UNITED NATIONS (NYTIMES) - China on Monday (Sept 23) made no new promises to take stronger climate action. The United States, having vowed to pull out of the Paris agreement, said nothing at all, demonstrating a lack of leadership from the biggest polluter in history. A host of presidents and prime ministers used the occasion to boast about what they were doing to reduce emissions but made only incremental promises.

That was the scene at the United Nations Climate Action Summit, which Secretary-General António Guterres had organised to highlight what he called "concrete" commitments to wean the global economy away from planet-warming fossil fuels and do more to help the most vulnerable cope with the effects of global warming.

There were, in fact, some concrete commitments. Some 60 countries promised to achieve net-zero emissions by 2050, and several asset fund managers said they would aim to get to a net-zero portfolio of investments by the same year.

President Donald Trump unexpectedly dropped into the General Assembly hall with Vice-President Mike Pence in the late morning.

Mr Michael Bloomberg, the former New York City mayor, welcomed Mr Trump's presence and addressed the President directly by saying, "Hopefully our discussions here will be useful for you when you formulate climate policy."

That was followed by laughter and applause. It signalled a sharp contrast from just a few years ago, when the United States had been credited with pushing other countries, including China, to take climate change seriously.

The US is not on track to meet its pledges under the Paris climate agreement, and the Trump administration has rolled back a host of environmental regulations, from automobile tailpipes, coal plants and oil and gas wells.

The Trump administration did not request a speaking slot at the summit.

But what really got the attention of delegates in the hall was when Swedish teenage climate activist Greta Thunberg, early in the day, lit into world leaders for their "business as usual" approach to a problem so grave.

"The eyes of all future generations are upon you," she said, her voice quavering with rage. "If you choose to fail us, I say we will never forgive you."

Rarely does anyone speak in this way at the world body. A bit later in the day, Thunberg watched with a look of fury in her eyes, as Mr Trump passed through a hall, a video clip posted on Twitter showed.

As for China, it did not signal its readiness to issue stronger, swifter targets to transition away from fossil fuels, as many had hoped.

Mr Wang Yi, a special representative for President Xi Jinping, noted that his country was keeping the promises it made under the 2015 Paris agreement and that "certain countries" - a clear reference to the United States, which has said it intends to withdraw - were not.

"China will faithfully fulfil its obligations," Mr Wang said.

President Emmanuel Macron of France also had a message for the United States, telling the assembly, "I don't want to see new trade negotiations with countries who are running counter to the Paris Agreement."

The statement could create a new stumbling block to trade agreements between the US and Europe, which are already plagued by deep differences over agriculture, the rules of the global trading system, and Mr Trump's potential tariffs on cars.

China's decision to not signal higher ambition reflects, in part, concerns about its own slowing economy against the backdrop of conflicts with the United States on trade.

It also reflected Beijing's reluctance to take stronger climate action in the absence of similar moves from richer countries.

The European Union has not signalled its intention to cut emissions faster either, and the US is nowhere on track to meet its original commitments under the Paris accord.

"There's no particular reason why China should do anything new now, because they're not getting any pressure from the United States and they're on track to achieve their commitments," said Dr Kelly Sims Gallagher, professor of energy and environmental policy at the Fletcher School of Law and Diplomacy at Tufts University.

"This extends the limbo while the rest of the world waits to see what the United States is going to be doing in 2020," she said.

Prime Minister Narendra Modi of India said his country would increase its share of renewable energy by 2022, without making any promises to reduce its dependence on coal.

Chancellor Angela Merkel of Germany promoted a new plan worth US$60 billion (S$82.66 billion) over 10 years to speed a transition to clean power.

Russia announced that it would ratify the Paris agreement, but nothing more about how to cut emissions from its sprawling state-owned petroleum industry.

The summit unfolded against the backdrop of new data that showed the quickening pace of warming.

The world is getting hotter, faster, the World Meteorological Organisation concluded in its latest report on Sunday, with the five-year period between 2014 and 2019 the warmest on record.

Emissions of carbon dioxide, a major contributor to global warming when it is pumped into the atmosphere, are at all time highs. The seas are rising rapidly. The average global temperature is 1.1 degrees Celsius higher than what it was in the mid-19th century, and at the current pace, average global temperatures will be 3 degrees Celsius higher by the end of the century.

"I will not be there, but my granddaughters will, and your grandchildren, too," Mr Guterres said in his opening remarks. "I refuse to be an accomplice in the destruction of their one and only home."

Mr Guterres' most direct call went to those countries that use money from their taxpayers to subsidise fossil fuel projects that, as he put it, "boost hurricanes, spread tropical diseases and heighten conflict".

"We are in a deep climate hole. To get out, we must first stop digging," he said.

"Is it common sense to build ever more coal plants that are choking our future? Is it common sense to reward pollution that kills millions with dirty air and makes it dangerous for people in cities around the world to sometimes even venture out of their homes?"

The pledges being delivered against the UN's green marbled backdrop stood in sharp contrast to the anger that spilled onto the streets on Friday, when masses of children and young people protested around the world.

On Monday, protesters blocked traffic in Washington to demand a swift pivot away from the world's fossil fuel-based economy.

According to the UN Environment Programme, the world's 20 largest economies, which account for 80 per cent of global greenhouse gas emissions, "are not yet taking on transformative climate commitments at the necessary breadth and scale".

Scientists and policymakers have said that even holding warming to a less-dangerous 1.5 degrees Celsius would entail a significant transformation of the global energy system, costing trillions of dollars.

But the cost of doing nothing is also staggeringly high.

Studies show that if emissions continue to rise at their current pace, the number of people needing humanitarian aid as a result of natural disasters could double by 2050.

And a sweeping report from 13 US federal agencies last year warned that failing to rein in warming could shave 10 per cent off the country's economy by century's end.