UNITED NATIONS (Reuters) - Teenage activist Greta Thunberg angrily denounced world leaders on Monday for failing to tackle climate change, unleashing the outrage felt by millions of her peers in the heart of the United Nations by demanding: “How dare you?”

The Swedish campaigner’s brief address electrified the start of a summit aimed at mobilising government and business to break international paralysis over carbon emissions, which hit record highs last year despite decades of warnings from scientists.

“This is all wrong. I shouldn’t be up here. I should be back in school on the other side of the ocean yet you all come to us young people for hope. How dare you?” said Thunberg, 16, her voice quavering with emotion.

“You have stolen my dreams and my childhood with your empty words,” she said.

Inspired by Thunberg’s solitary weekly protest outside the Swedish parliament a year ago, millions of young people poured onto the streets around the globe last Friday to demand governments attending the summit take emergency action.

“I was very struck by the emotion in the room when some of the young people spoke earlier,” French President Emmanuel Macron told the U.N. Climate Action Summit. “I also want to play my role in listening to them. I think that no political decision maker can remain deaf to this call for justice between generations.”

U.N. Secretary-General Antonio Guterres, who organised the one-day event to boost the 2015 Paris Agreement to combat global warming, had warned leaders only to turn up if they came armed with concrete action plans, not empty speeches.

“Nature is angry. And we fool ourselves if we think we can fool nature, because nature always strikes back, and around the world nature is striking back with fury,” said Guterres, a former Portuguese prime minister.

“There is a cost to everything. But the biggest cost is doing nothing. The biggest cost is subsidising a dying fossil fuel industry, building more and more coal plants, and denying what is plain as day: that we are in a deep climate hole, and to get out we must first stop digging,” he said.

Nevertheless, there were few new proposals from governments for the kind of rapid change climate scientists say is now needed to avert devastating impacts from warming. The summit has, by contrast, been marked by a flurry of pledges from business, pension funds, insurers and banks to do more.

"We have broken the cycle of life," said Emmanuel Faber, chief executive of French food group Danone DANO.PA, who announced a "One Planet" initiative with a group of 19 major food companies to transition towards more sustainable farming.

“We need your support for shifting agricultural subsidies from killing life into supporting biodiversity,” Faber said.

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TRUMP APPEARS

U.S. President Donald Trump, who questions climate science and has challenged every major U.S. regulation aimed at combating climate change, made a brief appearance in the audience of the summit along with Vice President Mike Pence and Secretary of State Mike Pompeo. He did not speak but he listened to remarks by German Chancellor Angela Merkel and Indian Prime Minister Narendra Modi.

Former New York Mayor Michael Bloomberg, who serves as a U.N. special envoy on climate action, called out Trump’s low-key appearance before he spoke on Monday: “Hopefully our deliberations will be helpful to you as you formulate climate policy,” he said to audience laughter.

Merkel announced Germany would double its contribution to a U.N. fund to support less developed countries to combat climate change to 4 billion euros from 2 billion euros.

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Among the day’s other initial announcements was one from the Marshall Islands, whose president Hilda Heine said she would seek parliamentary approval to declare a climate crisis on the low-lying atoll, already grappling with sea level rise.

Heine said her country and New Zealand, Sweden, Norway, Denmark and others who form the “High Ambition” bloc at U.N. climate negotiations, will commit to achieving net-zero greenhouse gas emissions by 2050.

With climate impacts such as extreme weather, thawing permafrost and sea-level rise unfolding much faster than expected, scientists say the urgency of the crisis has intensified since the Paris accord was agreed.

The agreement will enter a crucial implementation phase next year after another round of negotiations in Chile in December.

Existing pledges to curb emissions are nowhere near enough to avert catastrophic warming, say scientists, who warn that failing to change course could ultimately put the survival of industrial societies at risk.

Laurence Tubiana, a former French diplomat and an architect of the Paris accord, said she drew some comfort from more ambitious pledges by a nucleus of political and business leaders.

“When you look at the emergency and you see the level of the response, of course I cannot be happy,” Tubiana told reporters. “The golden nugget I see is this group of countries, companies and cities.”

Over the past year, Guterres has called for no new coal plants to be built after 2020, urged a phase-out of fossil fuel subsidies and asked countries to map out how to achieve carbon neutrality by 2050.

While some countries have made progress, some of the biggest emitting countries remain far behind, even as wildfires, heat waves and record temperatures have provided glimpses of the devastation that could lie in store in a warmer world.

In a measure of the gap between government action and the ever-louder alarms sounded by climate scientists, the United Nations Development Programme said that 14 nations representing a quarter of global emissions have signalled that they do not intend to revise current climate plans by 2020.

Pope Francis, in a message broadcast to the conference, called for honesty, responsibility and courage to face “one of the most serious and worrying phenomena of our time”.