Crunch UN talks on climate change kicked off in Copenhagen today, amid calls for a strong international agreement which delivers immediate action to tackle rising temperatures.
Thousands of officials, environmental campaigners, politicians and journalists are in the Danish capital for the bid to strike a political deal on curbing emissions and providing finance to support poor countries in the fight against climate change.
At the talk's opening ceremony the UN's chief climate official Yvo de Boer said the negotiations will only be a success if they deliver "significant and immediate action" on global warming.
And the chairman of the UN's expert Intergovernmental Panel on Climate Change, Dr Rajendra Pachauri, said as the evidence was "now overwhelming" the world needs to take early action on global warming.
He told the opening of the conference that the "global community has a moral and material responsibility" to limit the impacts of climate change on vulnerable countries who are most at risk of flooding, storms, droughts and rising sea levels brought on by increases in temperatures.
Connie Hedegaard, conference president and Denmark's former climate minister warned: "This is our chance. If we miss it, it could take years before we got a new and better one. If we ever do."
In the UK, the run up to the negotiations has been dogged by the leaking of stolen emails from the University of East Anglia's Climatic Research Unit which climate sceptics claim show evidence that researchers were manipulating data to support a theory of man-made global warming.
Energy and Climate Change Secretary Ed Miliband has warned that those who argue climate change is not the result of human actions are "profoundly irresponsible".
"The overwhelming consensus of scientists across the world is that climate change is real and is man-made and is happening," he told BBC Radio 4's Today Programme.
"The people who do somehow want to suggest that the science is in doubt are profoundly irresponsible."
Mr Miliband warned the next two weeks, during which the Copenhagen talks will be attended by more than 100 world leaders and representatives of 192 countries, were "crunch time for the planet".
He said the UK is pushing for the most ambitious, effective and fair deal possible and warned that if countries fail to take advantage of the current momentum to secure agreement on efforts to tackle rising temperatures, it would not get any easier to achieve at a later date.
Prime Minister Gordon Brown is talking by telephone today to Australian prime minister Kevin Rudd and Swiss president Hans-Rudolf Merz, and on Thursday to Chinese premier Wen Jiabao, as part of efforts to secure a strong deal.
The raft of offers of action from rich and developing countries that has materialised in the days running up to the conference, and the plans by more than 100 leaders - including US President Barack Obama - to come to the closing day of the talks has raised hopes of an agreement.
But there is still further to go before the pledges on curbing emissions will deliver the cuts in greenhouse gases scientists say are needed to keep temperatures rising by more than 2C.
And billions of pounds of finance has also to be agreed to help poor countries cope with the impacts of climate change and develop without increasing their pollution.
It it hoped the talks will deliver political agreement which can be turned into a legally binding treaty in the coming months.
Andy Atkins, executive director of Friends of the Earth, said: "As the Copenhagen talks kick off, the pressure is on rich nations to do what's fair and right for the planet and the billions of people that live on it.
"Rich countries must commit to legally-binding targets to slash their emissions, drop their support for a global carbon market and put new public money on the table for developing countries," he urged.
"The time for action is now - we need bold and decisive leadership from our leaders so that in two weeks we have a strong, fair agreement to tackle climate change."
The chief scientist for the UK's Department for Environment, Food and Rural Affairs (Defra), professor Bob Watson said: "It is extremely critical in Copenhagen that we have a strong agreement of all major emitters to reduce emissions of greenhouse gases.
"It is essential that governments agree how to reduce climate change by mitigating their emissions and vital we get an agreement on working together on how to adapt to climate change."