US President Barack Obama has urged negotiators at the UN climate conference in Paris to deliver a meaningful deal because the "next generation is watching".
Negotiators from 195 countries will try to reach a deal, within two weeks, aimed at reducing global carbon emissions and limiting global warming to 2°C.
More on the talks in Paris
In a series of opening addresses at the UN talks, heads of state and government exhorted each other to find common cause in the fortnight of bargaining to steer the global economy away from its dependence on fossil fuels.
Leaders from 147 nations, including New Zealand Prime Minster John Key have been delivering speeches to the meeting, known as COP21.
President Obama told delegates: "Climate change could define the contours of this century more than any other (challenge).
"I came here personally to say the United States not only recognises the problem but is committed to do something about it."
He added that recent years had shown that the global economy had grown while emissions had remained flat, breaking the old arguments for inaction "that economic growth and environmental protection were in conflict".
Russian President Vladimir Putin also addressed the conference.
During negotiations for the preceding Kyoto Protocol, Russia was the last industrialised nation to ratify the global agreement, allowing the landmark deal to come into force in 2001.
Echoing his US counterpart, Mr Putin said: "We have demonstrated we can ensure economic development and take care of our environment at the same time."
In a diplomatic play on semantics, probably to highlight the differing points of view between industrialised and emerging economies, Chinese President Xi Jinping told the conference he did not see the Paris talks as a turning point nor a "finish line, but a new starting point".
He said climate change went beyond national borders and that it was "a shared mission for all mankind", before reiterating China's pledge to start cutting its emissions from a peak in 2030.
Crucial 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," Mr Obama said after meeting President Xi Jinping on the sidelines of the summit.
"Tackling climate change is a shared mission for mankind," Mr Xi said.
Mr Obama said the two countries would work together at the summit to achieve an agreement that moved toward a low-carbon global economy this century and "robust" financial support for developing countries adapting to climate change.
British Prime Minister David Cameron used his three-minute address to consider how future generations would respond to the idea that it was "too difficult" for this generation of politicians to reach an agreement in 2015.
"Our grandchildren would ask why it was so difficult," he said, before listing how progress had been made in delivering climate policy, such as financing, carbon budgets and technological research and development.
"Instead of making excuses to our children and grandchildren, we should be taking action," Mr Cameron stated.
-BBC / Reuters