Addressing a climate change summit in Paris on Nov. 30, President Obama said the U.S. accepted its responsibility to help fix climate change, adding that global action need not damage economic growth. (Reuters)

President Obama joined leaders from 150 nations on Monday in pledging action against climate change, kicking off a historic two-week gathering that Obama called a “turning point” in the battle against one of humanity’s gravest threats.

Heads of state from industrial powers and tiny island nations assembled in a heavily guarded conference center north of Paris, where negotiators hope to forge a treaty to dramatically reduce emissions of greenhouse-gas pollution blamed for warming the planet.

Obama, speaking a few miles from the site of the Nov. 13 terrorist attack in the French capital, called for urgent action against a challenge that he suggested was greater even than the fight against terrorism.

“The growing threat of climate change could define the contours of this century more dramatically than any other,” Obama said in a speech at the Le Bourget conference center in Paris’s industrial outskirts. “What should give us hope that this is a turning point, that this is the moment we finally determined we would save our planet.”

Obama, who has staked his legacy on the fight against climate change, struck an ominous tone in describing the ravages of a warming planet, declaring that “no nation large or small, wealthy or poor, is immune.” He urged the leaders to take action even if the benefits were not evident for generations.

Citing Martin Luther King Jr., he warned that “there is such a thing as being too late.”

The remarks came during a day of ceremonial fanfare as well as substantive progress in marshaling resources to speed the shift to cleaner energy. U.S. officials formally announced the formation of a 20-nation initiative to spur funding on energy research, in tandem with a similar undertaking led by Microsoft co-founder Bill Gates and 27 of the world’s wealthiest private investors.

But there were also signs of discord as negotiators prepared to haggle over details of a complex treaty that requires all nations — even the poorest ones — to make a contribution to cutting greeenhouse-gas pollution. Some developing countries have insisted on compensation for economic and environmental damage stemming from decades of industrial emissions that came mostly from Western industrial powers.

French Foreign Minister Laurent Fabius opened the conference by calling on all governments to back an ambitious treaty.

“The stakes are too high and the menace of climate change is too great for us to be content with a minimalistic agreement,” he said.

Obama has already aligned most of the world’s largest greenhouse gas emitters behind substantial cuts in greenhouse gas emissions, but over the next dozen days international climate negotiators must still nail down the details of an agreement that would strengthen existing commitments and introduce a way for countries to review and expand their commitments in the near future.

The negotiations have been slowed by delegates from developing countries who believe that, for all of Obama’s efforts, the United States should do even more to help them grapple with the effects of climate change and lower global emissions.

Obama said that “the United States not only recognizes our role in creating this problem, we embrace our responsibility to do something about it.” But he stressed that all nations must act “right now.”

More than two weeks after terrorist attacks rocked this city and shocked the world, Obama described the gathering of world leaders here as “an act of defiance” and asked: “What greater rejection of those who would tear down our world than marshaling our best efforts to save it?”

With so many world leaders present, the summit also opened chances for top-level dialogue on other fronts.

Sideline talks included a meeting between Obama and Russian President Vladi­mir Putin to discuss Syria’s nearly five-year civil war and peace efforts in Ukraine between the Western-supported government and Moscow-backed rebels.

The two leaders agreed to continue talks among global stakeholders that began this fall in Vienna to forge a political solution to the fighting in Syria. But the two sides remain far apart on some key issues, and no clear progress appeared to have been made in Paris.

Russia says its military intervention in Syria seeks to cripple the Islamic State — a mutual enemy of Washington and its allies. But Moscow has heavily targeted rebel factions seeking to topple Syrian President Bashar al-Assad, a key Russian partner. In Paris, Obama repeated U.S. demands that Assad must eventually step down as part of any political transition.

Obama kicked off his day meeting with Chinese President Xi Jinping, hailing their mutual work on climate change but urging greater Chinese cooperation on cybercrime and “maritime” issues that include China’s military construction on disputed reefs in the South China Sea.

Putting China first on the schedule was an indication of China’s central role on global issues such as climate change.

It was just a year ago that Obama and Xi vowed to set definite limits on greenhouse gas emissions, laying the foundation for other countries to follow suit. On Monday, Obama said: “Our leadership on this issue has been absolutely vital.”

Other highlights included a meeting with Indian Prime Minister Narendra Modi, whose government has been pressing for more financing and technology transfer. U.S. officials expressed frustration that Modi had not pledged to do more to avoid sharp increases in coal-fired electricity to help the 240 million Indians who lack electricity. They hoped to make progress in the coming days.

Obama also attended the formal unveiling of an initiative to boost research and development spending on new technologies, led by Gates. The Microsoft founder, in an interview with The Washington Post, warned that the climate threat is too serious to allow technology to evolve at the usual slow pace.

“Historically, it takes over 50 years before you have a substantial shift in energy generation, but we need to do it more quickly,” Gates said. “We need to move faster than the energy sector ever has.”

Obama expressed belief that a breakthrough was needed in addition to current technologies, something some critics say is too optimistic. “We don’t know exactly what’s going to work best but we know if we put our best minds behind it and put our dollars behind it we’ll discover what works,” Obama said.

Some countries believe solutions lie closer. A group of countries — France, Canada, Chile, Ethiopia, Germany and Mexico — proposed putting a worldwide price on carbon to discourage the use of carbon intensive fuels, such as coal.

The chances of getting such a proposal through the Republican-controlled Congress, however, are remote. It is also unclear whether Congress will provide the additional research dollars Obama pledged as part of the Gates “innovation” initiative.

The summit will also feature a goal, heavily promoted by India and including 121 countries, of a massive investment in solar power in countries with the greatest amounts of sunlight.

“We have broken the old arguments for inaction,” Obama said at the plenary session Monday. “We have proved that strong economic growth and stable investment no longer have to conflict with one another.”

Obama is one of about 150 world leaders at the Paris summit — formally known as the 21st Conference of Participants. Those leaders also delivered speeches.

“A political moment like this may not come again,” said U.N. Secretary General Ban Ki-moon. “We have never faced such a test. But neither have we encountered such great opportunity.”

In the wake of terrorist attacks that left 130 dead, the conference, taking place in a center located at an old airport in Le Bourget, has become a major security headache, and traffic has been blocked in Paris until Tuesday.

After arriving in Paris on Sunday, Obama’s motorcade glided along the Seine through largely deserted streets before stopping in front of Le Bataclan, the concert hall where scores of people were killed in the terrorist attacks.

Flanked by French President François Hollande and Paris Mayor Anne Hidalgo, Obama placed a white rose on the street in front of the concert hall, making a small addition to the mound of flowers and candles there. After standing for a minute of silence with his hands folded before him, Obama walked away, briefly placing a hand on the shoulders of Hollande and Hidalgo.

The United States announced that it will contribute $51.2 million to a $248 million Least Developed Countries Fund to help the world’s poorest countries adapt to climate change. Germany is the largest of 11 donors to the fund.

“We know the truth — that many nations have contributed little to climate change but will be the first to feel its most destructive effects,” Obama said.

But U.S. officials said they would resist calls by many small island states and other countries vulnerable to climate-change effects that developed nations pay reparations or damages because of their historic emissions. Those nations also favor setting a goal of limiting climate change to 1.5 degrees Celsius instead of 2 degrees. Obama will meet with leaders of island nations Tuesday.

Other countries used the conference to announce new details. Norway, Germany and Britain said they would provide $1 billion a year until 2020 in payments for verified emissions reductions from forests and land use in other countries.

Mufson reported from Paris and Warrick reported from Washington.