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UPDATE 2-Teenager Thunberg angrily tells U.N. climate summit 'you have stolen my dreams'

Valerie Volcovici and Matthew Green

my dreams'@ (Adds quotes from Michael Bloomberg, Trump appearance, early leader pledges, Pope's message)

UNITED NATIONS, Sept 23 (Reuters) - Teenage climate change activist Greta Thunberg on Monday opened the United Nations Climate Action Summit with an angry condemnation of world leaders for failing to take strong measures to combat climate change - "How dare you," she said.

Days after millions of young people took to the streets worldwide to demand emergency action on climate change, leaders gathered for the annual United Nations General Assembly aiming to inject fresh momentum into stalling efforts to curb carbon emissions.

A visibly emotional Thunberg, 16, said in stern remarks at the opening of the summit that the generations that have polluted the most have burdened her and her generation with the extreme impacts of climate change.

"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 the Swedish teenager, her voice quivering.

"You have stolen my dreams and my childhood with your empty words," Thunberg said, adding that the plans that leaders will unveil will not be enough to respond to the rate of the planet's warming.

Thunberg has galvanized a new wave of climate change activism through her weekly Fridays for Future school strikes, which she began with her weekly, solitary protests outside of the Swedish parliament.

U.N. Secretary-General Antonio Guterres had warned governments ahead of the event that they would have to offer action plans to qualify to speak at the summit, which is aimed at boosting the 2015 Paris Agreement to combat global warming.

In his opening remarks, he tried to capture the urgency of climate change and called out the fossil fuel industry.

"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," Guterres said.

"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.

TRUMP APPEARS

U.S. President Donald Trump, a climate change denier who has undone 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 give remarks 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 the U.N. special envoy on climate action, called out Trump's stealth 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 Germany's contribution to a U.N. fund to support less developed countries to combat climate change to 4 billion euros from 2 billion euros.

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.

Over 60 world leaders and CEOs of energy and financial companies are expected to address the conference and announce climate finance measures and transitioning from coal power.

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. Pledges made so far under the agreement are nowhere near enough to avert catastrophic warming, scientists say, and last year carbon emissions hit a record high.

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". (Reporting by Matthew Green and Valerie Volcovici; additional reporting by Arshad Mohammed; editing by Mary Milliken and Grant McCool)