expressing the extreme concern of Africa in light of the increase of disasters related to climate change. Africa suffers the loss and damage on a daily basis.”

“It is now time for the developed countries to live up to their responsibilities,” said Baron Waqa, president of Nauru and representing small island states. “Lack of resources is the problem.”

The talks in Bonn have progressed reasonably smoothly, without the drama and walkouts of previous summits. But one issue has flared up – whether rich nations are doing enough before 2020 to cut their emissions and to help poorer nations cope with the impacts of global warming.

It has been forced to the top of the agenda by the devastating floods and hurricanes seen around the world this year – as well as the meeting being run by the vulnerable island nation of Fiji. Negotiators agreed a deal on this on Wednesday, involving new stock takes of action in 2018 and 2019 and on progress towards the $100bn a year in climate funding that rich nations have promised to deliver by 2020.

The divisive issue of “loss and damage” – compensation for poor nations for the climate change damage – has been defused for now, with an expert meeting to address concerns next year. Progress has been made in drawing up the draft rules for implementing the Paris agreement, an essential step ahead of agreeing them in 2018.

Merkel, Guterres and Macron were almost upstaged by the first speech of the high-level session, given by 12-year-old Timoci Naulusala from Fiji, without any hint of nerves. Referring to the impact of Cyclone Winston in 2016, he said: “My home, my school, sources of food, money, water, were totally destroyed. My once beautiful village, which I called home, is a barren waste. Climate change is real, not a dream.”

Frank Bainimarama, Fiji’s prime minister and president of the summit, said: “We are not simply negotiating words on a page, but we are representing all our people and the places they call home.”

Merkel, who as an environment minister chaired the first climate summit 23 years ago, has been under pressure this week to phase out Germany’s large coal-fired power stations, which are likely to bust Germany’s climate targets.

“We still use a lot of coal, particularly lignite,” she said, acknowledging the issue is controversial, but she said jobs had to be taken into account too. She added that progress was expected in the next few days as she settles the terms of a new governing coalition with the Green and Liberal parties.

However, Prof John Schellnhuber, at the Potsdam Institute for Climate Impact Research in Germany, and who has advised both Merkel and the pope, said the 20,000 German jobs in coal would be lost to mechanisation in any case, and were a small number compared to the 600,000 created in the wider economy each year.

Macron set out unusual detail for a head of state in his speech, saying France would close all its coal power plants by 2021 and would ban all new exploration for fossil fuel in its territories. He also said France would fund interconnectors and energy storage technology to spread renewable energy around Europe and work to push the cost of CO2 emissions to €30 a tonne, which would end the viability coal and drive out gas.