The UN has expressed optimism over international talks on climate change that have opened in Denmark, billing it as a "turning point" in a bid to slow down global warming.
Yvo de Boer, the head of the UN framework convention on climate change, said he was confident the summit in Copenhagen, the Danish capital, would be a success.
"Copenhagen is already a turning point in the international response to climate change," he said.
The high-stakes talks are aimed at agreeing a deal for measures to help curb greenhouse gas emissions and the transfer of hundreds of billions of dollars from rich to poor countries to help developing nations adapt to climate change over the coming years.
Alan Fisher, Al Jazeera's correspondent in Copenhagen, said: "To get a 192 countries all pointing in the same direction, all with the same goal, it is, to use one phrase, 'like herding cats,' it's very difficult.
"But if you look around you can see why we need to do this, in Africa there is drought, there are people starving, and in one area, the Arctic Circle, we've seen temperatures rise there faster than anywhere else.
"In fact, they're talking about a rise in sea levels of somewhere between 5-7cm over the next 20 to 30 years."
A major challenge facing the two-week conference, which ends with a meeting of 105 world leaders on December 18, is to overcome deep distrust between rich and poor nations about sharing out the burden of costly curbs on emissions.
But the planned attendance of heads of state and pledges to curb emissions by all the top emitters – led by China, the US, Russia and India – have raised hopes for an accord after sluggish negotiations in the past two years.
Lars Lokke Rasmussen, the Danish prime minister, and Rajendra Pachauri, head of the UN's climate expert panel, are among the speakers at Monday's opening session.
Manmohan Singh, the Indian prime minister whose country is the world's fourth-largest greenhouse gas emitter, announced on Saturday that he would attend the closing summit, joining 104 other leaders, including Barack Obama, the US president.
World leaders did not attend the last time the world's environment ministers agreed on the existing UN climate pact, known as the Kyoto Protocol, in 1997.
Copenhagen is the biggest climate change meeting in history, with 15,000 participants from more than 190 nations.
New money needed
De Boer said developing nations need to take new action to slow the rise of their emissions.
He also wants rich nations to accept deep cuts to their emissions by 2020 and come up with at least $10bn a year in aid to the poor countries to kick off a deal, saying it has to be "new money, real and significant".
On Sunday he tried to allay criticism among climate change sceptics after emails from climate scientists that appeared to cast doubt on their research were recently leaked to the public.
De Boer acknowledged the emails did serious damage, but said the review process by some 2,500 scientists of climate change research was thorough and credible.
China, India, Brazil and South Africa earlier rejected a Danish suggestion to set a goal of halving world emissions by 2050, saying rich nations which have burnt fossil fuels since the industrial revolution must first cut their own emissions.
But South Africa added new impetus on Sunday, saying it was willing to cut its carbon emissions to 34 per cent below expected levels by 2020, provided that rich countries furnish financial and technological help.
"At the deal's heart must be a settlement between the rich world and the developing world. Social justice demands that the industrialised world digs deep into its pockets. Many of us, particularly in the developed world, will have to change our lifestyles"
Joint editorial by major world newspapers
The climate talks have sparked protests in many European cities, adding to the pressure world leaders are under to reduce rising emissions that the UN says will cause desertification, mudslides, more powerful cyclones, rising sea levels and the extinction of species.
The existing Kyoto pact obliges industrialised nations to cut emissions until 2012, and the idea behind the Copenhagen talks is to get action from all major emitters, including China and India, which were exempt from the Kyoto agreement.
The meeting will test how far developing nations will stick to entrenched positions, for example that rich nations must cut their greenhouse gases by at least 40 per cent by 2020 – far deeper than targets on offer.
On Monday some 56 newspapers from 45 countries published a joint editorial urging rich and poor nations to unite in Copenhagen.
"At the deal's heart must be a settlement between the rich world and the developing world," it said.
"Social justice demands that the industrialised world digs deep into its pockets. Many of us, particularly in the developed world, will have to change our lifestyles."