PARIS: World leaders launched an ambitious attempt on Monday to hold back rising temperatures, with the United States and China leading calls for the climate summit in Paris to mark a decisive turn in the fight against global warming.

In a series of opening addresses to the UN talks, heads of state and government exhorted each other to find common cause in two weeks of bargaining to steer the global economy away from its dependence on fossil fuels. French President Francois Hollande said the world was at a "breaking point".

The leaders arrived in Paris with high expectations and armed with promises to act. After decades of struggling negotiations and the failure of a summit in Copenhagen six years ago, some form of agreement - likely to be the strongest global climate pact yet - appears all but assured by mid-December.

"What should give us hope that this is a turning point, that this is the moment we finally determined we would save our planet, is the fact that our nations share a sense of urgency about this challenge and a growing realization that it is within our power to do something about it," said US President Barack Obama, one of the first leaders to speak at the summit.

The leaders gathered in a vast conference centre at Le Bourget airfield. In all, 195 countries are part of the unwieldy negotiating process, with a variety of leadership styles and ideologies that has made consensus elusive in the past.

Key issues, notably how to divide the global bill to pay for a shift to renewable energy, are still contentious.

"Climate justice demands that the little carbon space we still have, developing countries should have enough room to grow," said India's Prime Minister Narendra Modi, a key player because of his country's size and its heavy dependence on coal.

One difference this time may be the partnership between the United States and China, the two biggest carbon emitters, who between them account for almost 40 percent of the world's greenhouse gas emissions, according to the World Resources Institute think-tank.

Once far apart on climate issues, they agreed in 2014 to jointly kick-start a transition away from fossil fuels, each at its own speed and in its own way.

The United States and China "have both determined that it is our responsibility to take action," Obama said after meeting his Chinese counterpart Xi Jinping on the sidelines of the summit.

"Tackling climate change is a shared mission for mankind," Xi responded in his own remarks.

Obama said the two countries would work together at the summit to achieve an agreement that moves towards a low-carbon global economy this century and "robust" financial support for developing countries adapting to climate change.

Flying home to Rome on the papal plane after a visit to Africa, Pope Francis told journalists: "Every year the problems are getting worse. We are at the limits. If I may use a strong word I would say that we are at the limits of suicide."

Most scientists say failure to agree on strong measures in Paris would doom the world to ever-hotter average temperatures, deadlier storms, more frequent droughts and rising sea levels as polar ice caps melt.

SMOG OVER CHINA AND INDIA

Facing such alarming projections, the leaders of nations responsible for about 90 percent of the world's greenhouse gas emissions have come bearing pledges to reduce their national carbon output, through different measures at different rates.

As the summit opened in Paris, the capitals of the world's two most populous nations, China and India, were blanketed in hazardous, choking smog, with Beijing on an "orange" pollution alert, the second-highest level.

The deal will mark a momentous step in the often frustrating quest for global agreement, albeit one that on its own is not believed to be enough to prevent the earth's temperatures from rising past a damaging threshold. How and when nations should review their goals - and then set higher, more ambitious ones - is another issue to be resolved at the talks.

"The Paris conference is not the finishing line but a new starting point," Xi said.

The gathering is being held in a sombre city. Security has been tightened after Islamist militants killed 130 people on Nov. 13, and Hollande said he could not separate "the fight with terrorism from the fight against global warming". Leaders must face both challenges, leaving their children "a world freed of terror" as well as one "protected from catastrophes", he said.

On the eve of the summit, an estimated 785,000 people around the world joined the biggest day of climate change activism in history, telling world leaders there was "No Planet B" in the fight against global warming.

Signalling their determination to resolve the most intractable points, senior negotiators sat down on Sunday, a day earlier than planned, to begin their work.

The last attempt to get a global deal collapsed in chaos and acrimony in Copenhagen in 2009.

Anxious to avoid a re-run of the Copenhagen disaster, major powers have tried this time to smooth some of the bumps in the way of an agreement before they arrive.

The presidents, prime ministers and princes were making their cameo appearances at the outset of the conference rather than swooping in at the end.

The old goal of seeking a legally binding international treaty, certain to be dead on arrival in the Republican-controlled U.S. Congress, has been replaced by a system of national pledges to reduce emissions.

Some are presented as best intentions, others as measures legally enforced by domestic laws and regulations.

WHO WILL PAY?

If a signed deal now appears likely, so too is the prospect that it will not be enough to prevent the world's average temperature from rising beyond 2 degrees Celsius (3.6 Fahrenheit) above pre-industrial levels. That is widely viewed as a threshold for dangerous and potentially catastrophic changes in the planet's climate system.

Obama called for an "enduring framework for human progress", one that would compel countries to steadily ramp up their carbon-cutting goals and openly track progress against them.

The US-China agreement has been a balm for the main source of tension that characterised previous talks, in which the developing world argued that countries which had grown rich by industrialising on fossil fuels should pay the cost of shifting all economies to a renewable energy future.

The question of how richer nations can help cover the cost of switching to cleaner energy sources and offset climate-related damage must still be resolved,

A handful of the world's richest entrepreneurs, including Bill Gates, have pledged to double the $10 billion they collectively spend on clean energy research and development in the next five years.

"The climate bill has finally come due. Who will pay?" said Baron Waqa, president of the Pacific island nation Nauru.