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In the first international test for his climate-change strategy, President Barack Obama pressed world leaders Tuesday to follow the United States' lead on the issue, even as a United Nations summit revealed the many obstacles that still stand in the way of wider agreements to reduce heat-trapping pollution.

"The United States has made ambitious investments in clean energy and ambitious reductions in our carbon emissions," Obama said. "Today I call on all countries to join us, not next year or the year after that, but right now. Because no nation can meet this global threat alone."

But none of the pledges made at Tuesday's one-day meeting was binding. The largest-ever gathering of world leaders to discuss climate was designed to lay the groundwork for a new global climate-change treaty. It also revealed the sharp differences that divide countries on matters such as deforestation, carbon pollution and methane leaks from oil and gas production:

• Brazil, home to the Amazon rainforest, said it would not sign a pledge to halt deforestation by 2030.
   
• The United States decided not to join 73 countries in supporting a price on carbon, which Congress has indicated it would reject.
   
• And minutes after Obama said "nobody gets a pass," Chinese Vice Premier Zhang Gaoli insisted the world treat developing nations, including China, differently than developed nations, allowing them to release more heat-trapping pollution. China, the No. 1 carbon-polluting nation, signed on in support of pricing carbon and vowed to stop the rise of carbon-dioxide emissions as soon as possible.

"Today we must set the world on a new course," said United Nations Secretary-General Ban Ki-Moon, who added that pricing carbon was critical. "Climate change is the defining issue of our age. It is defining our present. Our response will define our future."

In some ways, the summit, which was part of the annual UN General Assembly, answered that call.

The European Union said its member nations next month were set to approve a plan that would cut greenhouse gases back to 40 per cent below 1990 levels by 2030. The EU also called for using renewable energy for 27 per cent of the bloc's power needs and increasing energy efficiency by 30 per cent.

The United States will not release its new emissions targets until early next year.

"There were not that many surprises," said Connie Hedegaard, the top climate official for the European Commission, referring to Obama's speech.

Hedegaard said the first-ever limits on carbon from power plants, proposed by Obama back in June, were "a good signal to send, but after today we will still have to wait until first quarter of 2015 to see how ambitious the United States will be."

By 2020, China will reduce its emissions per gross domestic product by 45 per cent from 2005 levels, Zhang said. But because economic growth in China has more than tripled since 2005, that means Chinese carbon pollution can continue to soar. Still, outside environmentalists hailed the country's promises because they went beyond any of China's previous statements.

More than 150 countries set the first-ever deadline to end deforestation by 2030, but that goal was thrown into doubt when Brazil said it would not join. Forests are important because they absorb the main greenhouse gas, carbon dioxide. The United States, Canada and the entire European Union signed onto a declaration to halve forest loss by 2020 and eliminate deforestation entirely by 2030.

If the forest goal is met, the UN says it would be the equivalent of taking every car in the world off the road. A group of companies, countries and nonprofits also pledged to restore more than 1 million square miles of forest worldwide by 2030. Norway promised to spend $350 million to protect forests in Peru and another $100 million in Liberia.

World leaders pledged to spend at least $5 billion making the world more sustainable. France promised $1 billion. Korea pledged $100 million. Others, like Chile, pledged cuts in greenhouse gas emissions by 2020.

A 2009 agreement called the Copenhagen Accord called for developed countries to contribute $10 billion a year in 2010 and scale it up to $100 billion a year by 2020 to help developing countries cut emissions and adapt to a changing climate.

Venezuelan President Nicolas Maduro chastised "polluting powers" for causing an "evil of such planetary dimensions" and then trying to barter their way out of their responsibilities.

Seychelles President James Michel called small island nations like his "victims of this pollution" and said it was up to the countries that burn the most coal, oil and gas to do the most.

"If they don't do something, the Earth will not survive, and that will be the end of us all," Michel said in an interview before the start of the summit.

Ban, actor Leonardo DiCaprio, former U.S. Vice-President Al Gore and scientist Rajendra K. Pachauri warned that time was short. By 2020, Ban said, the world must reduce greenhouse gases to prevent an escalating level of warming. Five years ago, leaders pledged to keep world temperatures from increasing by another 3.6 degrees Celsius.

Pachauri, who headed a Nobel Prize-winning panel of scientists that studied the issue, and Ban told world leaders the effects of global warming are already here, pointing to a UN building that flooded during the devastating Superstorm Sandy in 2012. Pachauri said it will get worse with droughts, storms and food and water shortages. He foresaw even more violent climate-driven conflicts.

And, Pachauri said, "a steady rise in our death toll, especially among the world's poorest. How on Earth can we leave our children with a world like this?"