World leaders shared the spotlight with a 12-year-old Pacific islander on Wednesday to drive home a plea for urgent climate action despite the Trump administration's rejection of a planet rescue plan.

"My home, my school, my source of food, water, money was totally destroyed. My life was in chaos," Timoci Naulusala, whose village in Fiji's Tailevu province was hit by a devastating cyclone last year, told hundreds of delegates including some 25 heads of state and government.

"My once beautiful village is now a barren and empty wasteland... Climate change is here to stay unless you do something about it."

The leaders descended on Bonn to re-energise negotiations hamstrung by America's rejection of the 2015 Paris Agreement.

They were met in the morning by anti-fossil fuel protesters waving posters proclaiming: "Clean coal is a dirty lie" and "Stop corporate capture of the climate".

The Paris pact, which Donald Trump announced in June the US would abandon, took more than two decades to negotiate and envisions a drawdown of Earth- warming greenhouse gas emissions from burning coal, oil and gas.

Labelling climate change "the defining threat of our time", Guterres told delegates that continued investment in fossil fuels made no financial sense and was "counterproductive".

"We must stop making bets on an unsustainable future that will place savings and societies at risk," he said.

Macron, for his part, described climate change as "the most significant struggle of our time", and urged European countries to open their wallets for the UN's IPCC climate science panel, which is facing a critical budget shortfall.

- Central challenge -

Trump cut funding to the IPCC, to which the US traditionally contributed about $2 million (1.7 million euros) a year -- representing almost half of its 2016 budget.

A collaboration of thousands of experts worldwide, the panel issues reports every few years and is widely considered as the authority on global warming science.

"I can guarantee that starting in 2018 the IPCC will not be short a single euro," Macron said to loud applause.

Merkel agreed climate change was "a, if not the, central challenge of mankind," adding the world must "stand together to implement" the Paris Agreement.

The pact commits countries to limiting average global warming to under two degrees Celsius (3.6 degrees Fahrenheit) over Industrial Revolution levels, and 1.5 C if possible, to avert calamitous climate change-induced storms, drought and sea-level rises.

To bolster the agreement, nations submitted voluntary commitments to curb emissions.

But the 1 C mark has already been passed.

A report Wednesday said America's withdrawal would boost global temperatures, calculated on current country pledges, by nearly half a degree Celsius by 2100, for a total of 3.2 C.

To widespread disappointment, Merkel said the shift away from fossil fuel was not always "that easy".

Coal still provides about 40 percent of Germany's electricity needs, and the country is set to miss its own goal of cutting emissions by 40 percent by 2020 from 1990 levels.

"We have a long way to go yet," the chancellor conceded, adding that job retention and economic viability must be considered when weighing energy projects.

"Mrs Merkel... needed to come to Bonn and show she had heard the suffering of the people of the Pacific and around the world and would do the responsible thing and end coal. She did not deliver this," said Jennifer Morgan, a Greenpeace executive director.

Back home, the issue has dogged Merkel's efforts to form a coalition government of her conservative allies, the anti-coal Greens and the pro- industry Free Democrats.

- Lone outsider -

Since Monday last week, bureaucrats in Bonn have haggled over a Paris Agreement "rule book", which will specify how countries must calculate and report their emissions cuts.

Now it is the turn of energy and environment ministers to unlock issues above the pay grade of rank-and-file negotiators -- with climate finance from rich to poor countries the main sticking point.

The task has been complicated by the presence of White House officials who hosted a sideline event Monday defending the continued use of fossil fuels.

The highest ranking American at the talks is an acting assistant secretary of state, Judith Garber, who is due to address the conference on Thursday.

This week, Syria became the 196th country to formally adopt the pact, leaving America as the only nation in the UN climate convention to reject it.

Source: AFP