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, world leaders gathered at the United Nations in New York City on Monday to try to inject fresh momentum into stalling efforts to curb carbon emissions.

Thunberg, visibly emotional, said in shaky but 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," she said.

"You have stolen my dreams and my childhood with your empty words," Thunberg said.

Advising world leaders that they'd be watched closely, she added: "You are failing us, but young people are starting to understand your betrayal."

Prime Minister Narendra Modi of India, which produces more carbon emissions than all countries but the U.S. and China, detailed India's commitments to combat climate change on Monday in New York. (Lucas Jackson/Reuters)

UN Secretary General António Guterres has warned governments 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.

Leaders including German Chancellor Angela Merkel and Indian Prime Minister Narendra Modi addressed the one-day gathering, alongside companies working to promote renewable energy.

Merkel said Germany would double its spending to fight climate change to €4 billion ($5.8 billion Cdn), while Modi pledged to increase his country's commitment to renewable energy, as well as spending the equivalent of $50 billion Cdn through 2025 to shore up water conservation efforts.

U.S. President Donald Trump, a climate change denier who has undone every major U.S. regulation aimed at fighting climate change, made a brief appearance in the audience of the summit along with Vice-President Mike Pence and Secretary of State Mike Pompeo. Trump listened to the remarks by Modi and Merkel, and then departed.

Former New York City mayor Michael Bloomberg, detailing his organization's plan to promote clean energy projects around the globe with willing countries, thanked Trump for showing up and threw a jab the president's way, drawing some laughs.

"Hopefully our discussions here today will be useful for you when you formulate climate policy," said Bloomberg.

Watch: Montreal mayor Plante on how cities can fight climate change

Montreal Mayor Valérie Plante told the United Nations that cities are on the front lines in the fight against climate change. 4:15

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, which is already grappling with sea level rise.

Heine said her country and New Zealand, Sweden, Norway, Denmark and others that form the High Ambition bloc at UN 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.

As well, the UN Food and Agriculture Organization (FAO) unveiled plans to plant urban forests over an area four times the size of Hong Kong, seeking to make Africa and Asia's rapidly growing cities greener.

The initial plan is to support 90 cities in 30 countries in Africa and Asia to create green areas, said FAO director-general Qu Dongyu in a statement. More specific details are expected to be released later in the week.

If managed well, urban forests could reduce air temperatures and cut the cost of air conditioning, the FAO said.

The Paris agreement will enter a crucial implementation phase next year after another round of negotiations in Chile in December. Pledges made under the agreement are nowhere near enough to avert catastrophic warming, scientists say, and last year carbon emissions hit a record high.

While some countries have made progress, some of the biggest emitting ones 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.

Phase out fossil fuel support, Guterres urges

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

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.

"Is it common sense to give trillions in hard-earned taxpayers' money to the fossil fuel industry to boost hurricanes, spread tropical diseases, and heighten conflict?" said Guterres in his speech. "Is it common sense to build ever more coal plants that are choking our future?"

A group of 13 major oil companies charted out a plan on Monday to promote investments in carbon capture, use and storage (CCUS).

Millions of climate strikers all across the world took to the street on Sept 20th. And there’s another strike scheduled on Sept 27th, as well. Today on Front Burner, we talk to Naomi Klein, author of the new book “On Fire: The Burning Case for the Green New Deal” about Greta Thunberg, the Green New Deal, and why she thinks mass mobilization around climate change may be the only thing that can help us avoid global warming’s most devastating effects: “If you don’t believe in social movements, and if they make you kind of queasy and they seem kind of messy, then you should feel really pessimistic, because it’s actually our only hope.” 25:59

Oil chiefs grappling with growing demand for action to fight climate change have looked to invest in carbon-capture and sequestration techniques that some executives, including Occidental Petroleum Corp CEO Vicki Hollub, said could make drilling carbon neutral.

With fossil fuel development growing worldwide, the oil and gas industry faces growing criticism from activists concerned about accelerating climate impacts from melting ice caps to sea-level rise and extreme weather.

The group, formed in 2014 and known as the Oil and Gas Climate Initiative (OGCI), said it aims to double the amount of carbon dioxide stored globally by 2030. The group is also taking steps to reduce methane emissions.

The companies, which include Exxon Mobil Corp, Chevron Corp and BP PLC, account for 32 per cent of global oil and gas production. They have agreed to co-operate to accelerate reduction of greenhouse gas emissions.

Separately, almost 90 big companies in sectors from food to cement to telecommunications are pledging to slash greenhouse gas emissions, summit organizers said.

'The house is on fire': Protest in D.C.

Trump, in his first few months in office, signalled his intention to pull the United States out of the 2015 Paris accord and has rolled back Obama-era rules on emission cuts and wants to maximize U.S. energy output.

Activists seeking to pressure U.S. politicians to fight climate change blocked major traffic hubs in Washington, D.C. — chaining themselves to a sailboat in one location — as they sought to draw attention to the summit.

Police work to remove protesters from a boat being used to block traffic at an intersection near the White House in Washington, covering them in blankets to protect them from sparks. (Erin Scott/Reuters)

Police arrived with a power saw to free the protesters chained to the boat, draping them with heavy blankets to protect them from flying sparks, and called a truck to haul the boat away.

About 200 protesters chanted nearby: "It's dire! It's dire! The house is on fire!"

The protest was backed by about two dozen groups, including the Metro D.C. chapter of the Democratic Socialists of America, Extinction Rebellion D.C. and Black Lives Matter D.C.