NEW YORK — Days after millions of young people took to the streets around the world to protest for more aggressive action on climate change, dozens of world leaders arrived Monday at the United Nations facing a simple question: What’s your plan?

Monday’s U.N. climate summit was billed as an opportunity for nations to make high-profile commitments to do more. But it also underscored the growing tensions between a burgeoning activist movement and global leaders, many of whom acknowledge the seriousness of climate change but have yet to commit to the kind of transformational changes that scientists say are needed to avert its worst effects in coming decades.

“You are failing us,” teenage climate activist Greta Thunberg told heads of state as she spoke onstage, her face flushed with anger. “The eyes of all future generations are upon you. And if you choose to fail us, I say, we will never forgive you.”

U.N. Secretary General António Guterres had insisted that countries show up to a much-anticipated “climate action summit” not with lofty rhetoric but with concrete promises, such as vowing to reach net zero emissions by 2050, cutting fossil fuel subsidies and ceasing construction of coal-fired power plants.

“Dear friends, there is a cost to everything. But the biggest cost is doing nothing,” Guterres told the gathering Monday morning. “The biggest cost is subsidizing a dying fossil fuel industry, building more and more coal power 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.”

Promises flowed one after another Monday, in three-minute speech after three-minute speech, as leaders from France, Germany, India and other countries detailed plans to increase their use of renewable energy and curb fossil fuel burning linked to climate change. Overall, at least 65 countries have now indicated that they intend to strengthen their commitments they made under the 2015 Paris agreement by the end of next year.

“We cannot let our youth spend every Friday demonstrating for the climate and simply answer, ‘Everything is fine, we are doing everything right,’ ” said French President Emmanuel Macron. “We are still far from the account.”

But as the day wore on, there was a growing sense that Monday’s summit would not by itself offer the jolt organizers had hoped. While many businesses and investors announced ambitious climate plans, as well as smaller countries and island nations already feeling the effects of climate change, the world’s largest emitters once again stopped short of committing to far-reaching new goals to rein in emissions.

On Monday, Indian Prime Minister Narendra Modi committed his nation to more than doubling its renewable energy capacity by 2030, though the growing country still relies heavily on coal-fired power generation. German Chancellor Angela Merkel detailed a recently passed package of measures to invest billions in electric vehicles, ramp up renewable energy and phase out coal, though green groups have called the efforts too modest.

United Nations Secretary-General Antonio Guterres, far left, and young environmental activists look on as Greta Thunberg, of Sweden, far right, addresses the Climate Action Summit in the United Nations General Assembly, at U.N. headquarters, Monday, Sept. 23, 2019. AP Photo/Jason DeCrow

“Most of the major economies fell woefully short. Their lack of ambition stands in sharp contrast with the growing demand for action around the world,” Andrew Steer, head of the World Resources Institute, said in a statement. “We need far greater national leadership on climate action – and we need it now.”

It remains unclear whether nations can ultimately muster the collective political will to make the kind of shift away from fossil fuels in transportation, energy and agriculture that scientists say are necessary to limit global warming. The planet has already warmed 1 degree Celsius (1.8 degrees Fahrenheit) above preindustrial levels.

In Paris in late 2015, leaders from 195 nations banded together in an unprecedented agreement to collectively cut carbon emissions and hold the world’s warming to “well below” 2 degrees C (3.6 degrees F). Four years later, the euphoria of that moment has faded into a sober realism about the scale of the challenge and the many obstacles that exist.

Global emissions of carbon dioxide are still rising. Many countries are failing to live up to the pledges they made in Paris – promises that leaders acknowledge were not ambitious enough to begin with – even as leaders have been asked to devise more ambitious plans by the end of 2020. Scientists have continued to issue ever more dire warnings about melting permafrost, retreating glaciers, rising seas, more extreme weather, and the impacts such changes are having on societies.

In addition, the United States under President Trump has retreated from its role as a global leader on climate action, and instead is planning to withdraw from the Paris accord. On Monday, Trump swept through the halls of the U.N. on his way to a meeting, as Thunberg looked on sternly. But otherwise, the United States was virtually invisible during a day focused on climate action.

The Climate Action Tracker, a consortium of scientists that compare global climate pledges and policies to future temperature projections, issued an analysis last week that found that if current policies remain in place, the world will warm at least 1.5 degrees C around 2035, 2 degrees around 2053 and 3.2 degrees by the end of the century. Even if national governments achieve the emissions cuts they have committed to, it noted, the average global temperature is likely to rise 2.9 degrees C by 2100.

Niklas Höhne, a German climatologist and founding partner of NewClimate Institute who created the Climate Action Tracker, said in an interview that the announcements made Monday are unlikely to change the long-term projections for global temperature rise because many of those nations are not large carbon emitters.

“It has to be a large player, with a large amount of emissions, to change the temperature,” he said.

Höhne added that while the actions of many individual states and cities, as well as U.S. companies, have kept overall U.S. carbon emissions from changing course significantly under Trump, the administration’s promotion of natural gas exports to countries such as India could ultimately lead to higher emissions overseas. During Modi’s recent visit to Houston, India’s state-owned Petronet LNG announced that it would invest in Tellurian Inc.’s liquefied natural gas project on the Gulf Coast.

“We need less fossil fuel infrastructure, not more,” Höhne said.

Some smaller, low-lying countries that are already wrestling with the impacts of climate change took the opportunity Monday to chide the world’s largest economies for not doing more to address climate change.

“My appeal is that powerful countries must chip in and take this seriously,” said Pakistani Prime Minister Imran Khan, as he said his nation would try to plant 10 billion trees over the next five years.

Monday’s much-anticipated summit comes amid growing pressure on world leaders to act more aggressively – and swiftly – to combat climate change.

On Friday, in one of the largest youth-led demonstrations ever, millions of people across more than 150 countries and every continent took to the streets to protests for climate action. Among the largest of the demonstrations was in New York, where a massive crowd filled with sign-toting students and other protesters filled Lower Manhattan, led by Thunberg.

At the United Nations on Monday, she and 15 other young people filed a legal complaint with the U.N. Committee on the Rights of the Child arguing that major countries have known about the risks of climate change for decades but have failed to take sufficient action to curb their emissions.

“Right here, right now, is where we draw the line,” she said. “The world is waking up, and change is coming whether you like it or not.”

Guterres reminded world leaders Monday that on the current trajectory, the world will face at least 3 degrees Celsius of warming by the end of the century – a level that scientists agree could trigger catastrophic impacts around the global.

“I will not be there, but my granddaughters will. And your grandchildren, too,” he said. “I refuse to be an accomplice in the destruction of their one and only home.”

He added, “It is my obligation – our obligation – to do everything to stop the climate crisis before it stops us. Time is running out. But it is not too late.”